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In 2017, the dominant greenhouse gases released into Earth's atmosphere-carbon dioxide, methane, and nitrous oxide-reached new record highs. The annual global average carbon dioxide concentration at Earth's surface for 2017 was 405.0 ± 0.1 ppm, 2.2 ppm greater than for 2016 and the highest in the modern atmospheric measurement record and in ice core records dating back as far as 800 000 years. The global growth rate of CO2 has nearly quadrupled since the early 1960s. With ENSO-neutral conditions present in the central and eastern equatorial Pacific Ocean during most of the year and weak La Niña conditions notable at the start and end, the global temperature across land and ocean surfaces ranked as the second or third highest, depending on the dataset, since records began in the mid-to-late 1800s. Notably, it was the warmest non-El Niño year in the instrumental record. Above Earth's surface, the annual lower tropospheric temperature was also either second or third highest according to all datasets analyzed. The lower stratospheric temperature was about 0.2°C higher than the record cold temperature of 2016 according to most of the in situ and satellite datasets. Several countries, including Argentina, Uruguay, Spain, and Bulgaria, reported record high annual temperatures. Mexico broke its annual record for the fourth consecutive year. On 27 January, the temperature reached 43.4°C at Puerto Madryn, Argentina-the highest temperature recorded so far south (43°S) anywhere in the world. On 28 May in Turbat, western Pakistan, the high of 53.5°C tied Pakistan's all-time highest temperature and became the world-record highest temperature for May. In the Arctic, the 2017 land surface temperature was 1.6°C above the 1981-2010 average, the second highest since the record began in 1900, behind only 2016. The five highest annual Arctic temperatures have all occurred since 2007. Exceptionally high temperatures were observed in the permafrost across the Arctic, with record values reported in much of Alaska and northwestern Canada. In August, high sea surface temperature (SST) records were broken for the Chukchi Sea, with some regions as warm as +11°C, or 3° to 4°C warmer than the longterm mean (1982-present). According to paleoclimate studies, today's abnormally warm Arctic air and SSTs have not been observed in the last 2000 years. The increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 7 March, sea ice extent at the end of the growth season saw its lowest maximum in the 37-year satellite record, covering 8% less area than the 1981-2010 average. The Arctic sea ice minimum on 13 September was the eighth lowest on record and covered 25% less area than the long-term mean. Preliminary data indicate that glaciers across the world lost mass for the 38th consecutive year on record; the declines are remarkably consistent from region to region. Cumulatively since 1980, this loss is equivalent to slicing 22 meters off the top of the average glacier. Antarctic sea ice extent remained below average for all of 2017, with record lows during the first four months. Over the continent, the austral summer seasonal melt extent and melt index were the second highest since 2005, mostly due to strong positive anomalies of air temperature over most of the West Antarctic coast. In contrast, the East Antarctic Plateau saw record low mean temperatures in March. The year was also distinguished by the second smallest Antarctic ozone hole observed since 1988. Across the global oceans, the overall long-term SST warming trend remained strong. Although SST cooled slightly from 2016 to 2017, the last three years produced the three highest annual values observed; these high anomalies have been associated with widespread coral bleaching. The most recent global coral bleaching lasted three full years, June 2014 to May 2017, and was the longest, most widespread, and almost certainly most destructive such event on record. Global integrals of 0-700-m and 0-2000-m ocean heat content reached record highs in 2017, and global mean sea level during the year became the highest annual average in the 25-year satellite altimetry record, rising to 77 mm above the 1993 average. In the tropics, 2017 saw 85 named tropical storms, slightly above the 1981-2010 average of 82. The North Atlantic basin was the only basin that featured an above-normal season, its seventh most active in the 164-year record. Three hurricanes in the basin were especially notable. Harvey produced record rainfall totals in areas of Texas and Louisiana, including a storm total of 1538.7 mm near Beaumont, Texas, which far exceeds the previous known U.S. tropical cyclone record of 1320.8 mm. Irma was the strongest tropical cyclone globally in 2017 and the strongest Atlantic hurricane outside of the Gulf of Mexico and Caribbean on record with maximum winds of 295 km h-1. Maria caused catastrophic destruction across the Caribbean Islands, including devastating wind damage and flooding across Puerto Rico. Elsewhere, the western North Pacific, South Indian, and Australian basins were all particularly quiet. Precipitation over global land areas in 2017 was clearly above the long-term average. Among noteworthy regional precipitation records in 2017, Russia reported its second wettest year on record (after 2013) and Norway experienced its sixth wettest year since records began in 1900. Across India, heavy rain and flood-related incidents during the monsoon season claimed around 800 lives. In August and September, above-normal precipitation triggered the most devastating floods in more than a decade in the Venezuelan states of Bolívar and Delta Amacuro. In Nigeria, heavy rain during August and September caused the Niger and Benue Rivers to overflow, bringing floods that displaced more than 100 000 people. Global fire activity was the lowest since at least 2003; however, high activity occurred in parts of North America, South America, and Europe, with an unusually long season in Spain and Portugal, which had their second and third driest years on record, respectively. Devastating fires impacted British Columbia, destroying 1.2 million hectares of timber, bush, and grassland, due in part to the region's driest summer on record. In the United States, an extreme western wildfire season burned over 4 million hectares; the total costs of $18 billion tripled the previous U.S. annual wildfire cost record set in 1991.

In 2016, the dominant greenhouse gases released into Earth's atmosphere-carbon dioxide, methane, and nitrous oxide-continued to increase and reach new record highs. The 3.5 +/- 0.1 ppm rise in global annual mean carbon dioxide from 2015 to 2016 was the largest annual increase observed in the 58-year measurement record. The annual global average carbon dioxide concentration at Earth's surface surpassed 400 ppm (402.9 +/- 0.1 ppm) for the first time in the modern atmospheric measurement record and in ice core records dating back as far as 800000 years. One of the strongest El Nino events since at least 1950 dissipated in spring, and a weak La Nina evolved later in the year. Owing at least in part to the combination of El Nino conditions early in the year and a long-term upward trend, Earth's surface observed record warmth for a third consecutive year, albeit by a much slimmer margin than by which that record was set in 2015. Above Earth's surface, the annual lower troposphere temperature was record high according to all datasets analyzed, while the lower stratospheric temperature was record low according to most of the in situ and satellite datasets. Several countries, including Mexico and India, reported record high annual temperatures while many others observed near-record highs. A week-long heat wave at the end of April over the northern and eastern Indian peninsula, with temperatures surpassing 44 degrees C, contributed to a water crisis for 330 million people and to 300 fatalities. In the Arctic the 2016 land surface temperature was 2.0 degrees C above the 1981-2010 average, breaking the previous record of 2007, 2011, and 2015 by 0.8 degrees C, representing a 3.5 degrees C increase since the record began in 1900. The increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 24 March, the sea ice extent at the end of the growth season saw its lowest maximum in the 37-year satellite record, tying with 2015 at 7.2% below the 1981-2010 average. The September 2016 Arctic sea ice minimum extent tied with 2007 for the second lowest value on record, 33% lower than the 1981-2010 average. Arctic sea ice cover remains relatively young and thin, making it vulnerable to continued extensive melt. The mass of the Greenland Ice Sheet, which has the capacity to contribute similar to 7 m to sea level rise, reached a record low value. The onset of its surface melt was the second earliest, after 2012, in the 37-year satellite record. Sea surface temperature was record high at the global scale, surpassing the previous record of 2015 by about 0.01 degrees C. The global sea surface temperature trend for the 21st century-to-date of +0.162 degrees C decade(-1) is much higher than the longer term 1950-2016 trend of +0.100 degrees C decade(-1). Global annual mean sea level also reached a new record high, marking the sixth consecutive year of increase. Global annual ocean heat content saw a slight drop compared to the record high in 2015. Alpine glacier retreat continued around the globe, and preliminary data indicate that 2016 is the 37th consecutive year of negative annual mass balance. Across the Northern Hemisphere, snow cover for each month from February to June was among its four least extensive in the 47-year satellite record. Continuing a pattern below the surface, record high temperatures at 20-m depth were measured at all permafrost observatories on the North Slope of Alaska and at the Canadian observatory on northernmost Ellesmere Island. In the Antarctic, record low monthly surface pressures were broken at many stations, with the southern annular mode setting record high index values in March and June. Monthly high surface pressure records for August and November were set at several stations. During this period, record low daily and monthly sea ice extents were observed, with the November mean sea ice extent more than 5 standard deviations below the 1981-2010 average. These record low sea ice values contrast sharply with the record high values observed during 2012-14. Over the region, springtime Antarctic stratospheric ozone depletion was less severe relative to the 1991-2006 average, but ozone levels were still low compared to pre-1990 levels. Closer to the equator, 93 named tropical storms were observed during 2016, above the 1981-2010 average of 82, but fewer than the 101 storms recorded in 2015. Three basins-the North Atlantic, and eastern and western North Pacific-experienced above-normal activity in 2016. The Australian basin recorded its least active season since the beginning of the satellite era in 1970. Overall, four tropical cyclones reached the Saffir-Simpson category 5 intensity level. The strong El Nino at the beginning of the year that transitioned to a weak La Nina contributed to enhanced precipitation variability around the world. Wet conditions were observed throughout the year across southern South America, causing repeated heavy flooding in Argentina, Paraguay, and Uruguay. Wetter-than-usual conditions were also observed for eastern Europe and central Asia, alleviating the drought conditions of 2014 and 2015 in southern Russia. In the United States, California had its first wetter-than-average year since 2012, after being plagued by drought for several years. Even so, the area covered by drought in 2016 at the global scale was among the largest in the post-1950 record. For each month, at least 12% of land surfaces experienced severe drought conditions or worse, the longest such stretch in the record. In northeastern Brazil, drought conditions were observed for the fifth consecutive year, making this the longest drought on record in the region. Dry conditions were also observed in western Bolivia and Peru; it was Bolivia's worst drought in the past 25 years. In May, with abnormally warm and dry conditions already prevailing over western Canada for about a year, the human-induced Fort McMurray wildfire burned nearly 590000 hectares and became the costliest disaster in Canadian history, with $3 billion (U.S. dollars) in insured losses.

International audience; This chapter discusses a few general properties of Antarctica ice sheet and shows why ice sheet topography is one of the most pertinent parameters for studying the dynamics of the ice sheet. It describes specific aspects of the altimetry technique applied for studying land ice and discusses specifics of measurements of polar ice sheets by radar altimeters. The chapter addresses various areas to which radar and laser altimetry have made notable contributions: surface climatology, ice dynamics, and monitoring of ice volume variations. It summarizes average characteristics of the ice sheet, assuming in first approximation that it is in a state of mass balance. Surface topography is one of the most pertinent parameters for monitoring an ice sheet but also for constraining ice flow models. When using altimetric series, the error due to the poor temporal repetition of the satellite ground track and the presence of surface slope must be corrected.

Satellite gravimetry is a unique measurement technique for observing mass transport processes in the Earth system on a global scale, providing essential indicators of both subtle and dramatic global change. Although past and current satellite gravity missions have achieved spectacular science results, due to their limited spatial and temporal resolution as well as limited length of the available time series numerous important questions are still unresolved. Therefore, it is important to move from current demonstration capabilities to sustained observation of the Earth’s gravity field. In an international initiative performed under the umbrella of the International Union of Geodesy and Geophysics, consensus on the science and user needs for a future satellite gravity observing system has been derived by an international panel of scientists representing the main fields of application, i.e., continental hydrology, cryosphere, ocean, atmosphere and solid Earth. In this paper the main results and findings of this initiative are summarized. The required target performance in terms of equivalent water height has been identified as 5 cm for monthly fields and 0.5 cm/year for long-term trends at a spatial resolution of 150 km. The benefits to meet the main scientific and societal objectives are investigated, and the added value is demonstrated for selected case studies covering the main fields of application. The resulting consolidated view on the required performance of a future sustained satellite gravity observing system represents a solid basis for the definition of technological and mission requirements, and is a prerequisite for mission design studies of future mission concepts and constellations.

SxviAUGUST 2016|ABSTRACT—J. BLUNDEN AND D. S. ARNDTIn 2015, the dominant greenhouse gases released into Earth’s atmosphere—carbon dioxide, methane, and nitrous oxide—all continued to reach new high levels. At Mauna Loa, Hawaii, the annual CO2 concentration increased by a record 3.1 ppm, exceeding 400 ppm for the first time on record. The 2015 global CO2 average neared this threshold, at 399.4 ppm. Additionally, one of the strongest El Niño events since at least 1950 developed in spring 2015 and continued to evolve through the year. The phenomenon was far reaching, impacting many regions across the globe and affecting most aspects of the climate system.Owing to the combination of El Niño and a long-term up-ward trend, Earth observed record warmth for the second con-secutive year, with the 2015 annual global surface temperature surpassing the previous record by more than 0.1°C and exceed-ing the average for the mid- to late 19th century—commonly considered representative of preindustrial conditions—by more than 1°C for the first time. Above Earth’s surface, lower troposphere temperatures were near-record high.Across land surfaces, record to near-record warmth was reported across every inhabited continent. Twelve countries, including Russia and China, reported record high annual tem-peratures. In June, one of the most severe heat waves since 1980 affected Karachi, Pakistan, claiming over 1000 lives. On 27 October, Vredendal, South Africa, reached 48.4°C, a new global high temperature record for this month. In the Arctic, the 2015 land surface temperature was 1.2°C above the 1981–2010 average, tying 2007 and 2011 for the high-est annual temperature and representing a 2.8°C increase since the record began in 1900. Increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 25 February 2015, the lowest maximum sea ice extent in the 37-year satel-lite record was observed, 7% below the 1981–2010 average. Mean sea surface temperatures across the Arctic Ocean dur-ing August in ice-free regions, representative of Arctic Ocean summer anomalies, ranged from ~0°C to 8°C above average. As a consequence of sea ice retreat and warming oceans, vast walrus herds in the Pacific Arctic are hauling out on land rather than on sea ice, raising concern about the energetics of females and young animals. Increasing temperatures in the Barents Sea are linked to a community-wide shift in fish populations: boreal communities are now farther north, and long-standing Arctic species have been almost pushed out of the area.Above average sea surface temperatures are not confined to the Arctic. Sea surface temperature for 2015 was record high at the global scale; however, the North Atlantic southeast of Greenland remained colder than average and colder than 2014. Global annual ocean heat content and mean sea level also reached new record highs. The Greenland Ice Sheet, with the capacity to contribute ~7 m to sea level rise, experienced melting over more than 50% of its surface for the first time since the record melt of 2012.Other aspects of the cryosphere were remarkable. Alpine glacier retreat continued, and preliminary data indicate that 2015 is the 36th consecutive year of negative annual mass balance. Across the Northern Hemisphere, late-spring snow cover extent continued its trend of decline, with June the sec-ond lowest in the 49-year satellite record. Below the surface, record high temperatures at 20-m depth were measured at all permafrost observatories on the North Slope of Alaska, increasing by up to 0.66°C decade–1 since 2000. In the Antarctic, surface pressure and temperatures were lower than the 1981–2010 average for most of the year, consis-tent with the primarily positive southern annular mode, which saw a record high index value of +4.92 in February. Antarctic sea ice extent and area had large intra-annual variability, with a shift from record high levels in May to record low levels in August. Springtime ozone depletion resulted in one of the largest and most persistent Antarctic ozone holes observed since the 1990s.Closer to the equator, 101 named tropical storms were observed in 2015, well above the 1981–2010 average of 82. The eastern/central Pacific had 26 named storms, the most since 1992. The western north Pacific and north and south Indian Ocean basins also saw high activity. Globally, eight tropical cyclones reached the Saffir–Simpson Category 5 intensity level.Overlaying a general increase in the hydrologic cycle, the strong El Niño enhanced precipitation variability around the world. An above-normal rainy season led to major floods in Paraguay, Bolivia, and southern Brazil. In May, the United States recorded its all-time wettest month in its 121-year national record. Denmark and Norway reported their second and third wettest year on record, respectively, but globally soil moisture was below average, terrestrial groundwater storage was the lowest in the 14-year record, and areas in “severe” drought rose from 8% in 2014 to 14% in 2015. Drought conditions prevailed across many Caribbean island nations, Colombia, Venezuela, and northeast Brazil for most of the year. Several South Pacific countries also experienced drought. Lack of rainfall across Ethiopia led to its worst drought in decades and affected millions of people, while prolonged drought in South Africa severely affected agricultural production. Indian summer monsoon rainfall was just 86% of average. Extremely dry conditions in Indonesia resulted in intense and widespread fires during August–November that produced abundant car-bonaceous aerosols, carbon monoxide, and ozone. Overall, emissions from tropical Asian biomass burning in 2015 were almost three times the 2001–14 average.

We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth's energy imbalance and heat flux into most of the global ocean's surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10-40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (500-2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6-9 m with evidence of extreme storms while Earth was less than 1 °C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50-150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.

© 2015, Springer-Verlag Berlin Heidelberg. Changing rainfall patterns have significant effect on water resources, agriculture output in many countries, especially the country like India where the economy depends on rain-fed agriculture. Rainfall over India has large spatial as well as temporal variability. To understand the variability in rainfall, spatial–temporal analyses of rainfall have been studied by using 107 (1901–2007) years of daily gridded India Meteorological Department (IMD) rainfall datasets. Further, the validation of IMD precipitation data is carried out with different observational and different reanalysis datasets during the period from 1989 to 2007. The Global Precipitation Climatology Project data shows similar features as that of IMD with high degree of comparison, whereas Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation data show similar features but with large differences, especially over northwest, west coast and western Himalayas. Spatially, large deviation is observed in the interior peninsula during the monsoon season with National Aeronautics Space Administration-Modern Era Retrospective-analysis for Research and Applications (NASA-MERRA), pre-monsoon with Japanese 25 years Re Analysis (JRA-25), and post-monsoon with climate forecast system reanalysis (CFSR) reanalysis datasets. Among the reanalysis datasets, European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) shows good comparison followed by CFSR, NASA-MERRA, and JRA-25. Further, for the first time, with high resolution and long-term IMD data, the spatial distribution of trends is estimated using robust regression analysis technique on the annual and seasonal rainfall data with respect to different regions of India. Significant positive and negative trends are noticed in the whole time series of data during the monsoon season. The northeast and west coast of the Indian region shows significant positive trends and negative trends over western Himalayas and north central Indian region.

Most of the dozens of essential climate variables monitored each year in this report continued to follow their long-term trends in 2014, with several setting new records. Carbon dioxide, methane, and nitrous oxide-the major greenhouse gases released into Earth's atmosphere-once again all reached record high average atmospheric concentrations for the year. Carbon dioxide increased by 1.9 ppm to reach a globally averaged value of 397.2 ppm for 2014. Altogether, 5 major and 15 minor greenhouse gases contributed 2.94 W m-2 of direct radiative forcing, which is 36% greater than their contributions just a quarter century ago. Accompanying the record-high greenhouse gas concentrations was nominally the highest annual global surface temperature in at least 135 years of modern record keeping, according to four independent observational analyses. The warmth was distributed widely around the globe's land areas, Europe observed its warmest year on record by a large margin, with close to two dozen countries breaking their previous national temperature records; many countries in Asia had annual temperatures among their 10 warmest on record; Africa reported above-average temperatures across most of the continent throughout 2014; Australia saw its third warmest year on record, following record heat there in 2013; Mexico had its warmest year on record; and Argentina and Uruguay each had their second warmest year on record. Eastern North America was the only major region to observe a below-average annual temperature. But it was the oceans that drove the record global surface temperature in 2014. Although 2014 was largely ENSO-neutral, the globally averaged sea surface temperature (SST) was the highest on record. The warmth was particularly notable in the North Pacific Ocean where SST anomalies signaled a transition from a negative to positive phase of the Pacific decadal oscillation. In the winter of 2013/14, unusually warm water in the northeast Pacific was associated with elevated ocean heat content anomalies and elevated sea level in the region. Globally, upper ocean heat content was record high for the year, reflecting the continued increase of thermal energy in the oceans, which absorb over 90% of Earth's excess heat from greenhouse gas forcing. Owing to both ocean warming and land ice melt contributions, global mean sea level in 2014 was also record high and 67 mm greater than the 1993 annual mean, when satellite altimetry measurements began. Sea surface salinity trends over the past decade indicate that salty regions grew saltier while fresh regions became fresher, suggestive of an increased hydrological cycle over the ocean expected with global warming. As in previous years, these patterns are reflected in 2014 subsurface salinity anomalies as well. With a now decade-long trans-basin instrument array along 26°N, the Atlantic meridional overturning circulation shows a decrease in transport of-4.2 ± 2.5 Sv decade-1. Precipitation was quite variable across the globe. On balance, precipitation over the world's oceans was above average, while below average across land surfaces. Drought continued in southeastern Brazil and the western United States. Heavy rain during April-June led to devastating floods in Canada's Eastern Prairies. Above-normal summer monsoon rainfall was observed over the southern coast of West Africa, while drier conditions prevailed over the eastern Sahel. Generally, summer monsoon rainfall over eastern Africa was above normal, except in parts of western South Sudan and Ethiopia. The south Asian summer monsoon in India was below normal, with June record dry. Across the major tropical cyclone basins, 91 named storms were observed during 2014, above the 1981-2010 global average of 82. The Eastern/Central Pacific and South Indian Ocean basins experienced significantly above-normal activity in 2014; all other basins were either at or below normal. The 22 named storms in the Eastern/Central Pacific was the basin's most since 1992. Similar to 2013, the North Atlantic season was quieter than most years of the last two decades with respect to the number of storms, despite the absence of El Niño conditions during both years. In higher latitudes and at higher elevations, increased warming continued to be visible in the decline of glacier mass balance, increasing permafrost temperatures, and a deeper thawing layer in seasonally frozen soil. In the Arctic, the 2014 temperature over land areas was the fourth highest in the 115-year period of record and snow melt occurred 20-30 days earlier than the 1998-2010 average. The Greenland Ice Sheet experienced extensive melting in summer 2014. The extent of melting was above the 1981-2010 average for 90% of the melt season, contributing to the second lowest average summer albedo over Greenland since observations began in 2000 and a record-low albedo across the ice sheet for August. On the North Slope of Alaska, new record high temperatures at 20-m depth were measured at four of five permafrost observatories. In September, Arctic minimum sea ice extent was the sixth lowest since satellite records began in 1979. The eight lowest sea ice extents during this period have occurred in the last eight years. Conversely, in the Antarctic, sea ice extent countered its declining trend and set several new records in 2014, including record high monthly mean sea ice extent each month from April to November. On 20 September, a record large daily Antarctic sea ice extent of 20.14 × 106 km2 occurred. The 2014 Antarctic stratospheric ozone hole was 20.9 million km2 when averaged from 7 September to 13 October, the sixth smallest on record and continuing a decrease, albeit statistically insignificant, in area since 1998.

© 2015 Elsevier Inc. Using GPS radio occultation refractivity data collected over the period of 2002-2013, we present a new method for identification of the intertropical convergence zone (ITCZ). The ITCZ is identified by estimating the maximum in the monthly meridional refractivity and specific humidity field by applying a Gaussian fit at each longitude. The interannual variability and climatology of the ITCZ is presented from 12. years of refractivity data. This new method captures all the general features of ITCZ extent and its variability. We also examine the effects of the ITCZ vertically during different seasons. The ITCZ is observed mostly at eastern Pacific in May month, and it is zonally distributed in the September and October months of each year. The zonal variability is large between lower and higher levels, particularly over the Indian monsoon and western Pacific. The latitudinal difference in the vertical extent between 850. hPa and higher levels is larger during the northern hemisphere (NH) summer than NH winter.

Glacial isostatic adjustment (GIA) represents a source of uncertainty for ice sheet mass balance estimates from the Gravity Recovery and Climate Experiment (GRACE) time-variable gravity measurements. We evaluate Greenland GIA corrections from Simpson et al (2009 Quat. Sci. Rev. 28 1631-57), A et al (2013 Geophys. J. Int. 192 557-72) and Wu et al (2010 Nature Geosci. 3 642-6) by comparing the spatial patterns of GRACE-derived ice mass trends calculated using the three corrections with volume changes from ICESat (Ice, Cloud, and land Elevation Satellite) and OIB (Operation IceBridge) altimetry missions, and surface mass balance products from the Regional Atmospheric Climate Model (RACMO). During the period September 2003-August 2011, GRACE ice mass changes obtained using the Simpson et al (2009 Quat. Sci. Rev. 28 1631-57) and A et al (2013 Geophys. J. Int. 192 557-72) GIA corrections yield similar spatial patterns and amplitudes, and are consistent with altimetry observations and surface mass balance data. The two GRACE estimates agree within 2% on average over the entire ice sheet, and better than 15% in four subdivisions of Greenland. The third GRACE estimate corrected using the (Wu et al 2010 Nature Geosci. 3 642-6)) GIA shows similar spatial patterns, but produces an average ice mass loss for the entire ice sheet that is 64 - 67 Gt yr-1 smaller. In the Northeast the recovered ice mass change is 46-49 Gt yr -1 (245-270%) more positive than that deduced from the other two corrections. By comparing the spatial and temporal variability of the GRACE estimates with trends of volume changes from altimetry and surface mass balance from RACMO, we show that the Wu et al (2010 Nature Geosci. 3 642-6) correction leads to a large mass increase in the Northeast that is inconsistent with independent observations. © 2014 IOP Publishing Ltd.

This study investigates how warming and changes in precipitation may affect the cycling of carbon (C) in tundra soils, and between high Arctic tundra and the atmosphere. We quantified ecosystem respiration (Reco) and soil pore space CO2 in a polar semi-desert in northwestern Greenland under current and future climate conditions simulated by long-term experimental warming (+2 °C, +4 °C), water addition (+50% summer precipitation), and a combination of both (+4 °C × +50% summer precipitation). We also measured the 14C content of Reco and soil CO2 to distinguish young C cycling rapidly between the atmosphere and the ecosystem from older C stored in the soil for centuries to millennia. We identified changes in the amount and timing of precipitation as a key control of the magnitude, seasonality and sources of Reco in a polar semi-desert. Throughout each summer, small (4 mm), more winter snow and experimental irrigation were associated with higher Reco fluxes and the release of recently fixed (young) C. Warmer summers and experimental warming also resulted in higher Reco fluxes (+2 °C > +4 °C), but coincided with losses of older C. We conclude that in high Arctic, dry tundra systems, future magnitudes and patterns of old C emissions will be controlled as much by the summer precipitation regime and winter snowpack as by warming. The release of older soil C is of concern, as it may lead to net C losses from the ecosystem. Therefore, reliable predictions of precipitation amounts, frequency, and timing are required to predict the changing C cycle in the high Arctic.

In this paper, GPS RO stratospheric temperatures are compared with different satellite and different model data sets. The data sets used for comparison include Japanese 25-year Reanalysis (JRA-25), UK Met office (MetO), ERA-Interim, GEOS5, and satellite temperatures are AIRS_Aqua, HIRDLS, Aura_MLS and SABER. Apart from the comparison, we also studied the seasonal variation of temperature during summer and winter in both the hemispheres. The seasonal and latitudinal variation of temperature by GPS (CHAMP+COSMIC) with other reanalysis (JRA-25, GEOS5 and METO) and satellite measurements (AURA_MLS and SABER) for both the hemispheres show reasonably good agreement. The difference of about ±0.5 to ±0.75 K is observed at 20 km, ±1K to ±1.75K at 35 km while the high values of ~3 to 4 K are observed at upper stratosphere and lower mesosphere height regions (50 km to 60 km). The GPS RO temperatures above 35 km are noticed to be warmer than reanalysis and satellite data sets in SH region. This tendency increases with increasing height and reaches its maximum at 60 km, with magnitude of 3 K to 4 K with reanalysis data sets and 2 K to 3.5 K with satellite measurements. The calculated SAO and AO amplitudes based on GPS data are found to be comparable with the earlier results.

The evolution of ocean temperature measurement systems is presented with a focus on the development and accuracy of two critical devices in use today (expendable bathythermographs and conductivity-temperature-depth instruments used on Argo floats). A detailed discussion of the accuracy of these devices and a projection of the future of ocean temperature measurements are provided. The accuracy of ocean temperature measurements is discussed in detail in the context of ocean heat content, Earth's energy imbalance, and thermosteric sea level rise. Up-to-date estimates are provided for these three important quantities. The total energy imbalance at the top of atmosphere is best assessed by taking an inventory of changes in energy storage. The main storage is in the ocean, the latest values of which are presented. Furthermore, despite differences in measurement methods and analysis techniques, multiple studies show that there has been a multidecadal increase in the heat content of both the upper and deep ocean regions, which reflects the impact of anthropogenic warming. With respect to sea level rise, mutually reinforcing information from tide gauges and radar altimetry shows that presently, sea level is rising at approximately 3 mm yr−1 with contributions from both thermal expansion and mass accumulation from ice melt. The latest data for thermal expansion sea level rise are included here and analyzed. Fil: Abraham, J. P.. University of St. Thomas. School of Engineering; Estados Unidos Fil: Baringer, M.. National Oceanic and Atmospheric Administration; Estados Unidos Fil: Bindoff, N. L.. University of Tasmania; Australia. Commonwealth Scientific and Industrial Research Organisation. Marine and Atmospheric Research; Australia Fil: Boyer, T.. National Oceanographic Data Center; Estados Unidos Fil: Cheng, L. J.. Chinese Academy of Sciences; República de China Fil: Church, J. A.. Centre for Australian Weather and Climate Research. Marine and Atmospheric Research; Australia Fil: Conroy, J. L.. Georgia Institute of Techology; Estados Unidos Fil: Domingues, C. M.. University of Tasmania; Australia Fil: Fasullo, J. T.. National Center for Atmospheric Research; Estados Unidos Fil: Gilson, J.. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos Fil: Goni, G.. National Oceanic and Atmospheric Administration; Estados Unidos Fil: Good, S. A.. Met Office Hadley Centre; Reino Unido Fil: Gorman, J. M.. University of St. Thomas; Estados Unidos Fil: Gouretski, V.. Universitat Hamburg; Alemania Fil: Ishii, M.. Meteorological Research Institute. Climate Research Department; Japón Fil: Johnson, G. C.. National Oceanic and Atmospheric Administration. Pacific Marine Environmental Laboratory; Estados Unidos Fil: Kizu, S.. Tohoku University. Department of Geophysics; Japón Fil: Lyman, J. M.. University of Hawaii at Manoa; Estados Unidos. National Oceanic and Atmospheric Administration. Pacific Marine Environmental Laboratory; Estados Unidos Fil: MacDonald, A. M.. Woods Hole Oceanographic Institution; Estados Unidos Fil: Minkowycz, W. J.. University of Illinois; Estados Unidos Fil: Moffitt, S. E.. Bodega Marine Laboratory; Estados Unidos. University of California at Davis; Estados Unidos Fil: Palmer, M. D.. Met Office Hadley Centre; Estados Unidos Fil: Piola, Alberto Ricardo. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval. Departamento Oceanografía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Reseghetti, F.. Italian National Agency for New Technologies; Italia Fil: Schuckmann, K.. Institut Français Recherche Exploitation Mer; Francia Fil: Trenberth, K. E.. National Center for Atmospheric Research; Estados Unidos Fil: Velicogna, I.. University of California at Irvine; Estados Unidos. California Institute of Technology; Estados Unidos Fil: Willis, J. K.. California Institute of Technology; Estados Unidos

Time-variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) mission have been available since 2002 to estimate the mass balance of the Greenland and Antarctic Ice Sheets. We analyze current progress and uncertainties in GRACE estimates of ice sheet mass balance. We discuss the impacts of errors associated with spherical harmonic truncation, spatial averaging, temporal sampling, and leakage from other time-dependent signals (e.g., glacial isostatic adjustment (GIA)). The largest sources of error for Antarctica are the GIA correction, the omission of l=1 terms, nontidal changes in ocean mass, and measurement errors. For Greenland, the errors come mostly from the uncertainty in the scaling factor. Using Release 5.0 (RL05) GRACE fields for January 2003 through November 2012, we find a mass change of −258 ± 41 Gt/yr for Greenland, with an acceleration of −31 ± 6 Gt/yr2, and a loss that migrated clockwise around the ice sheet margin to progressively affect the entire periphery. For Antarctica, we report changes of −83 ± 49 and −147 ± 80 Gt/yr for two GIA models, with an acceleration of −12 ± 9 Gt/yr2 and a dominance from the southeast pacific sector of West Antarctica and the Antarctic Peninsula.

We use monthly measurements of time-variable gravity from the GRACE (Gravity Recovery and Climate Experiment) satellite mission to quantify changes in terrestrial water storage (TWS) in the Lena river basin, Eurasia, during the period April 2002 to September 2010. We estimate a TWS increase of 32 10 km 3/yr for the entire basin, equivalent to an increase in water thickness of 1.3 0.4cm/yr over a basin of 2.4 million km2. We compare TWS estimates from GRACE with time series of precipitation (P) minus evapotranspiration (ET) from ERA-Interim reanalysis minus observational river discharge (R). We find an excellent agreement in annual and inter-annual variability between the two time series. Furthermore, we find that a bias of -20 10% in P-ET is sufficient to effectively close the water budget with GRACE. When we account for this bias, the time series of cumulative TWS from GRACE and climatological data agree to within 3.8 cm of water thickness, or 9% of the mean annual P. The TWS increase is not uniform across the river basin and exhibits a peak, over an area of 502,400 km2, centered at 118.5E, 62.5N, and underlain by discontinuous permafrost. In this region, we attribute the observed TWS increase of 68 19 km3 to an increase in subsurface water storage. This large subsurface water signal will have a significant impact on the terrestrial hydrology of the region, including increased baseflow and alteration of seasonal runoff. © 2012 by the American Geophysical Union.

Ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques, over different time periods, and at various levels of spatial detail. Considerable disparity remains between these estimates due to the inherent uncertainties of each method, the lack of detailed comparison between independent estimates, and the effect of temporal modulations in ice sheet surface mass balance. Here, we present a consistent record of mass balance for the Greenland and Antarctic ice sheets over the past two decades, validated by the comparison of two independent techniques over the last 8 years: one differencing perimeter loss from net accumulation, and one using a dense time series of time-variable gravity. We find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss. In 2006, the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise. Notably, the acceleration in ice sheet loss over the last 18 years was 21.9 ± 1 Gt/yr2 for Greenland and 14.5 ± 2 Gt/yr2 for Antarctica, for a combined total of 36.3 ± 2 Gt/yr2. This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr2). If this trend continues, ice sheets will be the dominant contributor to sea level rise in the 21st century.

Here, we present a 20‐year record of monthly ice sheet mass balance for Greenland and Antarctica. We examine and reconcile two independent methods for estimating temporal variations in ice sheet mass balance, the mass budget method (MB) and the gravity method, during the last 8 years. The MBM compares the surface mass balance (SMB; i.e., the sum of snowfall minus surface ablation) reconstructed from regional atmospheric models with perimeter loss (D; ice discharge) calculated from a time series of glacier velocity and ice thickness to deduce the rate of mass change, dM/dt [Rignot and Kanagaratnam, 2006; Howat et al., 2007; Rignot et al., 2008a; van den Broeke et al., 2009]. The gravity method employs a monthly time series of time‐variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) to estimate the relative mass as a function of time, M(t) [e.g., Velicogna and Wahr, 2006]. We resolve the differences between the two methods in terms of mass balance, dM(t)/dt, and acceleration of mass loss, d2M/dt2, and conclude by discussing the contribution of the ice sheets to sea level in recent and forthcoming decades.

We review the sea-level and energy budgets together from 1961, using recent and updated estimates of all terms. From 1972 to 2008, the observed sea-level rise (1.8 ± 0.2 mm yr−1 from tide gauges alone and 2.1 ± 0.2 mm yr−1 from a combination of tide gauges and altimeter observations) agrees well with the sum of contributions (1.8 ± 0.4 mm yr−1) in magnitude and with both having similar increases in the rate of rise during the period. The largest contributions come from ocean thermal expansion (0.8 mm yr−1) and the melting of glaciers and ice caps (0.7 mm yr−1), with Greenland and Antarctica contributing about 0.4 mm yr−1. The cryospheric contributions increase through the period (particularly in the 1990s) but the thermosteric contribution increases less rapidly. We include an improved estimate of aquifer depletion (0.3 mm yr−1), partially offsetting the retention of water in dams and giving a total terrestrial storage contribution of −0.1 mm yr−1. Ocean warming (90% of the total of the Earth's energy increase) continues through to the end of the record, in agreement with continued greenhouse gas forcing. The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as −0.8 ± 0.4 W m−2 for the 1980s and early 1990s. It increases in the late 1990s, as is required for consistency with little surface warming over the last decade. This increase is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity.

Here, we present a 20‐year record of monthly ice sheet mass balance for Greenland and Antarctica. We examine and reconcile two independent methods for estimating temporal variations in ice sheet mass balance, the mass budget method (MB) and the gravity method, during the last 8 years. The MBM compares the surface mass balance (SMB; i.e., the sum of snowfall minus surface ablation) reconstructed from regional atmospheric models with perimeter loss (D; ice discharge) calculated from a time series of glacier velocity and ice thickness to deduce the rate of mass change, dM/dt [Rignot and Kanagaratnam, 2006; Howat et al., 2007; Rignot et al., 2008a; van den Broeke et al., 2009]. The gravity method employs a monthly time series of time‐variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) to estimate the relative mass as a function of time, M(t) [e.g., Velicogna and Wahr, 2006]. We resolve the differences between the two methods in terms of mass balance, dM(t)/dt, and acceleration of mass loss, d2M/dt2, and conclude by discussing the contribution of the ice sheets to sea level in recent and forthcoming decades.

We review the sea-level and energy budgets together from 1961, using recent and updated estimates of all terms. From 1972 to 2008, the observed sea-level rise (1.8 ± 0.2 mm yr−1 from tide gauges alone and 2.1 ± 0.2 mm yr−1 from a combination of tide gauges and altimeter observations) agrees well with the sum of contributions (1.8 ± 0.4 mm yr−1) in magnitude and with both having similar increases in the rate of rise during the period. The largest contributions come from ocean thermal expansion (0.8 mm yr−1) and the melting of glaciers and ice caps (0.7 mm yr−1), with Greenland and Antarctica contributing about 0.4 mm yr−1. The cryospheric contributions increase through the period (particularly in the 1990s) but the thermosteric contribution increases less rapidly. We include an improved estimate of aquifer depletion (0.3 mm yr−1), partially offsetting the retention of water in dams and giving a total terrestrial storage contribution of −0.1 mm yr−1. Ocean warming (90% of the total of the Earth's energy increase) continues through to the end of the record, in agreement with continued greenhouse gas forcing. The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as −0.8 ± 0.4 W m−2 for the 1980s and early 1990s. It increases in the late 1990s, as is required for consistency with little surface warming over the last decade. This increase is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity.

The annual exchange of water between the continents and oceans is observed by GPS, gravimetry, and altimetry. However, the global average amplitude of this annual cycle (observed amplitude of ∼8 mm) is not representative of the effects that would be observed at individual tide gauges or at ocean bottom pressure recorders because of self-attraction and loading effects (SAL). In this paper, we examine the spatial variation of sea level change caused by the three main components that load the Earth and contribute to the water cycle: hydrology (including snow), the atmosphere, and the dynamic ocean. The SAL effects cause annual amplitudes at tide gauges (modeled here with a global average of ∼9 mm) to vary from less than 2 mm to more than 18 mm. We find a variance reduction (global average of 3 to 4%) after removing the modeled time series from a global set of tide gauges. We conclude that SAL effects are significant in places (e.g., the south central Pacific and coastal regions in Southeast Asia and west central Africa) and should be considered when interpreting these data sets and using them to constrain ocean circulation models.

Widespread glacier acceleration has been observed in Greenland in the past few years associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean. These glaciers thin both at the surface, from warm air temperatures, and along their submerged faces in contact with warm ocean waters. Little is known about the rates of submarine melting and how they may affect glacier dynamics. Here we present measurements of ocean currents, temperature and salinity near the calving fronts of the Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry and geographical positions. We calculate water-mass and heat budgets that reveal summer submarine melt rates ranging from 0.7±0.2 to 3.9±0.8 m d -1. These rates of submarine melting are two orders of magnitude larger than surface melt rates, but comparable to rates of iceberg discharge. We conclude that ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs, and suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics. © 2010 Macmillan Publishers Limited. All rights reserved.

Widespread glacier acceleration has been observed in Greenland in the past few years associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean. These glaciers thin both at the surface, from warm air temperatures, and along their submerged faces in contact with warm ocean waters. Little is known about the rates of submarine melting and how they may affect glacier dynamics. Here we present measurements of ocean currents, temperature and salinity near the calving fronts of the Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry and geographical positions. We calculate water-mass and heat budgets that reveal summer submarine melt rates ranging from 0.7±0.2 to3.9±0.8md−1. These rates of submarine melting are two orders of magnitude larger than surface melt rates, but comparable to rates of iceberg discharge. We conclude that ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs, and suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics.

Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge

Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge

MEM. SOC. GEOLO. IT.

Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment satellites, we determined mass variations of the Antarctic ice sheet during 2002-2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 ± 80 cubic kilometers of ice per year, which is equivalent to 0.4 ± 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.

We use 22 monthly GRACE (Gravity Recovery and Climate Experiment) gravity fields to estimate the linear trend in Greenland ice mass during 2002-2004. We recover a decrease in total ice mass of 82 ± 28 km3of ice per year, consistent with estimates from other techniques. Our uncertainty estimate is dominated by the effects of GRACE measurement errors and errors in our post glacial rebound (PG) correction. The main advantages of GRACE are that it is sensitive to the entire ice sheet, and that it provides mass estimates with only minimal use of supporting physical assumptions or ancillary data. Copyright 2005 by the American Geophysical Union.

Eleven monthly GRACE gravity field solutions are now available for analyses. We show those fields can be used to recover monthly changes in water storage, both on land and in the ocean, to accuracies of 1.5 cm of water thickness when smoothed over 1000 km. The amplitude of the annually varying signal can be determined to 1.0 cm. Results are 30% better for a 1500 km smoothing radius, and 40% worse for a 750 km radius. We estimate the annually varying component of water storage for three large drainage basins (the Mississippi, the Amazon, and a region draining into the Bay of Bengal), to accuracies of 1.0–1.5 cm.

In recent years, great improvements have been made toward understanding the modern dynamics and recent history of the ice sheets. Several recently-launched satellite missions promise to make geodesy the most powerful tool for investigation of the changing ice sheets, including their past history and their present behavior. Mathematical description of ice sheet behavior from geodetic data requires accurate modeling of all the processes which may affect the measurements. Most geodetic tools measure changes in elevation of the ice sheets, which can include Post Glacial Rebound (PGR), the current Ice Mass Trend (IMT) consisting of both accumulation and glacial outflux, and processes of compaction within the firn column. Consequently it is necessary for mathematical models of geodetic data to separate the effects of IMT, PGR, and compaction. Satellite measurements of the time-variable geoid are insensitive to compaction effects and depend on IMT and PGR differently than do height measurements. Two methodological approaches have been proposed to separate these effects using measurements of height and time-variable geoid: 1- direct inversion for ice mass variability (Wu et al., 2002), which requires a priori assumptions about either the Earth’s rheology or the ice load history; 2- iterative solution for the fields, which theoretically is more approximate but is computationally much simpler and less dependent on a priori assumptions. In this paper we analyze how we can learn about IMT and PGR by combining geodetic measurements, and we assess the conditions required to optimally combine satellite and ground-based data sets.

Measurements of ice elevation from the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud, and Land Elevation Satellite can be combined with time-variable geoid measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to learn about ongoing changes in polar ice mass and viscoelastic rebound of the lithosphere under the ice sheet. We estimate the accuracy in recovering the spatially varying ice mass trend and postglacial rebound signals for Antarctica, from combining 5 years of simulated GRACE and GLAS data. We obtain root-mean square accuracies of 5.3 and 19.9 mm yr−1 for postglacial rebound and ice mass trend, respectively, when smoothed over 250 km scales. The largest source of error in the combined signals is the effect of the unknown time-variable accumulation on the density of the ice column. To estimate this contribution and so obtain better estimates of ice mass trend and postglacial rebound, we add Global Positioning System (GPS) measurements of vertical velocities as additional constraints. Using an empirical relation between the errors in postglacial rebound and ice mass trend that result from the unknown density variation within the ice column, we are able to solve for all three unknowns in the problem: ice mass trend, postglacial rebound, and the snow compaction trend. The addition of a plausible distribution of GPS measurements reduces the errors in estimates of postglacial rebound and ice mass trend to 3.4 and 15.9 mm yr−1, respectively.

The Earth's structure and the geod) nam ic processes that originate it are strictly interconnected: an accurate knowledge of the Earth·s structure and its physical properties is fundamental for the understanding of the dynamics of our planet and critical for realistic simulations of seismic ground motion. On the other side the separation and the definition of the different dynamic processes acting in a certain area help to better understand the structure and the physical properties of the investigated region. In recent years much emphasis has been given to the inversion or observed phase and waveform data for the best fitting set or model parameters at a variety of scale-lengths.

In the framework of several international research projects performed in Ross Sea (Antarctica) in the last years marine gravity data, high resolution reflection seismic line sand deep seismic sounding profiles have been acquired. The gravity data set has been used to compute maps of Bouguer anomaly, isostatic anomaly, isostatic correction and local geoid undulations. Forward modelling of seismic and gravity data has been also used to compute the depths of the Crust-Mantle discontinuity.

In 2017, the dominant greenhouse gases released into Earth's atmosphere-carbon dioxide, methane, and nitrous oxide-reached new record highs. The annual global average carbon dioxide concentration at Earth's surface for 2017 was 405.0 ± 0.1 ppm, 2.2 ppm greater than for 2016 and the highest in the modern atmospheric measurement record and in ice core records dating back as far as 800 000 years. The global growth rate of CO2 has nearly quadrupled since the early 1960s. With ENSO-neutral conditions present in the central and eastern equatorial Pacific Ocean during most of the year and weak La Niña conditions notable at the start and end, the global temperature across land and ocean surfaces ranked as the second or third highest, depending on the dataset, since records began in the mid-to-late 1800s. Notably, it was the warmest non-El Niño year in the instrumental record. Above Earth's surface, the annual lower tropospheric temperature was also either second or third highest according to all datasets analyzed. The lower stratospheric temperature was about 0.2°C higher than the record cold temperature of 2016 according to most of the in situ and satellite datasets. Several countries, including Argentina, Uruguay, Spain, and Bulgaria, reported record high annual temperatures. Mexico broke its annual record for the fourth consecutive year. On 27 January, the temperature reached 43.4°C at Puerto Madryn, Argentina-the highest temperature recorded so far south (43°S) anywhere in the world. On 28 May in Turbat, western Pakistan, the high of 53.5°C tied Pakistan's all-time highest temperature and became the world-record highest temperature for May. In the Arctic, the 2017 land surface temperature was 1.6°C above the 1981-2010 average, the second highest since the record began in 1900, behind only 2016. The five highest annual Arctic temperatures have all occurred since 2007. Exceptionally high temperatures were observed in the permafrost across the Arctic, with record values reported in much of Alaska and northwestern Canada. In August, high sea surface temperature (SST) records were broken for the Chukchi Sea, with some regions as warm as +11°C, or 3° to 4°C warmer than the longterm mean (1982-present). According to paleoclimate studies, today's abnormally warm Arctic air and SSTs have not been observed in the last 2000 years. The increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 7 March, sea ice extent at the end of the growth season saw its lowest maximum in the 37-year satellite record, covering 8% less area than the 1981-2010 average. The Arctic sea ice minimum on 13 September was the eighth lowest on record and covered 25% less area than the long-term mean. Preliminary data indicate that glaciers across the world lost mass for the 38th consecutive year on record; the declines are remarkably consistent from region to region. Cumulatively since 1980, this loss is equivalent to slicing 22 meters off the top of the average glacier. Antarctic sea ice extent remained below average for all of 2017, with record lows during the first four months. Over the continent, the austral summer seasonal melt extent and melt index were the second highest since 2005, mostly due to strong positive anomalies of air temperature over most of the West Antarctic coast. In contrast, the East Antarctic Plateau saw record low mean temperatures in March. The year was also distinguished by the second smallest Antarctic ozone hole observed since 1988. Across the global oceans, the overall long-term SST warming trend remained strong. Although SST cooled slightly from 2016 to 2017, the last three years produced the three highest annual values observed; these high anomalies have been associated with widespread coral bleaching. The most recent global coral bleaching lasted three full years, June 2014 to May 2017, and was the longest, most widespread, and almost certainly most destructive such event on record. Global integrals of 0-700-m and 0-2000-m ocean heat content reached record highs in 2017, and global mean sea level during the year became the highest annual average in the 25-year satellite altimetry record, rising to 77 mm above the 1993 average. In the tropics, 2017 saw 85 named tropical storms, slightly above the 1981-2010 average of 82. The North Atlantic basin was the only basin that featured an above-normal season, its seventh most active in the 164-year record. Three hurricanes in the basin were especially notable. Harvey produced record rainfall totals in areas of Texas and Louisiana, including a storm total of 1538.7 mm near Beaumont, Texas, which far exceeds the previous known U.S. tropical cyclone record of 1320.8 mm. Irma was the strongest tropical cyclone globally in 2017 and the strongest Atlantic hurricane outside of the Gulf of Mexico and Caribbean on record with maximum winds of 295 km h-1. Maria caused catastrophic destruction across the Caribbean Islands, including devastating wind damage and flooding across Puerto Rico. Elsewhere, the western North Pacific, South Indian, and Australian basins were all particularly quiet. Precipitation over global land areas in 2017 was clearly above the long-term average. Among noteworthy regional precipitation records in 2017, Russia reported its second wettest year on record (after 2013) and Norway experienced its sixth wettest year since records began in 1900. Across India, heavy rain and flood-related incidents during the monsoon season claimed around 800 lives. In August and September, above-normal precipitation triggered the most devastating floods in more than a decade in the Venezuelan states of Bolívar and Delta Amacuro. In Nigeria, heavy rain during August and September caused the Niger and Benue Rivers to overflow, bringing floods that displaced more than 100 000 people. Global fire activity was the lowest since at least 2003; however, high activity occurred in parts of North America, South America, and Europe, with an unusually long season in Spain and Portugal, which had their second and third driest years on record, respectively. Devastating fires impacted British Columbia, destroying 1.2 million hectares of timber, bush, and grassland, due in part to the region's driest summer on record. In the United States, an extreme western wildfire season burned over 4 million hectares; the total costs of $18 billion tripled the previous U.S. annual wildfire cost record set in 1991.