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The Earth system responds to solar variability on a wide range of timescales. Knowledge of total solar irradiance (TSI) and solar spectral irradiance (SSI) spanning minutes to centuries is needed by scientists studying a broad array of research applications. For these purposes, the NOAA National Centers for Environmental Information (NCEI) Climate Data Record Program established the Solar Irradiance Climate Data Record. Version 2 of the Naval Research Laboratory's solar variability models that are derived from and demonstrate consistency with irradiance observations specifies TSI and SSI for the Solar Irradiance Climate Data Record. We establish the veracity of the Naval Research Laboratory models on the timescales and over the wavelength range for which the Sun is known to vary and, thereby, specify the utility of these models. Through comparisons with irradiance observations and independent models, we validate NRLTSI2 estimates of TSI on solar rotational (~27‐day), solar cycle (~11‐year), and multidecadal (spacecraft era) variability timescales. Similarly, we validate NRLSSI2 estimates of SSI rotational variability in the ultraviolet through the mid‐visible spectrum. Validation of NRLSSI2 estimates at longer wavelengths, particularly in the near‐infrared, and for the full spectrum at solar cycle timescales and longer is not possible with the current observational record due to instrumental noise and instrument instability. We identify where key new data sets, such as observations from the Total and Spectral Solar Irradiance Sensor‐1, are expected to provide a fuller understanding of total and spectral solar irradiance variability on multiple timescales.

We present the construction of a new white-light coronal brightness index (CBI) from the entire archive of observations recorded by the Large Angle Spectrometric Coronagraph (LASCO) C2 camera between 1996 and 2017, comprising two full solar cycles. We reduce all fully calibrated daily C2 observations of the white light corona into a single daily coronal brightness observation for every day of observation recorded by the instrument, with mean daily brightness values binned into 0.1 Rsun radial x 1 degree angular regions from 2.4 -- 6.2 Rsun for a full 360-degrees. As a demonstration of the utility of the CBI, we construct a new solar irradiance proxy that correlates well with a variety of direct solar irradiance observations, with correlations shown to be in the range of 0.77-0.89. We also present a correlation mapping technique to show how irradiance correlations depend on, and relate to, coronal structure/locations, and to demonstrate how the LASCO CBI can be used to perform long-term "spatial correlation" studies to investigate relationships between the solar corona and any arbitrary concurrent geophysical index. Using this technique we find possible relationships between coronal brightness and plasma temperature, interplanetary magnetic field magnitude and (very weakly) proton density., Comment: 35 pages, 9 Figures. Accepted to Solar Physics

A wide variety of research applications require knowledge of total solar irradiance (TSI) and solar spectral irradiance (SSI) on time scales from minutes to centuries. The current satellite data re...

We describe the historical evolution of the conceptualization, formulation, quantification, application and utilization of “radiative forcing (RF, see e.g., IPCC, 1990)” of Earth’s climate.\ud \ud Basic theories of shortwave and long wave radiation were developed through the 19th and 20th centuries, and established the analytical framework for defining and quantifying the perturbations to the Earth’s radiative energy balance by natural and anthropogenic influences. The insight that the Earth’s climate could be radiatively forced by changes in carbon dioxide, first introduced in the 19th century, gained empirical support with sustained observations of the atmospheric concentrations of the gas beginning in 1957. Advances in laboratory and field measurements, theory, instrumentation, computational technology, data and analysis of well-mixed greenhouse gases and the global climate system through the 20th Century enabled the development and formalism of RF; this allowed RF to be related to changes in global-mean surface temperature with the aid of increasingly sophisticated models. This in turn led to RF becoming firmly established as a principal concept in climate science by 1990.\ud \ud The linkage with surface temperature has proven to be the most important application of the RF concept, enabling a simple metric to evaluate the relative climate impacts of different agents. The late 1970s and 1980s saw accelerated developments in quantification including the first assessment of the effect of the forcing due to doubling of carbon dioxide on climate (the “Charney” report, National Research Council, 1979). The concept was subsequently extended to a wide variety of agents beyond well-mixed greenhouse gases (WMGHGs: carbon dioxide, methane, nitrous oxide, and halocarbons) to short-lived species such as ozone. The WMO (1986) and IPCC (1990) international assessments began the important sequence of periodic evaluations and quantifications of the forcings by natural (solar irradiance changes and stratospheric aerosols resulting from volcanic eruptions) and a growing set of anthropogenic agents (WMGHGs, ozone, aerosols, land surface changes, contrails). From 1990s to the present, knowledge and scientific confidence in the radiative agents acting on the climate system has proliferated. The conceptual basis of RF has also evolved as both our understanding of the way radiative forcing drives climate change, and the diversity of the forcing mechanisms, have grown. This has led to the current situation where “Effective Radiative Forcing (ERF, e.g., IPCC, 2013)” is regarded as the preferred practical definition of radiative forcing in order to better capture the link between forcing and global-mean surface temperature change. The use of ERF, however, comes with its own attendant issues, including challenges in its diagnosis from climate models, its applications to small forcings, and blurring of the distinction between rapid climate adjustments (fast responses) and climate feedbacks; this will necessitate further elaboration of its utility in the future. Global climate model simulations of radiative perturbations by various agents have established how the forcings affect other climate variables besides temperature e.g., precipitation. The forcing-response linkage as simulated by models, including the diversity in the spatial distribution of forcings by the different agents, has provided a practical demonstration of the effectiveness of agents in perturbing the radiative energy balance and causing climate changes.\ud \ud The significant advances over the past half-century have established, with very high confidence, that the global-mean ERF due to human activity since preindustrial times is positive (the 2013 IPCC assessment gives a best estimate of 2.3 W m-2, with a range from 1.1 to 3.3 W m-2; 90% confidence interval). Further, except in the immediate aftermath of climatically-significant volcanic eruptions, the net anthropogenic forcing dominates over natural radiative forcing mechanisms. Nevertheless, the substantial remaining uncertainty in the net anthropogenic ERF leads to large uncertainties in estimates of climate sensitivity from observations and in predicting future climate impacts. The uncertainty in the ERF arises principally from the incorporation of the rapid climate adjustments in the formulation, the well-recognized difficulties in characterizing the preindustrial state of the atmosphere, and the incomplete knowledge of the interactions of aerosols with clouds. This uncertainty impairs the quantitative evaluation of climate adaptation and mitigation pathways in the future. A grand challenge in Earth System science lies in continuing to sustain the relatively simple essence of the radiative forcing concept in a form similar to that originally devised, and at the same time improving the quantification of the forcing. This, in turn, demands an accurate, yet increasingly complex and comprehensive, accounting of the relevant processes in the climate system.

We present a new climate data record for total solar irradiance and solar spectral irradiance between 1610 and the present day with associated wavelength and time-dependent uncertainties and quarterly updates. The data record, which is part of the National Oceanic and Atmospheric Administration’s (NOAA) Climate Data Record (CDR) program, provides a robust, sustainable, and scientifically defensible record of solar irradiance that is of sufficient length, consistency, and continuity for use in studies of climate variability and climate change on multiple time scales and for user groups spanning climate modeling, remote sensing, and natural resource and renewable energy industries. The data record, jointly developed by the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP) and the Naval Research Laboratory (NRL), is constructed from solar irradiance models that determine the changes with respect to quiet sun conditions when facular brightening and sunspot darkening features are present on the solar disk where the magnitude of the changes in irradiance are determined from the linear regression of a proxy magnesium (Mg) II index and sunspot area indices against the approximately decade-long solar irradiance measurements of the Solar Radiation and Climate Experiment (SORCE). To promote long-term data usage and sharing for a broad range of users, the source code, the dataset itself, and supporting documentation are archived at NOAA’s National Centers for Environmental Information (NCEI). In the future, the dataset will also be available through the LASP Interactive Solar Irradiance Data Center (LISIRD) for user-specified time periods and spectral ranges of interest.

Climate change detection and attribution have proven unexpectedly challenging during the 21st century. Earth’s global surface temperature increased less rapidly from 2000 to 2015 than during the last half of the 20th century, even though greenhouse gas concentrations continued to increase. A probable explanation is the mitigation of anthropogenic warming by La Nina cooling and declining solar irradiance. Physical climate models overestimated recent global warming because they did not generate the observed phase of La Nina cooling and may also have underestimated cooling by declining solar irradiance. Ongoing scientific investigations continue to seek alternative explanations to account for the divergence of simulated and observed climate change in the early 21st century, which IPCC termed a “global warming hiatus.” Amplified by media commentary, the suggestions by these studies that “missing” mechanisms may be influencing climate exacerbates confusion among policy makers, the public and other stakeholders about the causes and reality of modern climate change. Understanding and communicating the causes of climate change in the next 20 years may be equally challenging. Predictions of the modulation of projected anthropogenic warming by natural processes have limited skill. The rapid warming at the end of 2015, for example, is not a resumption of anthropogenic warming but rather an amplification of ongoing warming by El Nino. Furthermore, emerging feedbacks and tipping points precipitated by, for example, melting summer Arctic sea ice may alter Earth’s global temperature in ways that even the most sophisticated physical climate models do not yet replicate. This article is categorized under: Paleoclimates and Current Trends > Climate Forcing

We describe spectral and time-dependent variations in solar irradiance, discuss their implications for Earth science, and summarize ongoing work in developing the next generation of observationally-consistent models of solar irradiance variability.

The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N2, and O2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002–2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N2 densities are lower than the NRLMSISE-00 model, but O2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

We present a statistical attribution analysis of the changes in global annual average thermospheric mass density and ionospheric total electron content (TEC) between the cycle 22/23 solar minimum (which occurred at epoch 1996.4) and the prolonged cycle 23/24 minimum (2008.8). The mass density data are derived from orbital drag, and the TEC data are derived from ground-based GPS receivers. The interminima change in mass density was −36% relative to the 1996.4 yearly average. Considering each multiplicative forcing independently, lower average geomagnetic activity during the cycle 23/24 minimum produced an interminima density change of at least −14%, solar extreme ultraviolet (EUV) irradiance forcing produced a density change of −1% to −13%, and changes in thermospheric CO2 concentration produced a density change of −5%. There was essentially no interminima change in global TEC derived from ground-based GPS receivers or space-based altimeters, even though past behavior suggests that it should have changed −3% (0.2 TEC units (1 TECU = 1016 el m−2)) in response to lower geomagnetic activity and −1% to −9% (0.1–0.8 TECU) in response to lower EUV irradiance. There is large uncertainty in the interminima change of solar EUV irradiance; the mass density and TEC data suggest a plausible range of 0% to −6%.

The adjustment of parameterized gravity waves associated with model convection and finer vertical resolution has made possible the generation of the quasi-biennial oscillation (QBO) in two Goddard Institute for Space Studies (GISS) models, GISS Middle Atmosphere Global Climate Model III and a climate/middle atmosphere version of Model E2. Both extend from the surface to 0.002 hPa, with 2° × 2.5° resolution and 102 layers. Many realistic features of the QBO are simulated, including magnitude and variability of its period and amplitude. The period itself is affected by the magnitude of parameterized convective gravity wave momentum fluxes and interactive ozone (which also affects the QBO amplitude and variability), among other forcings. Although varying sea surface temperatures affect the parameterized momentum fluxes, neither aspect is responsible for the modeled variation in QBO period. Both the parameterized and resolved waves act to produce the respective easterly and westerly wind descent, although their effect is offset in altitude at each level. The modeled and observed QBO influences on tracers in the stratosphere, such as ozone, methane, and water vapor are also discussed. Due to the link between the gravity wave parameterization and the models' convection, and the dependence on the ozone field, the models may also be used to investigate how the QBO may vary with climate change.

Statistical models that account for the separate influences on total atmospheric ozone of ozone-depleting substances, anthropogenic greenhouse gases, and natural processes are formulated from the Merged Ozone Data (MOD V8 and V8.6) and used to explore scenarios for ozone’s evolution from 1900 to 2100. The statistical models based on MOD V8 project larger growth in total ozone during the twenty-first century than do coupled chemistry–climate models globally and in the tropics where the chemistry–climate models indicate persistent ozone depletion. The statistical models based on MOD V8.6 suggest, instead, that total ozone everywhere never (or barely) recovers to 1980 levels. Since the decline in ozone-depleting substances and the increase in greenhouse gas concentrations are both expected to increase ozone in the twenty-first century, these results suggest that downward instrumental drifts may be present in MOD V8.6. Instrumental drifts, of opposite sign, may also be present in MOD V8 since it is possible to reduce the projections of the corresponding statistical models to agree with those of the chemistry–climate models by altering the long-term trends of the MOD V8 data within the estimated long-term uncertainty. Alternatively, the chemistry–climate models may project excess tropical ozone depletion by overestimating trends in the upwelling of tropical (ozone poor) air associated with global warming and the resultant decline in mean age of air. This possibility is consistent with independent observations that the age of stratospheric air has not declined during the past three decades, as the globe has warmed 0.3°C, and that model parameterizations of tropical convection may be inadequate.

We simulate the ionosphere and thermosphere throughout the extended solar minimum epoch from 2008 to 2009 using geospace models, systematically validating the models with databases of observed geospace composition. We isolate and quantify observed changes of as much as 4 total electron content unit (TECU) (1 TECU = 1016 elections m−2) (~36%) in global (60°S−60°N) ionospheric total electron density and as much as 19 × 10−12 kg m−3 (~75%) in global thermospheric mass density at 250 km associated with fluctuating solar EUV radiation and geomagnetic activity during this nominally “quiet” period. Corresponding modeled responses to both solar EUV radiation and geomagnetic activity are about a factor of 2 smaller than is observed. We identify, as well, semiannual and annual oscillations that produce geospace variability comparable to that produced by external solar and geomagnetic influences, and which cause distinct differences among the three individual Whole Heliosphere Intervals. From the first Whole Heliosphere Interval (March–April 2008) to the third Whole Heliosphere Interval (June–July 2009) total electron content (60°S−60°N) decreased 3.6 TECU (~32%) and mass density at 250 km decreased 9 × 10−12 kg m−3 (~34%) due to these oscillations. Reliable attribution of the geospace base state during the 2008–2009 solar minimum epoch and geospace comparisons among the Whole Heliosphere Intervals thus requires that the semiannual and annual oscillations be properly distinguished in addition to the concurrent solar and heliospheric effects which have been the primary goal of the majority of Whole Heliosphere Interval (WHI) characterizations.

Simulations of the preindustrial and doubled CO2 climates are made with the GISS Global Climate Middle Atmosphere Model 3 using two different estimates of the absolute solar irradiance value: a higher value measured by solar radiometers in the 1990s and a lower value measured recently by the Solar Radiation and Climate Experiment. Each of the model simulations is adjusted to achieve global energy balance; without this adjustment the difference in irradiance produces a global temperature change of 0.4°C, comparable to the cooling estimated for the Maunder Minimum. The results indicate that by altering cloud cover the model properly compensates for the different absolute solar irradiance values on a global level when simulating both preindustrial and doubled CO2 climates. On a regional level, the preindustrial climate simulations and the patterns of change with doubled CO2 concentrations are again remarkably similar, but there are some differences. Using a higher absolute solar irradiance value and the requisite cloud cover affects the model’s depictions of high-latitude surface air temperature, sea level pressure, and stratospheric ozone, as well as tropical precipitation. In the climate change experiments it leads to an underestimation of North Atlantic warming, reduced precipitation in the tropical western Pacific, and smaller total ozone growth at high northern latitudes. Although significant, these differences are typically modest compared with the magnitude of the regional changes expected for doubled greenhouse gas concentrations. Nevertheless, the model simulations demonstrate that achieving the highest possible fidelity when simulating regional climate change requires that climate models use as input the most accurate (lower) solar irradiance value.

The distinctive spatial patterns of the ionosphere's total electron content (TEC) response to solar, seasonal, diurnal and geomagnetic influences are determined across the globe using a new statistical model constructed from 2-hourly TEC observations from 1998 to 2015. The model combines representations of the physical solar EUV photon and geomagnetic activity drivers with solar-modulated sinusoidal parameterizations of four seasonal cycles and solar- and seasonally-modulated parameterizations of three diurnal cycles. The average absolute residual of the data-model differences is 2.1 TECU (9%) and the root mean square error is 3.5 TECU (15%). Solar and geomagnetic variability, the semiannual oscillation and the diurnal and semidiurnal oscillations all impact TEC most at low magnetic latitudes where TEC itself maximizes, with differing degrees of longitudinal inhomogeneity. In contrast, the annual oscillation manifests primarily in the Southern Hemisphere with maximum amplitude over mid latitude South America, extending to higher southern latitudes in the vicinity of the Weddell Sea. Nighttime TEC levels in the vicinity of the Weddell Sea exceed daytime levels every year in southern hemisphere summer as a consequence of the modulation of the diurnal oscillations by the seasonal oscillations. The anomaly, which is present at all phases of the solar cycle, commences sooner and ends later under solar minimum conditions. The model minus data residuals maximize at tropical magnetic latitudes in four geographical regions similar to the ionosphere pattern generated by lower atmospheric meteorology. Enhanced residuals at northern mid latitudes during winter are consistent with an influence of atmospheric gravity waves.

Reliable historical records of total solar irradiance (TSI) are needed for climate change attribution and research to assess the extent to which long-term variations in the Sun's radiant energy incident on the Earth may exacerbate (or mitigate) the more dominant warming in recent centuries due to increasing concentrations of greenhouse gases. We investigate potential impacts of the new Sunspot Index and Long-term Solar Observations (SILSO) sunspot-number time series on model reconstructions of TSI. In contemporary TSI records, variations on time scales longer than about a day are dominated by the opposing effects of sunspot darkening and facular brightening. These two surface magnetic features, retrieved either from direct observations or from solar activity proxies, are combined in TSI models to reproduce the current TSI observational record. Indices that manifest solar-surface magnetic activity, in particular the sunspot-number record, then enable the reconstruction of historical TSI. Revisions to the sunspot-number record therefore affect the magnitude and temporal structure of TSI variability on centennial time scales according to the model reconstruction methodologies. We estimate the effects of the new SILSO record on two widely used TSI reconstructions, namely the NRLTSI2 and the SATIRE models. We find that the SILSO record has little effect on either model after 1885 but leads to greater amplitude solar-cycle fluctuations in TSI reconstructions prior, suggesting many 18th and 19th century cycles could be similar in amplitude to those of the current Modern Maximum. TSI records based on the revised sunspot data do not suggest a significant change in Maunder Minimum TSI values, and comparing that era to the present we find only very small potential differences in estimated solar contributions to climate with this new sunspot record., 18 pages, 6 figures, accepted for publication in Solar Physics, Jan. 2016

Recent observations made by the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) spacecraft suggest that the Sun’s visible and infrared spectral irradiance increased from 2004 to 2008, even as the total solar irradiance measured simultaneously by SORCE’s Total Irradiance Monitor (TIM) decreased. At the same time, solar ultraviolet (UV) irradiance decreased 3–10 times more than expected from prior observations and model calculations of the known effects of sunspot and facular solar features. Analysis of the SIM spectral irradiance observations during the solar minimum epoch of 2008, when solar activity was essentially invariant, exposes trends in the SIM observations relative to both total solar irradiance and solar activity that are unlikely to be solar in origin. The authors suggest that the SIM’s radically different solar variability characterization is a consequence of undetected instrument sensitivity drifts, not true solar spectrum changes. It is thus doubtful that simulations of climate and atmospheric change using SIM measurements are indicative of real terrestrial behavior.

How—indeed whether—the Sun's variable energy outputs influence Earth's climate has engaged scientific curiosity for more than a century. Early evidence accrued from correlations of assorted solar and climate indices, and from recognition that cycles near 11, 88 and 205 years are common in both the Sun and climate.1, 2 But until recently, an influence of solar variability on climate, whether through cycles or trends, was usually dismissed because climate simulations with (primarily) simple energy balance models indicated that responses to the decadal solar cycle would be so small as to be undetectable in observations.3 However, in the past decade modeling studies have found both resonant responses and positive feedbacks in the ocean-atmosphere system that may amplify the response to solar irradiance variations.4, 5 Today, solar cycles and trends are recognized as important components of natural climate variability on decadal to centennial time scales. Understanding solar-terrestrial linkages is requisite for the comprehensive understanding of Earth's evolving environment. The attribution of present-day climate change, interpretation of changes prior to the industrial epoch, and forecast of future decadal climate change necessitate quantitative understanding of how, when, where, and why natural variability, including by the Sun, may exceed, obscure or mitigate anthropogenic changes. Copyright © 2010 John Wiley & Sons, Ltd. For further resources related to this article, please visit the WIREs website.

We use a flux transport model to simulate the evolution of the Sun's total and open magnetic flux over the last 26 solar cycles (1713-1996). Polar field reversals are maintained by varying the meridional flow speed between 11 and 20 m s-1, with the poleward-directed surface flow being slower during low-amplitude cycles. If the strengths of the active regions are fixed but their numbers are taken to be proportional to the cycle amplitude, the open flux is found to scale approximately as the square root of the cycle amplitude. However, the scaling becomes linear if the number of active regions per cycle is fixed but their average strength is taken to be proportional to the cycle amplitude. Even with the inclusion of a secularly varying ephemeral region background, the increase in the total photospheric flux between the Maunder minimum and the end of solar cycle 21 is at most ~one-third of its minimum-to-maximum variation during the latter cycle. The simulations are compared with geomagnetic activity and cosmogenic isotope records and are used to derive a new reconstruction of total solar irradiance (TSI). The increase in cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W m-2. Because the diffusive decay rate accelerates as the average spacing between active regions decreases, the photospheric magnetic flux and facular brightness grow more slowly than the sunspot number and TSI saturates during the highest amplitude cycles.

Electromagnetic radiation from the Sun is Earth’s primary energy source. Space-based radiometric measurements in the past two decades have begun to establish the nature, magnitude and origins of its variability. An 11-year cycle with peak-to-peak amplitude of order 0.1 % is now well established in recent total solar irradiance observations, as are larger variations of order 0.2 % associated with the Sun’s 27-day rotation period. The ultraviolet, visible and infrared spectral regions all participate in these variations, with larger changes at shorter wavelengths. Linkages of solar radiative output variations with solar magnetism are clearly identified. Active regions alter the local radiance, and their wavelength-dependent contrasts relative to the quiet Sun control the relative spectrum of irradiance variability. Solar radiative output also responds to sub-surface convection and to eruptive events on the Sun. On the shortest time scales, total irradiance exhibits five minute fluctuations of amplitude $\approx 0.003$ %, and can increase to as much as 0.015 % during the very largest solar flares. Unknown is whether multi-decadal changes in solar activity produce longer-term irradiance variations larger than observed thus far in the contemporary epoch. Empirical associations with solar activity proxies suggest reduced total solar irradiance during the anomalously low activity in the seventeenth century Maunder Minimum relative to the present. Uncertainties in understanding the physical relationships between direct magnetic modulation of solar radiative output and heliospheric modulation of cosmogenic proxies preclude definitive historical irradiance estimates, as yet.

We simulate the evolution of the Sun's large-scale magnetic field during solar cycle 21, including the effect of surface transport processes and active region emergence. As an important new constraint on the model, we have scaled our source fluxes upward to be consistent with the average measured strength of the interplanetary magnetic field (IMF). By adopting a poleward bulk flow of amplitude ~20-25 m s-1 together with a supergranular diffusion rate of ~500 km2 s-1, we are then able to match the observed variation of the Sun's polar fields and open magnetic flux. The high meridional flow speeds, peaking at low latitudes, prevent the buildup of an overly strong axisymmetric dipole component at sunspot minimum, while accounting for the giant poleward surges of flux and accompanying polar field fluctuations observed near sunspot maximum. The present simulations also reproduce the large peak in the equatorial dipole and IMF strength recorded in 1982.

We use a magnetic flux transport model to simulate the evolution of the Sun's polar fields and open flux during solar cycles 13 through 22 (1888-1997). The flux emergence rates are assumed to scale according to the observed sunspot-number amplitudes. We find that stable polarity oscillations can be maintained if the meridional flow rate is allowed to vary from cycle to cycle, with higher poleward speeds occurring during the more active cycles. Our model is able to account for a doubling of the interplanetary field strength since 1900, as deduced by Lockwood, Stamper, & Wild from the geomagnetic aa index. We confirm our earlier conclusion that such a doubling of the open flux does not imply that the base level of the total photospheric flux has increased significantly over the last century.

Since November 1978 a complete set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 23 years. From measurements made by different space-based radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI is compiled with an overall precision of order 0.05 Wm–2 and a secular trend uncertainty of ±3 ppm/year. This time series is compared with an empirical model of irradiance variability based on sunspot darkening and brightening due to faculae and network. From this comparison the model is calibrated and used to estimate possible changes of TSI in the past, using historical proxies of solar activity. For this purpose, stellar observations provide information about the possible range of solar variability over the last millennium when changes of Earth's climate are well documented. Together, the paleo solar and climate data enable a discussion of the extent of global climate change that can be explained by a variable Sun.

International audience; Onboard the International Space Station (ISS), two instruments are observing the solar spectral irradiance (SSI) at wavelengths from 16 to 2900 nm. Although the ISS platform orientation generally precludes pointing at the Sun more than 10 - 14 days per month, in November/December 2012 a continuous period of measurements was obtained by implementing an ISS 'bridging' maneuver. This enabled observations to be made of the solar spectral irradiance (SSI) during a complete solar rotation. We present these measurements, which quantify the impact of active regions on SSI, and compare them with data simultaneously gathered from other platforms, and with models of spectral irradiance variability. Our analysis demonstrates that the instruments onboard the ISS have the capability to measure SSI variations consistent with other instruments in space. A comparison among all available SSI measurements during November-December 2012 in absolute units with reconstructions using solar proxies and observed solar activity features is presented and discussed in terms of accuracy.

Proper numerical simulation of the Earth's climate change requires reliable knowledge of solar irradiance and its variability on different time scales as well as the wavelength dependence of this variability. As new measurements of the solar spectral irradiance have become available so too have new reconstructions of historical solar irradiance variations based on different approaches. However these various solar spectral irradiance reconstructions have not yet been compared in detail to quantify differences in their absolute values variability and implications for climate and atmospheric studies. In this paper we quantitatively compare five different reconstructions of solar spectral irradiance changes during the past four centuries in order to document and analyze their differences. The impact on atmosphere and climate studies is discussed in terms of the calculation of short wave solar heating rates. © 2013 Springer Science+Business Media Dordrecht.

Annual levels of the Sun's irradiance are estimated until 2018 using parameterizations of the 10.7 cm radio flux. Peak irradiance during solar cycle 24 occurs in 2010, with levels comparable to or slightly lower than prior maxima in 2000, 1989 and 1981. Minima occur in 2006 and 2016. Future irradiance trends underlying the 11-year cycle, which are assumed not to exceed those in historical reconstructions during the past 350 years, are estimated to be less than ±0.4 Wm−2 per decade for total irradiance, ±0.01 Wm−2 per decade for UV radiation at 295–310 nm and ±0.04 Wm−2 per decade for UV radiation at 200–295 nm. When activity cycles and longer-term trend scenarios are combined, total solar irradiance forcing of climate between cycle minima in 1996 and 2016 is in the range ±0.1 Wm−2. For comparison, the forecast net anthropogenic climate forcing over this 22-year period is in the range 0.5 to 0.9 Wm−2.

We present a new model of solar irradiance variability at extreme ultraviolet wavelengths (EUV, 50–1200 A). In this model, quiet Sun, coronal hole, and active region intensities for optically thin emission lines are computed from emission measure distributions determined from spectrally and spatially resolved observations. For optically thick emission lines and continua, empirical values are used. The contribution of various solar features to the spectral irradiance variability is determined from a simple model of limb-brightening and full-disk solar images taken at the Big Bear Solar Observatory and by the Soft X-Ray Telescope on Yohkoh. To extend our irradiance model beyond the time period covered by the available images, we use correlations with proxies for solar activity. Comparisons with the available irradiance data from the Atmospheric Explorer E (AE-E) spacecraft show that our model is capable of reproducing the rotational modulation of the EUV irradiance near solar maximum. The AE-E data, however, show systematically more solar cycle variability than our model estimates.

Because of the dependence of the Sun's irradi- ance on solar activity, reductions from contemporary levels are expected during the seventeenth century Maunder Min- imum. New reconstructions of spectral irradiance are de- veloped since 1600 with absolute scales traceable to space- based observations. The long-term variations track the en- velope of group sunspot numbers and have amplitudes con- sistent with the range of Ca II brightness in Sun-like stars. Estimated increases since 1675 are 0.7%, 0.2% and0.07% in broadultraviolet, visible/nearinfraredandinfraredspectral bands, with a total irradiance increase of 0.2%.

The large-scale magnetic field of the Sun, including the open flux that extends into the interplanetary medium, originates in active regions but is redistributed over the photosphere by differential rotation, supergranular convection, and poleward meridional flow. We use simulations to clarify the role of the surface transport processes in the evolution of the total open flux, Φopen, which determines the strength of the radial interplanetary field component. Representing the initial photospheric field configuration by one or more bipolar magnetic regions (BMRs), we show that Φopen varies approximately as the net dipole strength, determined by vectorially summing the dipole moments of the individual BMRs. As meridional flow carries the BMR flux to higher latitudes, the equatorial dipole component is annihilated on a timescale ∼1 yr by the combined effect of rotational shearing and supergranular diffusion. The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strength of the original active regions. We discuss the implications of these results for the solar cycle evolution of Φopen.

The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φ open can be inferred from current-free extrapolations of the observed photospheric field. We derive the long-term variation of Φ open during 1971-1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower-latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φ open closely follows that of the Sun's total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large-scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φ open and the IMF strength lasting typically ∼1 yr.

The Naval Research Laboratory (NRL) has embarked on a development program to upgrade empirical models of the neutral upper atmosphere (thermosphere and upper mesosphere) and to apply these models to scientific and engineering problems. The program focus has been the Mass Spectrometer — Incoherent Scatter Radar (MSIS) model of composition and temperature. The new NRLMSIS model, due for release in 2000, has ingested additional data sets, including, for the first time, the drag and accelerometer data of Jacchia and others. The formulation in the lower thermosphere now has improved flexibility, and a new species, “anomalous oxygen,” allows for appreciable O + and hot atomic oxygen contributions to the total mass density at high altitudes. A new full disk proxy for the solar chromospheric EUV driver of thermospheric variability is available for studies in combination with the NRLMSIS model and database. This will determine the value of augmenting F 10.7 in the model formulation — a longstanding issue. Whereas F 10.7 correlates more closely with coronal EUV flux, chromospheric fluxes provide the primary thermospheric heating.

Indices of solar activity relevant for understanding and modelling solar irradiance variability are identified, and their temporal characteristics compared. Reproducing observed solar irradiance variability requires a minimum of two different types of indices — an index for irradiance depletion by sunspots and an index for global irradiance enhancement by faculae and network. When combined with appropriate wavelength-dependent parameterizations of sunspot and facular contrasts and center-to-limb functions, these indices permit the construction of empirical models of daily, monthly and annual solar total and spectral irradiances. The models are compared with observations at selected wavelengths and for the total irradiance. While the models replicate much of the rotational and 11-year cycle variance in contemporary irradiance databases, differences exist because of either the presence of variability mechanisms additional to solar magnetism, or of unresolved instrumental effects in the databases. The reconstruction of solar irradiance in the past requires speculation about the extent of intercycle fluctuations in the global facular index, or in other, as yet unspecified, variability mechanisms.

From the coldest period of the Little Ice Age to the present time, the surface temperature of the Earth increased by perhaps 0.8°C. Solar variability may account for part of this warming which, during the past 350 years, generally tracks fluctuating solar activity levels. While increases in greenhouse gas concentrations are widely assumed to be the primary cause of recent climate change, surface temperatures nevertheless varied significantly during pre-industrial periods, under minimal levels of greenhouse gas variations. A climate forcing of 0.3 W m−2 arising from a speculated 0.13% solar irradiance increase can account for the 0.3°C surface warming evident in the paleoclimate record from 1650 to 1790, assuming that climate sensitivity is 1°C W−1 m−2 (which is within the IPCC range). The empirical Sun–climate relationship defined by these pre-industrial data suggests that solar variability may have contributed 0.25°C of the 0.6°C subsequent warming from 1900 to 1990, a scenario which time dependent GCM simulations replicate when forced with reconstructed solar irradiance. Thus, while solar variability likely played a dominant role in modulating climate during the Little Ice Age prior to 1850, its influence since 1900 has become an increasingly less significant component of climate change in the industrial epoch. It is unlikely that Sun–climate relationships can account for much of the warming since 1970, not withstanding recent attempts to deduce long term solar irradiance fluctuations from the observational data base, which has notable occurrences of instrumental drifts. Empirical evidence suggests that Sun–climate relationships exist on decadal as well as centennial time scales, but present sensitivities of the climate system are insufficient to explain these short-term relationships. Still to be simulated over the time scale of the Little Ice Age to the present is the combined effect of direct radiative forcing, indirect forcing via solar-induced ozone changes in the atmosphere, and speculated charged particle mechanisms whose pathways and sensitivities are not yet specified.

We constructed gridded fields of diabatic heat storage changes in the upper ocean from 20°S to 60°N from historical temperature profiles collected from 1955 to 1996. We filtered these 42 year records for periods of 8 to 15 years and 15 to 30 years, producing depth-weighted vertical average temperature (DVT) changes from the sea surface to the top of the main pycnocline. Basin and global averages of these DVT changes reveal decadal and interdecadal variability in phase across the Indian, Pacific, Atlantic, and Global Oceans, each significantly correlated with changing surface solar radiative forcing at a lag of 0±2 years. Decadal and interdecadal changes in global average DVT are 0.06°±0.01°K and 0.04°K±0.01°K, respectively, the same as those expected from consideration of the Stefan-Boltzmann radiation balance (i.e., 0.3°K per W m−2) in response to 0.1% changes in surface solar radiative forcing of 0.2 W m−2 and 0.15 W m−2, respectively. Global spatial patterns of DVT changes are similar to temperature changes simulated in coupled ocean-atmosphere models, suggesting that natural modes of Earth's variability are phase-locked to the solar irradiance cycle. A trend in global average DVT of 0.15°K over this 42 year record cannot be explained by changing surface solar radiative forcing. But when we consider the 0.5 W m−2 increase in surface radiative forcing estimated from the increase in atmospheric greenhouse gas and aerosol (GGA) concentrations over this period [Intergovernmental Panel on Climate Change, 1995], the Stefan-Boltzmann radiation balance yields this observed change. Moreover, the sum of solar and GGA surface radiative forcing can explain the relatively sharp increase in global and basin average DVT in the late 1970's.

Measurements from the Halogen Occultation Experiment (HALOE) of the upper stratosphere show increases in HCl and H2O and decreases in CH4 and O3 during the period 1992–1995. These changes all coincide with the decline of solar cycle 22. Using a simple photochemical model, we find that 4 major components contribute to the O3 decrease: 1) an increase in total chlorine as indicated by increasing HCl 2) an additional increase in reactive chlorine due to repartitioning of Cly by the decreasing CH4 3) a decrease in odd oxygen production due to decreased solar flux and 4) an increase in odd hydrogen loss due to increasing H2O. At 2 mbar, the Cly repartitioning is the largest cause of O3 changes. Because the Cly repartitioning coincides with decreasing solar flux, some recent observational estimates of the long-term response of upper stratospheric O3 to solar UV irradiance variability may be too large. Compared with the HALOE O3 data, the model O3 exhibits a larger negative trend. This appears to be because the model O3 is more sensitive to the increased H2O than is the observed O3. The 20–30% decline in CH4 also implies a large increase in ClO which exceeds that expected from CFCs. Observations of ClO from the Microwave Limb Sounder (MLS) support this inference.

In a companion paper we presented a new reference spectrum for the EUV irradiance of the quiet Sun based primarily on intensities calculated from a quiet Sun emission measure distribution and recent compilations of atomic data. The contributions of optically thick emission lines and continua were included empirically. In this paper we present comparisons between this reference spectrum and independently measured irradiance observations, previous quiet Sun reference spectra, and the predictions of empirical EUV irradiance models. These comparisons indicate relatively good agreement among fluxes of emission lines formed in the solar chromosphere and transition region. The fluxes for coronal emission lines in previous quiet Sun reference spectra, however, do not agree with our calculated fluxes or with a recent irradiance observation taken at a low level of solar activity. Coronal emission lines in the Atmospheric Explorer E (AE-E) quiet Sun reference spectrum SC21REFW have fluxes that are typically smaller than our calculated fluxes by factors of 2 or more. We also identify inconsistencies in the earlier reference spectrum of Heroux and Hinteregger [1978] (F74113), where the fluxes of many coronal emission lines with wavelengths below 250 A are inconsistent with the fluxes of coronal emission lines at longer wavelengths. The fluxes of EUV continua in the various reference spectra and irradiance observations also differ significantly.

We present a quiet Sun irradiance spectrum from 50-1200 A based primarily on intensities computed from a newly constructed quiet Sun emission measure distribution. We derive the emission measure from a spectrum of a portion of the quiet solar disk measured with the Harvard instrument on Skylab and recent compilations of atomic data. For some specific emission lines and continua which are not optically thin and cannot be computed using an emission measure, we either use intensities from the Harvard spectrum directly or infer them from other observations. Application of a simple center-to-limb variation converts the intensities to equivalent full-disk quiet Sun fluxes.

We present scientific highlights from the Upper Atmosphere Research Satellite (UARS) pro- gram obtained during the first 5 years of the program. The UARS measures the chemical composition and meteorological state of the middle and upper atmo- sphere as well as the solar irradiance. The comprehen- sive suite of UARS geophysical parameters, their near- global coverage, and the multiyear time span have yielded new insights into the processes affecting our atmosphere. In this article we select seven scientific issues, including stratospheric ozone chemistry, the dy- namics of the middle and upper atmosphere, and solar irradiance variations, and discuss the significant impact that the UARS observations have had on our under- standing of each issue.

Using recently processed Ca K filtergrams, recorded with a 1 A filter at the Big Bear Solar Observatory (BBSO), we quantitatively assess the component of solar irradiance variability attributable to bright magnetic features on the Sun's disk. The Ca K filtergrams, flattened by removing instrumental effects and center-to-limb variations, provide information about bright sources of irradiance variability associated with magnetic activity in both active regions and dispersed active region remnants broadly distributed in the supergranule network (termed collectively faculae). Procedures are developed to construct both total and UV spectral solar irradiance variations explicitly from the processed Ca K filtergrams, independently of direct irradiance observations. The disk-integrated bolometric and UV facular brightness signals determined from the filtergrams between late 1991 and mid-1995 are compared with concurrent solar irradiance measurements made by high-precision solar radiometers on the Upper Atmosphere Research Satellite (UARS). The comparisons suggest that active-region and active-network changes can account for the measured variations. This good agreement during a period covering most of the decline in solar activity from the cycle 22 maximum to the impending solar minimum directly implicates magnetic features as the sources of the 11 yr irradiance cycle, apparently obviating the need for an additional component other than spots or faculae.

Nimbus 7 and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) spacecraft measurements of solar irradiance both exhibit variability at mid (200–300 nm) and near (300–400 nm) ultraviolet (UV) wavelengths that are attributable to the Sun's 27-day solar rotation, even though instrument sensitivity drifts obscure longer-term, 11-year cycle variations. Competing influences of dark sunspots and bright faculae are the dominant causes of this rotational modulation. Parameterizations of these influences using a newly developed UV sunspot darkening index and the Mg index facular proxy replicate the rotational modulation detected in both the broadband Nimbus 7 filter data (275–360 nm and 300–410 nm) and in SOLSTICE 1-nm spectra from 200 to 400 nm. Assuming that these rotational modulation influences scale linearly over the solar cycle, long–term databases of sunspot and global facular proxies permit estimation of 11-year cycle amplitudes of the mid– and near–UV solar spectrum, unmeasured at wavelengths longward of 300 nm because of insufficient long-term repeatability (relative accuracy) of state-of-the-art solar radiometers at these wavelengths. Reconstructions of UV irradiances throughout the 11-year solar cycle indicate variabilities of 0.173 W/m2 (1.1%) in the integrated radiation from 200 to 300 nm and 0.24 W/m2 (0.25%) in radiation from 300 to 400 nm. These two UV bands thus contribute about 13% and 18%, respectively, to the 1.34 W/m2 (0.1%) total (spectrally integrated) radiative output solar cycle. The parameterizations allow customization of UV irradiance time series for specific wavelength bands required as inputs to general circulation model simulations of solar cycle forcing of global climate change, and have practical implications regarding the long-term repeatability required for future solar monitoring.

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.

▪ Abstract To what extent are changes in the Earth's global environment linked with fluctuations in its primary energy source, the radiation from a variable star, the Sun? A firm scientific basis for policy making with regard to anthropogenic greenhouse warming of climate and chlorofluorocarbon depletion of ozone requires a reliable answer to this question. Reduction of the vulnerability of spacecraft operations and communications to space weather necessitates knowledge of solar induced variability in Earth's upper atmosphere. Toward these goals, solar radiation monitoring and studies of solar variability mechanisms facilitate an understanding of the sources and amplitudes of the Sun's changing radiation. Interdisciplinary studies that link these changes with a wide array of terrestrial phenomena over the longer time scales of global change and the shorter time scales of space weather address the relevance of solar radiation variability for Earth. However, although numerous associations are apparent between solar and terrestrial fluctuations, full comprehension of the physical mechanisms responsible for the many facets of radiative Sun-Earth coupling remains to be accomplished.

By focusing on time sequences of basin-average and global-average upper ocean temperature (i.e., from 40oS to 60oN) we find temperatures responding to changing solar irradiance in three separate frequency bands with periods of >100 years, 18-25 years, and 9-13 years. Moreover, we find them in two different data sets, that is, surface marine weather observations from 1990 to 1991 and bathythermograph (BT) upper ocean temperature profiles from 1955 to 1994. Band-passing basin-average temperature records find each frequency component in phase across the Indian, Pacific, and Atlantic Oceans, yielding global-average records with maximum amplitudes of 0.04 o _+ 0.01oK and 0.07 o _+ 0.01oK on decadal and interdecadal scales, respectively. These achieve maximum correlation with solar irradiance records (i.e., with maximum amplitude 0.5 W m -2 at the top of the atmosphere) at phase lags ranging from 30 o to 50 o. From the BT data set, solar signals in global-average temperature penetrate to 80-160 m, confined to the upper layer above the main pycnocline. Operating a global-average heat budget for the upper ocean yields sea surface temperature responses of 0.01o-0.03oK and 0.02o-0.05oK on decadal and interdecadal scales, respectively, from the 0.1 W m -2 penetration of solar irradiance to the sea surface. Since this is of the same order as that observed (i.e., 0.04o-0.07oK), we can infer that anomalous heat from changing solar irradiance is stored in the upper layer of the ocean.

We present the first climate prediction of the coming decade made with multiple models, initialized with prior observations. This prediction accrues from an international activity to exchange decadal predictions in near real-time, in order to assess differences and similarities, provide a consensus view to prevent over-confidence in forecasts from any single model, and establish current collective capability. We stress that the forecast is experimental, since the skill of the multi-model system is as yet unknown. Nevertheless, the forecast systems used here are based on models that have undergone rigorous evaluation and individually have been evaluated for forecast skill. Moreover, it is important to publish forecasts to enable open evaluation, and to provide a focus on climate change in the coming decade. Initialized forecasts of the year 2011 agree well with observations, with a pattern correlation of 0.62 compared to 0.31 for uninitialized projections. In particular, the forecast correctly predicted La Nia in the Pacific, and warm conditions in the north Atlantic and USA. A similar pattern is predicted for 2012 but with a weaker La Nia. Indices of Atlantic multi-decadal variability and Pacific decadal variability show no signal beyond climatology after 2015, while temperature in the Nio3 region is predicted to warm slightly by about 0.5 A degrees C over the coming decade. However, uncertainties are large for individual years and initialization has little impact beyond the first 4 years in most regions. Relative to uninitialized forecasts, initialized forecasts are significantly warmer in the north Atlantic sub-polar gyre and cooler in the north Pacific throughout the decade. They are also significantly cooler in the global average and over most land and ocean regions out to several years ahead. However, in the absence of volcanic eruptions, global temperature is predicted to continue to rise, with each year from 2013 onwards having a 50 % chance of exceeding the current observed record. Verification of these forecasts will provide an important opportunity to test the performance of models and our understanding and knowledge of the drivers of climate change.

We introduce a new model of solar irradiance variability at extreme ultraviolet (EUV) wavelengths. The model combines a spectral emission line database, solar emission measure distributions, and estimates from ground-based solar images of the fraction of the Sun covered by the various types of activity to synthesize the irradiance. Using Call K-line images, the model can be used to estimate the irradiance from EUV line emission formed in the upper chromosphere and lower transition region. Comparisons of this new model with existing empirical models reveal both similarities and disagreements in the absolute magnitude, the amplitude of the rotational modulation, and the intermediate-term solar cycle variability of the predicted fluxes.