Papers on the role of the Sun in recent global warming

This list contains papers that on Sun’s role in the recent climate change. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative. Generally no papers simply reporting solar activity measurements are included (those papers will get their own list in future) or other solar activity related papers that do not discuss recent climate change. Also indirect solar effects are mainly left to their own lists (for example through geomagnetism). There already are separate lists for solar cycle length and cosmic rays.

UPDATE (March 19, 2020): Seip & Grøn (2015) added, thanks to Barry for pointing it out.
UPDATE (September 3, 2013): Wang & Dickinson (2013) added. Thanks to JesusR for pointing it out.

A New method for identifying possible causal relationships between CO2, total solar irradiance and global temperature change – Seip & Grøn (2015) [Full text]
Abstract: We apply a novel method based upon “before” and “after” relationships to investigate and quantify interconnections between global temperature anomaly (GTA), as response variable, and greenhouse gases (CO2) and total solar irradiance (TSI) as candidate causal variables for the period 1880 to 2010. The most likely interpretations of our results for the 6 to 8 years cyclic components of the variables are that during the period 1929 to 1936, CO2 significantly leads GTA. However, during the period 1960–2003, GTA apparently leads CO2, that is, the peaks (and troughs) in GTA are in front of, and close to, the peaks (and troughs) in CO2. For time windows outside these periods, we did not find significant before or after-relations. An alternative interpretation is that there is a shift between short (≈1.5 year) and long (≈5 years) durations between cause and effect. Relationships between GTA and TSI suggest that “inertia” of the global sea, land, and atmosphere system leads to delays longer than half their common cycle length of about 10 years. Based on the interaction patterns between the variables GTA, CO2, and TSI, we suggest the possibility that a new regime for how the variables interact started around 1960. From trend forms, and not considering physical mechanisms, we found that the trend in CO2 contributes ≈ 90 %, and the trend in TSI ≈ 10 %, to the trend in GTA during the last 130 years.
Citation: Seip, K.L., Grøn, Ø. A New method for identifying possible causal relationships between CO2, total solar irradiance and global temperature change. Theor Appl Climatol 127, 923–938 (2017). https://doi.org/10.1007/s00704-015-1675-8.

Contribution of solar radiation to decadal temperature variability over land – Wang & Dickinson (2013) “Global air temperature has become the primary metric for judging global climate change. The variability of global temperature on a decadal timescale is still poorly understood. This paper examines further one suggested hypothesis, that variations in solar radiation reaching the surface (R_s) have caused much of the observed decadal temperature variability. Because R_s only heats air during the day, its variability is plausibly related to the variability of diurnal temperature range (daily maximum temperature minus its minimum). We show that the variability of diurnal temperature range is consistent with the variability of R_s at timescales from monthly to decadal. This paper uses long comprehensive datasets for diurnal temperature range to establish what has been the contribution of R_s to decadal temperature variability. It shows that R_s over land globally peaked in the 1930s, substantially decreased from the 1940s to the 1970s, and changed little after that. Reduction of R_s caused a reduction of more than 0.2 °C in mean temperature during May to October from the 1940s through the 1970s, and a reduction of nearly 0.2 °C in mean air temperature during November to April from the 1960s through the 1970s. This cooling accounts in part for the near-constant temperature from the 1930s into the 1970s. Since then, neither the rapid increase in temperature from the 1970s through the 1990s nor the slowdown of warming in the early twenty-first century appear to be significantly related to changes of R_s.” Kaicun Wang and Robert E. Dickinson, PNAS, 2013, doi: 10.1073/pnas.1311433110. [Full text]

Evidence of recent causal decoupling between solar radiation and global temperature – Pasini et al. (2012) “The Sun has surely been a major external forcing to the climate system throughout the Holocene. Nevertheless, opposite trends in solar radiation and temperatures have been empirically identified in the last few decades. Here, by means of an inferential method—the Granger causality analysis—we analyze this situation and, for the first time, show that an evident causal decoupling between total solar irradiance and global temperature has appeared since the 1960s.” Antonello Pasini et al 2012 Environ. Res. Lett. 7 034020 doi:10.1088/1748-9326/7/3/034020. [Full text]

Solar Influence on Global and Regional Climates – Lockwood (2012) “The literature relevant to how solar variability influences climate is vast—but much has been based on inadequate statistics and non-robust procedures. The common pitfalls are outlined in this review. The best estimates of the solar influence on the global mean air surface temperature show relatively small effects, compared with the response to anthropogenic changes (and broadly in line with their respective radiative forcings). However, the situation is more interesting when one looks at regional and season variations around the global means. In particular, recent research indicates that winters in Eurasia may have some dependence on the Sun, with more cold winters occurring when the solar activity is low. Advances in modelling “top-down” mechanisms, whereby stratospheric changes influence the underlying troposphere, offer promising explanations of the observed phenomena. In contrast, the suggested modulation of low-altitude clouds by galactic cosmic rays provides an increasingly inadequate explanation of observations.” Mike Lockwood, Surveys in Geophysics, July 2012, Volume 33, Issue 3-4, pp 503-534. [Full text]

Solar Forcing of Climate – de Jager (2012) “Solar activity is evident both in the equatorial activity centres and in the polar magnetic field variations. The total solar irradiance variation is due to the former component. During the extraordinarily long minimum of activity between sunspot cycles 23 and 24, the variations related to the equatorial field components reached their minimum values in the first half of 2008, while those related to the polar field variations had their extreme values rather at the end of 2009 and the first half of 2010. The explanation of this delay is another challenge for dynamo theories. The role of the open solar flux has so far been grossly underestimated in discussions of Sun-climate relations. The gradual increase in the average terrestrial ground temperature since 1610 is related both to the equatorial and polar field variations. The main component (0.077 K/century) is due to the variation of the total solar irradiance. The second component (0.040 K/century) waits for an explanation. The smoothed residual increase, presumably antropogenic, obtained after subtraction of the known components from the total increase was 0.31 K in 1999.” C. de Jager, Surveys in Geophysics, July 2012, Volume 33, Issue 3-4, pp 445-451. [Full text]

Solar activity–climate relations: A different approach – Stauning (2011) “The presentation of solar activity–climate relations is extended with the most recent sunspot and global temperature data series. The extension of data series shows clearly that the changes in terrestrial temperatures are related to sources different from solar activity after ∼1985. Based on analyses of data series for the years 1850–1985 it is demonstrated that, apart from an interval of positive deviation followed by a similar negative excursion in Earth’s temperatures between ∼1923 and 1965, there is a strong correlation between solar activity and terrestrial temperatures delayed by 3 years, which complies with basic causality principles. A regression analysis between solar activity represented by the cycle-average sunspot number, SSN_A, and global temperature anomalies, ΔT_A, averaged over the same interval lengths, but delayed by 3 years, provides the relation ΔT_A∼0.009 (±0.002) SSN_A. Since the largest ever observed SSN_A is ∼90 (in 1954–1965), the solar activity-related changes in global temperatures could amount to no more than ±0.4 °C over the past ∼400 years where the sunspots have been recorded. It is demonstrated that the small amplitudes of cyclic variations in the average global temperatures over the ∼11 year solar cycle excludes many of the various driver processes suggested in published and frequently quoted solar activity–climate relations. It is suggested that the in-cycle variations and also the longer term variations in global temperatures over the examined 135 years are mainly caused by corresponding changes in the total solar irradiance level representing the energy output from the core, but further modulated by varying energy transmission properties in the active outer regions of the Sun.” P. Stauning, Journal of Atmospheric and Solar-Terrestrial Physics, Volume 73, Issue 13, August 2011, Pages 1999–2012, http://dx.doi.org/10.1016/j.jastp.2011.06.011.

Are secular correlations between sunspots, geomagnetic activity, and global temperature significant? – Love et al. (2011) “Recent studies have led to speculation that solar-terrestrial interaction, measured by sunspot number and geomagnetic activity, has played an important role in global temperature change over the past century or so. We treat this possibility as an hypothesis for testing. We examine the statistical significance of cross-correlations between sunspot number, geomagnetic activity, and global surface temperature for the years 1868–2008, solar cycles 11–23. The data contain substantial autocorrelation and nonstationarity, properties that are incompatible with standard measures of cross-correlational significance, but which can be largely removed by averaging over solar cycles and first-difference detrending. Treated data show an expected statistically-significant correlation between sunspot number and geomagnetic activity, Pearson p < 10−4, but correlations between global temperature and sunspot number (geomagnetic activity) are not significant, p = 0.9954, (p = 0.8171). In other words, straightforward analysis does not support widely-cited suggestions that these data record a prominent role for solar-terrestrial interaction in global climate change. With respect to the sunspot-number, geomagnetic-activity, and global-temperature data, three alternative hypotheses remain difficult to reject: (1) the role of solar-terrestrial interaction in recent climate change is contained wholly in long-term trends and not in any shorter-term secular variation, or, (2) an anthropogenic signal is hiding correlation between solar-terrestrial variables and global temperature, or, (3) the null hypothesis, recent climate change has not been influenced by solar-terrestrial interaction." Love, J. J., K. Mursula, V. C. Tsai, and D. M. Perkins (2011), Geophys. Res. Lett., 38, L21703, doi:10.1029/2011GL049380. [Full text]

Solar influences on climate – Gray et al. (2010) “Understanding the influence of solar variability on the Earth’s climate requires knowledge of solar variability, solar-terrestrial interactions, and the mechanisms determining the response of the Earth’s climate system. We provide a summary of our current understanding in each of these three areas. Observations and mechanisms for the Sun’s variability are described, including solar irradiance variations on both decadal and centennial time scales and their relation to galactic cosmic rays. Corresponding observations of variations of the Earth’s climate on associated time scales are described, including variations in ozone, temperatures, winds, clouds, precipitation, and regional modes of variability such as the monsoons and the North Atlantic Oscillation. A discussion of the available solar and climate proxies is provided. Mechanisms proposed to explain these climate observations are described, including the effects of variations in solar irradiance and of charged particles. Finally, the contributions of solar variations to recent observations of global climate change are discussed.” Gray, L. J., et al. (2010), SOLAR INFLUENCES ON CLIMATE, Rev. Geophys., 48, RG4001, doi:10.1029/2009RG000282. [Full text]

An influence of solar spectral variations on radiative forcing of climate – Haigh et al. (2010) “The thermal structure and composition of the atmosphere is determined fundamentally by the incoming solar irradiance. Radiation at ultraviolet wavelengths dissociates atmospheric molecules, initiating chains of chemical reactions—specifically those producing stratospheric ozone—and providing the major source of heating for the middle atmosphere, while radiation at visible and near-infrared wavelengths mainly reaches and warms the lower atmosphere and the Earth’s surface. Thus the spectral composition of solar radiation is crucial in determining atmospheric structure, as well as surface temperature, and it follows that the response of the atmosphere to variations in solar irradiance depends on the spectrum. Daily measurements of the solar spectrum between 0.2 µm and 2.4 µm, made by the Spectral Irradiance Monitor (SIM) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite since April 2004, have revealed that over this declining phase of the solar cycle there was a four to six times larger decline in ultraviolet than would have been predicted on the basis of our previous understanding. This reduction was partially compensated in the total solar output by an increase in radiation at visible wavelengths. Here we show that these spectral changes appear to have led to a significant decline from 2004 to 2007 in stratospheric ozone below an altitude of 45 km, with an increase above this altitude. Our results, simulated with a radiative-photochemical model, are consistent with contemporaneous measurements of ozone from the Aura-MLS satellite, although the short time period makes precise attribution to solar effects difficult. We also show, using the SIM data, that solar radiative forcing of surface climate is out of phase with solar activity. Currently there is insufficient observational evidence to validate the spectral variations observed by SIM, or to fully characterize other solar cycles, but our findings raise the possibility that the effects of solar variability on temperature throughout the atmosphere may be contrary to current expectations.” Joanna D. Haigh, Ann R. Winning, Ralf Toumi & Jerald W. Harder, Nature, Volume: 467, Pages: 696–699, Date published: 07 October 2010, doi:10.1038/nature09426. [Full text]

Solar change and climate: an update in the light of the current exceptional solar minimum – Lockwood (2010) “Solar outputs during the current solar minimum are setting record low values for the space age. Evidence is here reviewed that this is part of a decline in solar activity from a grand solar maximum and that the Sun has returned to a state that last prevailed in 1924. Recent research into what this means, and does not mean, for climate change is reviewed.” Mike Lockwood, Proc. R. Soc. A 8 February 2010 vol. 466 no. 2114 303-329, doi: 10.1098/rspa.2009.0519. [Full text]

Cycles and trends in solar irradiance and climate – Lean (2010) “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. 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. 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. 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.” Judith L. Lean, Wiley Interdisciplinary Reviews: Climate Change, 1, 1, 111-122, DOI: 10.1002/wcc.18. [Full text]

Solar trends and global warming – Benestad & Schmidt (2009) “We use a suite of global climate model simulations for the 20th century to assess the contribution of solar forcing to the past trends in the global mean temperature. In particular, we examine how robust different published methodologies are at detecting and attributing solar-related climate change in the presence of intrinsic climate variability and multiple forcings. We demonstrate that naive application of linear analytical methods such as regression gives nonrobust results. We also demonstrate that the methodologies used by Scafetta and West (2005, 2006a, 2006b, 2007, 2008) are not robust to these same factors and that their error bars are significantly larger than reported. Our analysis shows that the most likely contribution from solar forcing a global warming is 7 ± 1% for the 20th century and is negligible for warming since 1980.” Benestad, R. E. and G. A. Schmidt (2009), Solar trends and global warming, J. Geophys. Res., 114, D14101, doi:10.1029/2008JD011639. [Full text]

Recent changes in solar outputs and the global mean surface temperature. III. Analysis of contributions to global mean air surface temperature rise – Lockwood (2008) “A multivariate fit to the variation in global mean surface air temperature anomaly over the past half century is presented. The fit procedure allows for the effect of response time on the waveform, amplitude and lag of each radiative forcing input, and each is allowed to have its own time constant. It is shown that the contribution of solar variability to the temperature trend since 1987 is small and downward; the best estimate is −1.3% and the 2σ confidence level sets the uncertainty range of −0.7 to −1.9%. The result is the same if one quantifies the solar variation using galactic cosmic ray fluxes (for which the analysis can be extended back to 1953) or the most accurate total solar irradiance data composite. The rise in the global mean air surface temperatures is predominantly associated with a linear increase that represents the combined effects of changes in anthropogenic well-mixed greenhouse gases and aerosols, although, in recent decades, there is also a considerable contribution by a relative lack of major volcanic eruptions. The best estimate is that the anthropogenic factors contribute 75% of the rise since 1987, with an uncertainty range (set by the 2σ confidence level using an AR(1) noise model) of 49–160%; thus, the uncertainty is large, but we can state that at least half of the temperature trend comes from the linear term and that this term could explain the entire rise. The results are consistent with the intergovernmental panel on climate change (IPCC) estimates of the changes in radiative forcing (given for 1961–1995) and are here combined with those estimates to find the response times, equilibrium climate sensitivities and pertinent heat capacities (i.e. the depth into the oceans to which a given radiative forcing variation penetrates) of the quasi-periodic (decadal-scale) input forcing variations. As shown by previous studies, the decadal-scale variations do not penetrate as deeply into the oceans as the longer term drifts and have shorter response times. Hence, conclusions about the response to century-scale forcing changes (and hence the associated equilibrium climate sensitivity and the temperature rise commitment) cannot be made from studies of the response to shorter period forcing changes.” Mike Lockwood, Proc. R. Soc. A 8 June 2008 vol. 464 no. 2094 1387-1404, doi: 10.1098/rspa.2007.0348. [Full text]

Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. II. Different reconstructions of the total solar irradiance variation and dependence on response time scale – Lockwood & Fröhlich (2008) “We have previously placed the solar contribution to recent global warming in context using observations and without recourse to climate models. It was shown that all solar forcings of climate have declined since 1987. The present paper extends that analysis to include the effects of the various time constants with which the Earth’s climate system might react to solar forcing. The solar input waveform over the past 100 years is defined using observed and inferred galactic cosmic ray fluxes, valid for either a direct effect of cosmic rays on climate or an effect via their known correlation with total solar irradiance (TSI), or for a combination of the two. The implications, and the relative merits, of the various TSI composite data series are discussed and independent tests reveal that the PMOD composite used in our previous paper is the most realistic. Use of the ACRIM composite, which shows a rise in TSI over recent decades, is shown to be inconsistent with most published evidence for solar influences on pre-industrial climate. The conclusions of our previous paper, that solar forcing has declined over the past 20 years while surface air temperatures have continued to rise, are shown to apply for the full range of potential time constants for the climate response to the variations in the solar forcings.” Mike Lockwood and Claus Fröhlich, Proc. R. Soc. A 8 June 2008 vol. 464 no. 2094 1367-1385, doi: 10.1098/rspa.2007.0347. [Full text]

Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature – Lockwood & Fröhlich (2007) “There is considerable evidence for solar influence on the Earth’s pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.” Mike Lockwood and Claus Fröhlich, Proc. R. Soc. A 8 October 2007 vol. 463 no. 2086 2447-2460, doi: 10.1098/rspa.2007.1880. [Full text]

Variations in solar luminosity and their effect on the Earth’s climate – Foukal et al. (2006) “Variations in the Sun’s total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun’s output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present.” P. Foukal, C. Fröhlich, H. Spruit and T. M. L. Wigley, Nature 443, 161-166 (14 September 2006) | doi:10.1038/nature05072. [Full text]

Phenomenological solar contribution to the 1900–2000 global surface warming – Scafetta & West (2006) “We study the role of solar forcing on global surface temperature during four periods of the industrial era (1900–2000, 1900–1950, 1950–2000 and 1980–2000) by using a sun-climate coupling model based on four scale-dependent empirical climate sensitive parameters to solar variations. We use two alternative total solar irradiance satellite composites, ACRIM and PMOD, and a total solar irradiance proxy reconstruction. We estimate that the sun contributed as much as 45–50% of the 1900–2000 global warming, and 25–35% of the 1980–2000 global warming. These results, while confirming that anthropogenic-added climate forcing might have progressively played a dominant role in climate change during the last century, also suggest that the solar impact on climate change during the same period is significantly stronger than what some theoretical models have predicted.” Scafetta, N. and B. J. West (2006), Phenomenological solar contribution to the 1900–2000 global surface warming, Geophys. Res. Lett., 33, L05708, doi:10.1029/2005GL025539. [Full text]

Measurement and Uncertainty of the Long-Term Total Solar Irradiance Trend – Dewitte et al. (2005) “A possible long-term trend of the total solar irradiance could be a natural cause for climate variations on Earth. Measurement of the total solar irradiance with space radiometers started in 1978. We present a new total solar irradiance composite, with an uncertainty of ± 0.35 W m⁻². From the minimum in 1995 to the maximum in 2002 the total solar irradiance increased by 1.6 W m⁻². In between the minima of 1987 and 1995 the total solar irradiance increased by 0.15 W m⁻².” Steven Dewitte, Dominique Crommelynck, Sabri Mekaoui, Alexandre Joukoff, Solar Physics, October 2004, Volume 224, Issue 1-2, pp 209-216, DOI: 10.1007/s11207-005-5698-7. [Full text]

Total solar irradiance and climate – Mendoza (2005) “The solar radiation is the fundamental source of energy that drives the Earth’s climate and sustains life. The variability of this output certainly affects our planet. In the last two decades an enormous advance in the understanding of the variability of the solar irradiance has been achieved. Space-based measurements indicate that the total solar irradiance changes at various time scales, from minutes to the solar cycle. Climate models show that total solar irradiance variations can account for a considerable part of the temperature variation of the Earth’s atmosphere in the pre-industrial era. During the 20th century its relative influence on the temperature changes has descended considerably. This means that other sources of solar activity as well as internal and man-made causes are contributing to the Earth’s temperature variability, particularly the former in the 20th century. Some very challenging questions concerning total solar irradiance variations and climate have been raised: are total solar irradiance variations from cycle to cycle well represented by sunspot and facular changes? Does total solar irradiance variations always parallel the solar activity cycle? Is there a long-term variation of the total solar irradiance, and closely related to this, is the total solar irradiance output of the quiet sun constant? If there is not a long-term trend of total solar irradiance variations, then we need amplifying mechanisms of total solar irradiance to account for the good correlations found between total solar irradiance and climate. The latter because the observed total solar irradiance changes are inconsequential when introduced in present climate models.” Blanca Mendoza, Advances in Space Research, Volume 35, Issue 5, 2005, Pages 882–890, http://dx.doi.org/10.1016/j.asr.2004.10.011. [Full text]

How unusual is today’s solar activity? (reply) – Solanki et al. (2005) “Muscheler et al. claim that the solar activity affecting cosmic rays was much higher in the past than we deduced from ¹⁴C measurements. However, this claim is based on a problematic normalization and is in conflict with independent results, such as the ⁴⁴Ti activity in meteorites and the ¹⁰Be concentration in ice cores.” S. K. Solanki, I. G. Usoskin, B. Kromer, M. Schüssler & J. Beer, Nature 436, E4-E5 (28 July 2005) | doi:10.1038/nature04046.

How unusual is today’s solar activity? – Muscheler et al. (2005) “To put global warming into context requires knowledge about past changes in solar activity and the role of the Sun in climate change. Solanki et al. propose that solar activity during recent decades was exceptionally high compared with that over the preceding 8,000 years. However, our extended analysis of the radiocarbon record reveals several periods during past centuries in which the strength of the magnetic field in the solar wind was similar to, or even higher than, that of today.” Raimund Muscheler, Fortunat Joos, Simon A. Müller & Ian Snowball, Nature 436, E3-E4 (28 July 2005) | doi:10.1038/nature04045.

Unusual activity of the Sun during recent decades compared to the previous 11,000 years – Solanki et al. (2004) “Direct observations of sunspot numbers are available for the past four centuries, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades.” S. K. Solanki, I. G. Usoskin, B. Kromer, M. Schüssler & J. Beer, Nature 431, 1084-1087 (28 October 2004) | doi:10.1038/nature02995.

Can solar variability explain global warming since 1970? – Solanki & Krivova (2003) “The magnitude of the Sun’s influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations and climate modeling. This approach suffers from uncertainties that are difficult to estimate. Such uncertainties are introduced because the employed models may not include important but complex processes or mechanisms or may treat these in too simplified a manner. Here we take a more empirical approach. We employ time series of the most relevant solar quantities, the total and UV irradiance between 1856 and 1999 and the cosmic rays flux between 1868 and 1999. The time series are constructed using direct measurements wherever possible and reconstructions based on models and proxies at earlier times. These time series are compared with the climate record for the period 1856 to 1970. The solar records are scaled such that statistically the solar contribution to climate is as large as possible in this period. Under this assumption we repeat the comparison but now including the period 1970–1999. This comparison shows without requiring any recourse to modeling that since roughly 1970 the solar influence on climate (through the channels considered here) cannot have been dominant. In particular, the Sun cannot have contributed more than 30% to the steep temperature increase that has taken place since then, irrespective of which of the three considered channels is the dominant one determining Sun-climate interactions: tropospheric heating caused by changes in total solar irradiance, stratospheric chemistry influenced by changes in the solar UV spectrum, or cloud coverage affected by the cosmic ray flux.” Solanki, S. K. and N. A. Krivova (2003), Can solar variability explain global warming since 1970?, J. Geophys. Res., 108(A5), 1200, doi:10.1029/2002JA009753. [Full text]

Do Models Underestimate the Solar Contribution to Recent Climate Change? – Stott et al. (2003) “Current attribution analyses that seek to determine the relative contributions of different forcing agents to observed near-surface temperature changes underestimate the importance of weak signals, such as that due to changes in solar irradiance. Here a new attribution method is applied that does not have a systematic bias against weak signals. It is found that current climate models underestimate the observed climate response to solar forcing over the twentieth century as a whole, indicating that the climate system has a greater sensitivity to solar forcing than do models. The results from this research show that increases in solar irradiance are likely to have had a greater influence on global-mean temperatures in the first half of the twentieth century than the combined effects of changes in anthropogenic forcings. Nevertheless the results confirm previous analyses showing that greenhouse gas increases explain most of the global warming observed in the second half of the twentieth century.” Stott, Peter A., Gareth S. Jones, John F. B. Mitchell, 2003: Do Models Underestimate the Solar Contribution to Recent Climate Change?. J. Climate, 16, 4079–4093. doi: http://dx.doi.org/10.1175/1520-0442(2003)0162.0.CO;2. [Full text]

The Sun’s Role in Climate Variations – Rind (2002) A review paper. “Is the Sun the controller of climate changes, only the instigator of changes that are mostly forced by the system feedbacks, or simply a convenient scapegoat for climate variations lacking any other obvious cause? This question is addressed for suggested solar forcing mechanisms operating on time scales from billions of years to decades. Each mechanism fails to generate the expected climate response in important respects, although some relations are found. The magnitude of the system feedbacks or variability appears as large or larger than that of the solar forcing, making the Sun’s true role ambiguous. As the Sun provides an explicit external forcing, a better understanding of its cause and effect in climate change could help us evaluate the importance of other climate forcings (such as past and future greenhouse gas changes).” D. Rind, Science 26 April 2002: Vol. 296 no. 5568 pp. 673-677, DOI: 10.1126/science.1069562. [Full text]

The effects of solar variability on the Earth’s climate – Haigh (2002) A review paper. “The absolute value of total solar irradiance is not known to better than ca. 0.3% but measurements from satellite instruments over the past two solar cycles have shown that it varies by ca. 0.1% on this time-scale. Over longer periods its value has been reconstructed using proxy measures of solar activity, and these suggest that during the Maunder minimum in solar activity of the late 17th century it was 3-4 W m^-2 lower than at present. Observational data suggest that the Sun has influenced temperatures on decadal, centennial and millennial time-scales, but radiative forcing considerations and the results of energy-balance models and general circulation models suggest that the warming during the latter part of the 20th century cannot be ascribed entirely to solar effects. However, chemical and dynamical processes in the middle atmosphere may act to amplify the solar impact. An analysis of zonal mean temperature data shows that solar effects may be differentiated from those associated with other factors such as volcanic eruptions and the El Niño Southern Oscillation.” Joanna D. Haigh, Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 361, No. 1802, Science and Applications of the Space Environment: New Results and Interdisciplinary Connections (Jan. 15, 2003), pp. 95-111. [Full text]

Solar Variability and the Earth’s Climate: Introduction and Overview – Reid (2000) “Numerous attempts have been made over the years to link various aspects of solar variability to changes in the Earth’s climate. There has been growing interest in this possible connection in recent years, spurred largely by the need to understand the natural causes of climate change, against which the expected global warming due to man’s activities will have to be detected. The time scale of concern here is that of decades to centuries, and excludes the longer millennial scale in which orbital variations play a dominant role. The field has long been plagued by the lack of an acceptable physical mechanism by which solar variability can affect climate, but the discovery of variability in the Sun’s total irradiance (the solar “constant” of meteorology) by spacecraft instruments has pointed to a direct mechanism. Other less direct mechanisms that have been suggested involve variations in the Sun’s ultraviolet flux and in the plasma outflow of the solar wind. The purpose of this paper is to summarize the current state of the field, emphasizing the proposed mechanisms as an introduction to the more detailed papers that follow. The particular case of sea-surface temperature data will be used as an illustration.” George C. Reid, Space Science Reviews, November 2000, Volume 94, Issue 1-2, pp 1-11, DOI: 10.1023/A:1026797127105.

The Sun’s total irradiance: Cycles, trends and related climate change uncertainties since 1976 – Fröhlich & Lean (1998) “A composite record of the Sun’s total irradiance compiled from measurements made by five independent space‐based radiometers since 1978 exhibits a prominent 11‐year cycle with similar levels during 1986 and 1996, the two most recent minimum epochs of solar activity. This finding contradicts recent assertions of a 0.04% irradiance increase from the 1986 to 1996 solar minima and suggests that solar radiative output trends contributed little of the 0.2°C increase in the global mean surface temperature in the past decade. Nor does our 18‐year composite irradiance record support a recent upward irradiance trend inferred from solar cycle length, a parameter used to imply a close linkage in the present century between solar variability and climate change.” Fröhlich, C. and J. Lean (1998), The Sun’s total irradiance: Cycles, trends and related climate change uncertainties since 1976, Geophys. Res. Lett., 25(23), 4377–4380, doi:10.1029/1998GL900157. [Full text]

Dependence of global temperatures on atmospheric CO₂ and solar irradiance – Thomson (1997) “Changes in global average temperatures and of the seasonal cycle are strongly coupled to the concentration of atmospheric CO₂. I estimate transfer functions from changes in atmospheric CO₂ and from changes in solar irradiance to hemispheric temperatures that have been corrected for the effects of precession. They show that changes from CO₂ over the last century are about three times larger than those from changes in solar irradiance. The increase in global average temperature during the last century is at least 20 times the SD of the residual temperature series left when the effects of CO₂ and changes in solar irradiance are subtracted.” David J. Thomson, PNAS August 5, 1997 vol. 94 no. 16 8370-8377. [Full text]

The Seasons, Global Temperature, and Precession – Thomson (1995) “Analysis of instrumental temperature records beginning in 1659 shows that in much of the world the dominant frequency of the seasons is one cycle per anomalistic year (the time from perihelion to perihelion, 365.25964 days), not one cycle per tropical year (the time from equinox to equinox, 365.24220 days), and that the timing of the annual temperature cycle is controlled by perihelion. The assumption that the seasons were timed by the equinoxes has caused many statistical analyses of climate data to be badly biased. Coherence between changes in the amplitude of the annual cycle and those in the average temperature show that between 1854 and 1922 there were small temperature variations, probably of solar origin. Since 1922, the phase of the Northern Hemisphere coherence between these quantities switched from 0° to 180° and implies that solar variability cannot be the sole cause of the increasing temperature over the last century. About 1940, the phase patterns of the previous 300 years began to change and now appear to be changing at an unprecedented rate. The average change in phase is now coherent with the logarithm of atmospheric CO2 concentration.” David J. Thomson, Science 7 April 1995: Vol. 268 no. 5207 pp. 59-68, DOI: 10.1126/science.268.5207.59.