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Lightning is a key driver of wildfire activity in Alaska. Quantifying its historical variability and trends has been challenging because of changes in the observational network, but understanding historical and possible future changes in lightning activity is important for fire management planning. Dynamically downscaled reanalysis and global climate model (GCM) data were used to statistically assess lightning data in geographic zones used operationally by fire managers across Alaska. Convective precipitation was found to be a key predictor of weekly lightning activity through multiple regression analysis, along with additional atmospheric stability, moisture, and temperature predictor variables. Model-derived estimates of historical June–July lightning since 1979 showed increasing but lower-magnitude trends than the observed record, derived from the highly heterogeneous lightning sensor network, over the same period throughout interior Alaska. Two downscaled GCM projections estimate a doubling of lightning activity over the same June–July season and geographic region by the end of the twenty-first century. Such a substantial increase in lightning activity may have significant impacts on future wildfire activity in Alaska because of increased opportunities for ignitions, although the final outcome also depends on fire weather conditions and fuels.

Seasonality in photosynthetic activity is a critical component of seasonal carbon, water and energy cycles in the Earth system. This characteristic is a consequence of plant's adaptive evolutionary processes to a given set of environmental conditions. Changing climate in northern lands (>30°N) alters the state of climatic constraints on plant growth, and therefore, changes in the seasonality and carbon accumulation are anticipated. However, how photosynthetic seasonality evolved to its current state, and what role climatic constraints and their variability played in this process and ultimately in carbon cycle is still poorly understood due to its complexity. Here, we take the 'laws of minimum' as a basis and introduce a new framework where the timing (Day of Year) of peak photosynthetic activity (DOYP max ) acts as a proxy for plant's adaptive state to climatic constraints on its growth. Our analyses confirm that spatial variations in DOYP max reflect spatial gradients in climatic constraints as well as seasonal maximum and total productivity. We find a widespread warming-induced advance in DOYP max (-1.66 ± 0.30 days decade-1 , P < 0.001) across northern lands, indicating a spatio-temporal dynamism of climatic constraints to plant growth. We show that the observed changes in DOYP max are associated with an increase in total gross primary productivity through enhanced carbon assimilation early in the growing season, which leads to an earlier phase shift in land-atmosphere carbon fluxes and an increase in their amplitude. Such changes are expected to continue in the future based on our analysis of Earth System Model (ESM) projections. Our study provides a simplified, yet realistic framework based on first principles for the complex mechanisms by which various climatic factors constrain plant growth in northern ecosystems.

Climate change impacts the electric power system by affecting both the load and generation. It is paramount to understand this impact in the context of renewable energy as their market share has increased and will continue to grow. This study investigates the impact of climate change on the supply of renewable energy through applying novel metrics of intermittency, power production and storage required by the renewable energy plants as a function of historical climate data variability. Here we focus on and compare two disparate locations, Palma de Mallorca in the Balearic Islands and Cordova, Alaska. The main results of this analysis of wind, solar radiation and precipitation over the 1950–2020 period show that climate change impacts both the total supply available and its variability. Importantly, this impact is found to vary significantly with location. This analysis demonstrates the feasibility of a process to evaluate the local optimal mix of renewables, the changing needs for energy storage as well as the ability to evaluate the impact on grid reliability regarding both penetration of the increasing renewable resources and changes in the variability of the resource. This framework can be used to quantify the impact on both transmission grids and microgrids and can guide possible mitigation paths., P.C. and D.G. acknowledge financial support from Ministerio de Ciencia e Innovación (Spain), the Agencia Estatal de Investigación (AEI, Spain), and the Fondo Europeo de Desarrollo Regional (FEDER, EU) under grant PACSS (RTI2018-093732-B-C22) and the Maria de Maeztu program for Units of Excellence in R&D (MDM-2017-0711). D.N. gratefully acknowledges support from DOE Project GMLC 1.5.02—Resilient Alaskan Distribution system Improvements using Automation, Network analysis, Control, and Energy storage (RADIANCE). U.S.B. acknowledges support from the National Science Foundation under award #OIA-1753748 and by the State of Alaska for material which this work is based upon.

The ice formed by cold-season rainfall or rain on snow (ROS) has striking impacts on the economy and ecology of Alaska. An understanding of the atmospheric drivers of ROS events is required to better predict them and plan for environmental change. The spatially/temporally sparse network of stations in Alaska makes studying such events challenging, and gridded reanalysis or remote sensing products are necessary to fill the gaps. Recently developed dynamically downscaled climate data provide a new suite of high-resolution variables for investigating historical and projected ROS events across all of Alaska from 1979 to 2100. The dynamically downscaled reanalysis data of ERA-Interim replicated the seasonal patterns of ROS events but tended to produce more rain events than in station observations. However, dynamical downscaling reduced the bias toward more rain events in the coarse reanalysis. ROS occurred most frequently over southwestern and southern coastal regions. Extreme events with the heaviest rainfall generally coincided with anomalous high pressure centered to the south/southeast of the locations receiving the event and warm-air advection from the resulting southwesterly wind flow. ROS events were projected to increase in frequency overall and for extremes across most of the region but were expected to decline over southwestern/southern Alaska. Increases in frequency were projected as a result of more frequent winter rainfall, but the number of ROS events may ultimately decline in some areas as a result of temperatures rising above the freezing threshold. These projected changes in ROS can significantly affect wildlife, vegetation, and human activities across the Alaska landscape.

Key observational indicators of climate change in the Arctic, most spanning a 47 year period (1971–2017) demonstrate fundamental changes among nine key elements of the Arctic system. We find that, coherent with increasing air temperature, there is an intensification of the hydrological cycle, evident from increases in humidity, precipitation, river discharge, glacier equilibrium line altitude and land ice wastage. Downward trends continue in sea ice thickness (and extent) and spring snow cover extent and duration, while near-surface permafrost continues to warm. Several of the climate indicators exhibit a significant statistical correlation with air temperature or precipitation, reinforcing the notion that increasing air temperatures and precipitation are drivers of major changes in various components of the Arctic system. To progress beyond a presentation of the Arctic physical climate changes, we find a correspondence between air temperature and biophysical indicators such as tundra biomass and identify numerous biophysical disruptions with cascading effects throughout the trophic levels. These include: increased delivery of organic matter and nutrients to Arctic near‐coastal zones; condensed flowering and pollination plant species periods; timing mismatch between plant flowering and pollinators; increased plant vulnerability to insect disturbance; increased shrub biomass; increased ignition of wildfires; increased growing season CO2 uptake, with counterbalancing increases in shoulder season and winter CO2 emissions; increased carbon cycling, regulated by local hydrology and permafrost thaw; conversion between terrestrial and aquatic ecosystems; and shifting animal distribution and demographics. The Arctic biophysical system is now clearly trending away from its 20th Century state and into an unprecedented state, with implications not only within but beyond the Arctic. The indicator time series of this study are freely downloadable at AMAP.no. Key indicators of Arctic climate change: 1971–2017

The late-season extreme fire activity in Southcentral Alaska during 2019 was highly unusual and consequential. Firefighting operations had to be extended by a month in 2019 due to the extreme conditions of hot summer temperature and prolonged drought. The ongoing fires created poor air quality in the region containing most of Alaska&rsquo, s population, leading to substantial impacts to public health. Suppression costs totaled over $70 million for Southcentral Alaska. This study&rsquo, s main goals are to place the 2019 season into historical context, provide an attribution analysis, and assess future changes in wildfire risk in the region. The primary tools are meteorological observations and climate model simulations from the NCAR CESM Large Ensemble (LENS). The 2019 fire season in Southcentral Alaska included the hottest and driest June&ndash, August season over the 1979&ndash, 2019 period. The LENS simulation analysis suggests that the anthropogenic signal of increased fire risk had not yet emerged in 2019 because of the CESM&rsquo, s internal variability, but that the anthropogenic signal will emerge by the 2040&ndash, 80 period. The effect of warming temperatures dominates the effect of enhanced precipitation in the trend towards increased fire risk.

By extending the record of Alaskan divisional temperature and precipitation back in time, regional variations and trends of temperature and precipitation over 1920–2012 are documented. The use of the divisional framework highlights the greater spatial coherence of temperature variations relative to precipitation variations. The divisional time series of temperature are characterized by large interannual variability superimposed upon low-frequency variability, as well as by an underlying trend. Low-frequency variability corresponding to the Pacific decadal oscillation (PDO) includes Alaska’s generally warm period of the 1920s and 1930s, a cold period from the late 1940s through the mid-1970s, a warm period from the late 1970s through the early 2000s, and a cooler period in the most recent decade. An exception to the cooling of the past decade is the North Slope climate division, which has continued to warm. There has been a gradual upward trend of Alaskan temperatures relative to the PDO since 1920, resulting in a statewide average warming of about 1°C. In contrast to temperature, variations of precipitation are less consistent across climate divisions and have much less multidecadal character. Thirty-year trends of both variables are highly sensitive to the choice of the subperiod within the overall 93-yr period. The trends also vary seasonally, with winter and spring contributing the most to the annual trends.

Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics, and land surface changes in a broader perspective. We show that, during the last decade (2000s), atmospheric and sea surface temperatures are reaching levels last encountered millennia ago, when northern high latitude summer insolation was higher due to a different orbital configuration. Records from lake sediments in southern Greenland document major environmental and climatic conditions during the last 10,000 years, highlighting the role of soil dynamics in past vegetation changes, and stressing the growing anthropogenic impacts on soil erosion during the recent decades. Furthermore, past and present changes in atmospheric and oceanic heat advection appear to strongly influence both regional climate and ice sheet dynamics. Projections from climate models are investigated to quantify the magnitude and rates of future changes in Greenland temperature, which may be faster than past abrupt events occurring under interglacial conditions. Within one century, in response to increasing greenhouse gas emissions, Greenland may reach temperatures last time encountered during the last interglacial period, approximately 125,000 years ago. We review and discuss whether analogies between the last interglacial and future changes are reasonable, because of the different seasonal impacts of orbital and greenhouse gas forcings. Over several decades to centuries, future Greenland melt may act as a negative feedback, limiting regional warming albeit with global sea level and climatic impacts. 2012 John Wiley & Sons, Ltd. How to cite this article

Global models are the most widely used tools for understanding and assessing climatic variability and changes. However, coarse-resolution limits their capability to capture detailed finer-scale meteorological features, including heterogeneous spatial distributions and high-frequency temporal variability. In this study, the mesoscale Weather Research and Forecasting (WRF) model is used to dynamically downscale a CMIP5 global model simulation (CCSM MOAR output) for a portion of the Arctic marginal zone, encompassing Alaska and surrounding areas, with the aim to improve understanding, representation, and future projection of high-resolution climate changes in the area. Dynamic downscaling of the twentieth century simulation was conducted for the period 1991–2005 and validated against in situ observations archived by the NCDC. Downscaled results generally capture observed conditions well. However, cold biases exist across most of the study area, except for a weak warm bias along the western and northern Alaskan coasts. In addition, downscaled winds are stronger than observations and precipitation is overestimated along the Alaskan panhandle. The biases in the downscaled temperature, wind speed, and precipitation are correctable. The downscaled temperature bias exhibits strong seasonality, with a warm bias in the cold months and a cold bias in the warm months, particularly along the western and northern Alaskan coasts. Seasonality in the wind speed and precipitation biases, however, is relatively small. Under the RCP6 scenario, downscaled regional climate over Alaska and the surrounding areas demonstrate a significant warming trend over the entire study area during the twenty-first century, with the strongest warming occurring over the Arctic Ocean. Precipitation is also projected to increase along Alaska’s coastal areas and over the Arctic Ocean. Interior Alaska, on the other hand, becomes drier in the future climate scenario.

Arctic cyclone activity is investigated in the context of climate change and variability by using a modified automated cyclone identification and tracking algorithm, which differs from previously used algorithms by single counting each cyclone. The investigation extends earlier studies by lengthening the time period to 55 yr (1948– 2002) with a 6-hourly time resolution, by documenting the seasonality and the dominant temporal modes of variability of cyclone activity, and by diagnosing regional activity as quantified by the cyclone activity index (CAI). The CAI integrates information on cyclone intensity, frequency, and duration into a comprehensive index of cyclone activity. Arctic cyclone activity has increased during the second half of the twentieth century, while midlatitude activity generally decreased from 1960 to the early 1990s, in agreement with previous studies. New findings include the following. 1) The number and intensity of cyclones entering the Arctic from the midlatitudes has incre...

This synthesis study assesses recent changes of Arctic Ocean physical parameters using a unique collection of observations from the 2000s and places them in the context of long-term climate trends and variability. Our analysis demonstrates that the 2000s were an exceptional decade with extraordinary upper Arctic Ocean freshening and intermediate Atlantic water warming. We note that the Arctic Ocean is characterized by large amplitude multi-decadal variability in addition to a long-term trend, making the link of observed changes to climate drivers problematic. However, the exceptional magnitude of recent high-latitude changes (not only oceanic, but also ice and atmospheric) strongly suggests that these recent changes signify a potentially irreversible shift of the Arctic Ocean to a new climate state. These changes have important implications for the Arctic Ocean’s marine ecosystem, especially those components that are dependent on sea ice or that have temperature-dependent sensitivities or thresholds. Addressing these and other questions requires a carefully orchestrated combination of sustained multidisciplinary observations and advanced modeling.

The Yamal Peninsula in northwest Siberia is undergoing some of the most rapid land-cover and land-use changes in the Arctic due to a combination of gas development, reindeer herding, and climate change. Unusual geological condi- tions (nutrient-poor sands, massive ground ice and extensive landslides) exacerbate the impacts. These changes will likely increase markedly as transportation corridors are built to transport the gas to market. Understanding the nature, extent, causes and consequences (i.e., the cumulative effects) of the past and ongoing rapid changes on the Yamal is important for effective, long-term decision-making and planning. The cumulative effects to vegetation are the focus of this chapter because the plants are a critical component of the Yamal landscape that support the indigenous Nenets people and their reindeer and also protect the underlying ice-rich permafrost from melting. We are using a combination of ground-based studies (a transect of five loca- tions across the Yamal), remote-sensing studies, and analyses of Nenets land-use activities to develop vegetation-change models that can be used to help anticipate future states of the tundra and how those changes might affect traditional reindeer herding practices and the thermal state of the permafrost. This chapter provides an overview of the approach, some early results, and recommendations for expanding the concept of cumulative-effects analysis to include examining the simultaneous and interactive effects of multiple drivers of change.

This chapter provides an overview of observed changes in vegetation productivity in Arctic tundra and boreal forest ecosystems over the past 3 decades, based on satellite remote sensing and other observational records, and relates these to climate variables and sea ice conditions. The emerging patterns and relationships are often complex but clearly reveal a contrast in the response of the tundra and boreal biomes to recent climate change, with the tundra showing increases and undisturbed boreal forests mostly reductions in productivity. The possible reasons for this divergence are discussed and the consequences of continued climate warming for the vegetation in the Arctic region assessed using ecosystem models, both at the biome-scale and at high spatial resolution focussing on plant functional types in the tundra and the tundra-forest ecotones.

The causes of a greening trend detected in the Arctic using the normalized difference vegetation index (NDVI) are still poorly understood. Changes in NDVI are a result of multiple ecological and social factors that affect tundra net primary productivity. Here we use a 25 year time series of AVHRR-derived NDVI data (AVHRR: advanced very high resolution radiometer), climate analysis, a global geographic information database and ground-based studies to examine the spatial and temporal patterns of vegetation greenness on the Yamal Peninsula, Russia. We assess the effects of climate change, gas-field development, reindeer grazing and permafrost degradation. In contrast to the case for Arctic North America, there has not been a significant trend in summer temperature or NDVI, and much of the pattern of NDVI in this region is due to disturbances. There has been a 37% change in early-summer coastal sea-ice concentration, a 4% increase in summer land temperatures and a 7% change in the average time-integrated NDVI over the length of the satellite observations. Gas-field infrastructure is not currently extensive enough to affect regional NDVI patterns. The effect of reindeer is difficult to quantitatively assess because of the lack of control areas where reindeer are excluded. Many of the greenest landscapes on the Yamal are associated with landslides and drainage networks that have resulted from ongoing rapid permafrost degradation. A warming climate and enhanced winter snow are likely to exacerbate positive feedbacks between climate and permafrost thawing. We present a diagram that summarizes the social and ecological factors that influence Arctic NDVI. The NDVI should be viewed as a powerful monitoring tool that integrates the cumulative effect of a multitude of factors affecting Arctic land-cover change.

Satellite-based measurements of the normalized difference vegetation index (NDVI; an index of vegetation greenness and photosynthetic capacity) indicate that tundra environments are generally greening and becoming more productive as climates warm in the Arctic. The greening, however, varies and is even negative in some parts of the Arctic. To help interpret the space-based observations, the International Polar Year (IPY) Greening of the Arctic project conducted ground-based surveys along two >1500 km transects that span all five Arctic bioclimate subzones. Here we summarize the climate, soil, vegetation, biomass, and spectral information collected from the North America Arctic transect (NAAT), which has a more continental climate, and the Eurasia Arctic transect (EAT), which has a more oceanic climate. The transects have broadly similar summer temperature regimes and overall vegetation physiognomy, but strong differences in precipitation, especially winter precipitation, soil texture and pH, disturbance regimes, and plant species composition and structure. The results indicate that summer warmth and NDVI increased more strongly along the more continental transect.

[1] Arctic variability is dominated by multi-decadal fluctuations. Incomplete sampling of these fluctuations results in highly variable arctic surface-air temperature (SAT) trends. Modulated by multi-decadal variability, SAT trends are often amplified relative to northern-hemispheric trends, but over the 125-year record we identify periods when arctic SAT trends were smaller or of opposite sign than northern-hemispheric trends. Arctic and northern-hemispheric air-temperature trends during the 20th century (when multi-decadal variablity had little net effect on computed trends) are similar, and do not support the predicted polar amplification of global warming. The possible moderating role of sea ice cannot be conclusively identified with existing data. If long-term trends are accepted as a valid measure of climate change, then the SAT and ice data do not support the proposed polar amplification of global warming. Intrinsic arctic variability obscures long-term changes, limiting our ability to identify complex feedbacks in the arctic climate system.

Climate fluctuations in the North Atlantic Ocean have wide-spread implications for Europe, Africa, and the Americas. This study assesses the relative contribution of the long-term trend and variability of North Atlantic warming using EOF analysis of deep-ocean and near-surface observations. Our analysis demonstrates that the recent warming over the North Atlantic is linked to both long-term (including anthropogenic and natural) climate change and multidecadal variability (MDV, ~50–80 years). Our results suggest a general warming trend of 0.031 ± 0.006°C/decade in the upper 2,000 m North Atlantic over the last 80 years of the twentieth century, although during this time there are periods in which short-term trends were strongly amplified by MDV. For example, MDV accounts for ~60% of North Atlantic warming since 1970. The single-sign basin-scale pattern of MDV with prolonged periods of warming (cooling) in the upper ocean layer and opposite tendency in the lower layer is evident from observations. This pattern is associated with a slowdown (enhancement) of the North Atlantic thermohaline overturning circulation during negative (positive) MDV phases. In contrast, the long-term trend exhibits warming in tropical and mid-latitude North Atlantic and a pattern of cooling in regions associated with major northward heat transports, consistent with a slowdown of the North Atlantic circulation as evident from observations and confirmed by selected modeling results. This localized cooling has been masked in recent decades by warming during the positive phase of MDV. Finally, since the North Atlantic Ocean plays a crucial role in establishing and regulating the global thermohaline circulation, the multidecadal fluctuations discussed here should be considered when assessing long-term climate change and variability, both in the North Atlantic and at global scales.

Results from a suite of 30-year simulations (after spin-up) of the fully coupled Community Climate System Model version 2.0.1 are analyzed to examine the impact of doubling CO2 on interactions between the global water cycle and the regional water cycles of four similar-size, but hydrologically and thermally different study regions (the Yukon, Ob, St Lawrence, and Colorado river basins and their adjacent land). A heuristic evaluation based on published climatological data shows that the model generally produces acceptable results for the control 1× CO2 concentration, except for mountainous regions where it performs like other modern climate models. After doubling CO2, the Northern Hemisphere receives significantly (95% confidence level) more moisture from the Southern Hemisphere during the boreal summer than under 1× CO2 conditions, and the phase of the annual cycle of net moisture transport to areas north of 60°N shifts to a month later than in the reference simulation. Precipitation and evapotranspiration in the doubled CO2 simulation increase for the Yukon, Ob, and St Lawrence, but decrease, on average, for the Colorado region compared to the reference simulation. For all regions, interaction between global and regional water cycles increases under doubled CO2, because the amount of moisture entering and leaving the regions increases in the warmer climate. The degree of change in this interaction depends on region and season, and is related to slight shifts in the position/strength of semi-permanent highs and lows for the Yukon, Ob, and St Lawrence; in the Colorado region, higher temperatures associated with doubling CO2 and the anticyclone located over the region increase the persistence of dry conditions.