According to the IPCC (2007), changes in climate occur as a consequence of variations in the Earth’s radiation budget (solar energy absorbed by versus leaving the surface). Changes in the Earth’s radiation budget occur for 3 primary reasons; two of those three reasons involve solar forcing.
“Global climate is determined by the radiation balance of the planet. There are three fundamental ways the Earth’s radiation balance can change, thereby causing a climate change:
(1) changing the incoming solar radiation (e.g., by changes in the Earth’s orbit or in the Sun itself),
(2) changing the fraction of solar radiation that is reflected (this fraction is called the albedo – it can be changed, for example, by changes in cloud cover, small particles called aerosols or land cover), and
(3) altering the longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations).”
Reason (3) is, of course, the one that gets nearly all the attention from those who wish to characterize climate changes as primarily influenced by — or caused by — human activity. That’s where the 100 parts per million change in atmospheric CO2 concentration since 1900 comes in. According to the latest IPCC report, the total amount of radiative forcing attributed to changes in atmospheric CO2 concentrations since 1750 (through 2011) is just 1.8 W m-2. Again, that’s the total accumulated radiative effect attributed to CO2-forcing of climate changes over the last 260 years.
To put this into context, consider that the total amount of radiative forcing attributed to the +22 parts per million CO2 increase for the 2000-2010 period is claimed to be just 0.2 W m-2 by Feldman and co-authors (2015):
“Here we present observationally based evidence of clear-sky CO2 surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric CO2. … The time series both show statistically significant trends of 0.2 W m−2 per decade (with respective uncertainties of ±0.06 W m−2 per decade and ±0.07 W m−2 per decade)”
Remember that. CO2 climate-forcing amounts to merely 0.2 W m-2 per decade with a 22 parts per million increase in atmospheric concentration during the first decade of the 21st century, when there was a pause in global warming.
Reason (1) above, which is essentially changes in the Sun itself that affect its direct output, or irradiance (referred to as total solar irradiance, or TSI), is the second-most talked about explanatory reason attributed to climate changes. This one is controversial. While there are many scientists who are increasingly concluding that long-term changes in the Sun’s output (as recorded by sunspot variations) are responsible for centennial-scale warming and cooling periods, including the modern warming (see here for references to 18 such papers published in 2016 alone), there are still many doubters who believe such seemingly small changes in the Sun’s irradiance cannot have a significant effect on the Earth’s climate.
So let’s focus on Reason (2) as an explanatory factor for changes in the Earth’s radiation budget. This one rarely ever gets much attention. Most casual observers don’t think of clouds as an important factor affecting changes in climate. But they are – far more influential than CO2 within the longwave greenhouse effect. The prominent influence of clouds encompasses both Reason (2) and Reason (3), both albedo/shortwave reflectance and longwave (greenhouse) forcing.
As an example of the dominance of clouds in influencing climate relative to CO2 concentration variations, Ramanathan et al. (1989), in their seminal paper (1,300+ citations) on cloud radiative forcing, write:
“The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO2. The shortwave and longwave components of cloud forcing are about ten times as large as those for a CO2 doubling.”
Even on the RealClimate blog — founded by Michael Mann and Gavin Schmidt (among others) — there is an acknowledgement that the climate influence from changes in cloud cover are far more influential affecting the radiation budget than variations in CO2 (100 W m-2 for clouds versus just 4 W m-2 for doubled [600 ppm] CO2):
“Of course the range of net infrared forcing caused by changing cloud conditions (~100W/m2) is much greater than that caused by increasing levels of greenhouse gases (e.g. doubling pre-industrial CO2 levels will increase the net forcing by ~4W/m2)”
As mentioned, clouds influence the climate in both the shortwave (reflecting more or less solar radiation back to space depending upon cloud height and coverage) and longwave (via “trapping” heat at the surface, or the greenhouse effect). Of the two forcing pathways, the shortwave effects of clouds out-radiate the longwave effects of clouds such that increasing cloud cover leads to cooling, whereas decreasing cloud cover leads to warming.
“Satellite measurements and numerical forecast model reanalysis data are used to compute an updated estimate of the cloud radiative effect on the global multi-annual mean radiative energy budget of the atmosphere and surface. The cloud radiative cooling effect through reflection of short wave radiation dominates over the long wave heating effect, resulting in a net cooling of the climate system of − 21 Wm−2.”
So the net effect of reducing cloud cover is warming. And, not coincidentally, there has been a significant reduction in cloud cover on a net global scale since the 1980s which has allowed more solar radiative energy to warm the Earth’s surface (oceans primarily). This shortwave cloud radiative forcing since the 1980s (approximately 1 to 4 W m-2 per decade on a global scale on average) is several times greater than the alleged CO2 forcing value of just 0.2 W m-2 per decade for the +22 ppm CO2 increase for 2000 to 2010. In other words, decadal-scale changes in cloud cover maintain a dominant influence on the net radiation budget, easily outclassing CO2 as the primary source of radiative change since the 1980s.
“The decrease in the Earth’s reflectance [cloud cover] from 1984 to 2000 suggested by Fig. 4, translates into … an additional global shortwave forcing of 6.8 Wm2. To put that in perspective, the latest IPCC report (IPCC, 2001) argues for a 2.4 Wm2 increase in CO2 longwave forcing since 1850. The temporal variations in the albedo are closely associated with changes in the cloud cover.
“The reduction in total cloud cover of 6.8% [between 1984 – 2009] means that 5.4 Wm−2 (6.8% of 79) is no longer being reflected but acts instead as an extra forcing into the atmosphere… To put this [5.4 Wm-2 of solar radiative forcing via cloud cover reduction between 1984-2009] into context, the IPCC Fifth Assessment Report…states that the total anthropogenic radiative forcing for 2011 relative to 1750 is 2.29 Wm−2 for all greenhouse gases and for carbon dioxide alone is 1.68 Wm−2. The increase in radiative forcing caused by the reduction in total cloud cover over 10 years is therefore more than double the IPCC’s estimated radiative forcing for all greenhouse gases and more than three times greater than the forcing by carbon dioxide alone [from 1750 to present]. … According to the energy balance described by Trenberth et al. (2009), the reduction in total cloud cover accounts for the increase in temperature since 1987, leaving little, if any, of the temperature change to be attributed to other forcings.“
Radiation Budget Changes Are Primarily Caused By Changes In Cloud Cover, Not CO2
Changes in cloud cover do not need to be large to affect change in the radiation budget. Even if there are no changes in the Sun’s output (solar irradiance), a tiny reduction in cloud cover can still have a significant radiative effect and lead to climatic warming.
“Even a small change in the cloud cover modifies the transparency/absorption/reflectance of the atmosphere and affects the amount of absorbed solar radiation, even with no changes in the solar irradiance. Since the flux of CR [cosmic rays, which influences cloud cover changes] is modulated by the solar magnetic activity, this provides a link between solar variability and climate.”
“As in the prior studies, which were restricted to lower latitudes, there is a significant increase in the TOA outgoing longwave radiation during the period 1985 to 1999 together with an increase in solar (shortwave) radiation absorbed by the climate system. It is suggested that these changes are related to an observed reduction in cloud cover.”
“It is widely assumed that variations in Earth’s radiative energy budget at large time and space scales are small. We present new evidence from a compilation of over two decades of accurate satellite data that the top-of-atmosphere (TOA) tropical radiative energy budget is much more dynamic and variable than previously thought. Results indicate that the radiation budget changes are caused by changes in tropical mean cloudiness.”
“We will show that changes of relative humidity or low cloud cover explain the major changes in the global mean temperature. We will present the evidence of this argument using the observed relative humidity between years 1970 and 2011 and the observed low cloud cover between years 1983 and 2008. One percent increase in relative humidity or in low cloud cover decreases the temperature by 0.15 °C and 0.11 °C, respectively. In the time periods mentioned before the contribution of the CO2 increase was less than 10% to the total temperature change.”
A recently-published paper even suggests that the explosive increase in anthropogenic CO2 emissions since the early 1990s has had essentially no effect on the overall greenhouse forcing. Instead, changes in cloud cover explain the variance in the greenhouse effect since the 1990s. See “New Paper Documents Imperciptible CO2 Influence On The Greenhouse Effect Since 1992“.
Surface Solar Radiation, Modified By Clouds, Explain The 1980s-Present Warming
Again, CO2 forcing is claimed to yield about 0.2 W m-2 of radiative forcing per decade, which is the net forcing associated with an increase of +22 ppm between 2000 and 2010 (Feldman et al., 2015). In contrast, global-scale surface solar radiation (SSR) since the 1980s, modified by decadal-scale changes cloud cover trends, has been observed (via satellites) to produce a radiative forcing of between 0.8 W m-2 and 6 W m-2 per decade, depending on the start and end points. In other words, SSR dominates the changes in the radiation budget, with CO2 only a bit player.
“Long-term variations in solar radiation at Earth’s surface (S) can affect our climate, the hydrological cycle, plant photosynthesis, and solar power. Sustained decreases in S [surface solar radiation] have been widely reported from about the year 1960 to 1990. Here we present an estimate of global temporal variations in S by using the longest available satellite record. We observed an overall increase in S [surface solar radiation] from 1983 to 2001 at a rate of 0.16 watts per square meter (0.10%) per year [1.6 W m-2 per decade].”
“[T]here has been a global net decrease in 340 nm cloud plus aerosol reflectivity [1979-2011]. … Applying a 3.6% cloud reflectivity perturbation to the shortwave energy balance partitioning given by Trenberth et al. (2009) corresponds to an increase of 2.7 W m−2 of solar energy reaching the Earth’s surface and an increase of 1.4% or 2.3 W m−2 absorbed by the surface.”
“A similar reversal to brightening in the 1990s has been found on a global scale in a recent study that estimates surface solar radiation from satellite data. This indicates that the surface measurements may indeed pick up a largescale signal. The changes in both satellite derived and measured surface insolation data are also in line with changes in global cloudiness provided by the International Satellite Cloud Climatology Project (ISCCP), which show an increase until the late 1980s and a decrease thereafter, on the order of 5% from the late 1980s to 2002. A recent reconstruction of planetary albedo based on the earthshine method, which also depends on ISCCP cloud data, reports a similar decrease during the 1990s. Over the period covered so far by BSRN (1992 to 2001), the decrease in earth reflectance corresponds to an increase of 6 W m-2 in absorbed solar radiation by the globe.”
Atmospheric impacts on climatic variability of surface incident solar radiation
“The Earth’s climate is driven by surface incident solar radiation (Rs). Direct measurements have shown that Rs has undergone significant decadal variations. … By merging direct measurements collected by Global Energy Budget Archive with those derived from SunDu [sunshine duration], we obtained a good coverage of Rs [surface incident solar radiation] over the Northern Hemisphere. From this data, the average increase of Rs [surface incident solar radiation] from 1982 to 2008 is estimated to be 0.87 W m−2 per decade [2.3 W/m-2 total]”
“Traditionally the Earth’s reflectance has been assumed to be roughly constant, but large decadal variability, not reproduced by current climate models, has been reported lately from a variety of sources. There is a consistent picture among all data sets by which the Earth’s albedo has decreased over the 1985-2000 interval. The amplitude of this decrease ranges from 2-3 W/m2 to 6-7 W/m2 but any value inside these ranges is highly climatologically significant and implies major changes in the Earth’s radiation budget.”
Conclusion: “Global solar irradiance showed a significant fluctuation during the last 90 years. It increased from 1920 to 1940s/1950s, thereafter it decreased toward late 1980s. In early 1990s 75% of the globe indicated the increasing trend of solar irradiance, while the remaining area continued to lose solar radiation. The magnitudes of the variation are estimated at +12 W m 2 [1920-1940s/1950s], – 8 W m 2 [1950s-1980s], and +8 Wm2 [1990-2005], for the first brightening, for the dimming, and the recent brightening periods, respectively. … During the 15 years from 1990 to 2005 the sunshine duration hours over the five sites increased by 0.4 h/d which corresponds to the decrease in total cloud amount of 4%. The present analysis shows that the increase in 2.5 W m2 in global solar radiation was caused by the reduction of the total cloud amount by 4%.”
3 New Papers Reveal Dominance Of Solar, Cloud Forcing On Climate
The above scientific papers reference a global- or hemispheric-scale surface solar radiation (SSR). Documentation of regional SSR trends are widely available in the scientific literature. For example, in Hawaii, Longman et al., (2014) found that a 5-11% per decade reduction in cloud cover led to a 9 to 18 W m-2 per decade increase in SSR over the period 1988-2012. For Europe, Posselt et al. (2014) found SSR forcing for Europe was 4.35 W m-2 per decade for 1983-2010. The following three papers document the dominance of solar and cloud forcing on climate for Spain, the Mediterranean, and the Iberian Peninsula respectively since the 1980s.
“The linear trend in the mean annual series of global solar radiation shows a significant increase since the 1980s of around 10 Wm-2 over the whole 32-year study period. Similar significant increases are observed in the mean seasonal series, with the highest rate of absolute (relative) change during summer (autumn). These results are in line with the widespread increase of global solar radiation, also known as the brightening period, reported at many worldwide observation sites (e.g. Wild, 2009; Sanchez-Lorenzo et al., 2013b). On the other hand, the annual mean diffuse solar radiation series shows a significant decrease during the last three decades, but it is disturbed by strong increases in 1983 and 1991-1992, which might reflect the effects of the El Chichón and Pinatubo volcanic eruptions as a result of enhanced scattering of the aerosols emitted during these large volcanic eruptions. Summarizing, all these results point towards a diminution of clouds and/or aerosols in Spain since the 1980s.”
“[T]his work investigates the evolution and trends in the surface net short-wave radiation (NSWR, surface solar radiation – reflected) over the Mediterranean Basin during the period 1979 − 2012 using monthly re-analysis datasets from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) and aims to shed light on the specific role of clouds on the NSWR trends. The solar dimming/brightening phenomenon is temporally and spatially analyzed over the Mediterranean Basin. The spatially-averaged NSWR over the whole Mediterranean Basin was found to increase in MERRA by +0.36 Wm−2 per decade, with higher rates over the western Mediterranean (+0.82 Wm−2 per decade), and especially during spring (March-April-May; +1.3 Wm−2 per decade). … The increasing trends in NSWR are mostly associated with decreasing ones in cloud optical depth (COD), especially for the low (<700 hPa) clouds. The decreasing COD trends (less opaque clouds and/or decrease in absolute cloudiness) are more pronounced during spring, thus controlling the increasing tendency in NSWR. The NSWR trends for cloudless (clear) skies are influenced by changes in the water-vapor content or even variations in surface albedo to a lesser degree, whereas aerosols are temporally constant in MERRA.”
“The present paper describes how the entire series of global solar radiation (1987–2014) and diffuse radiation (1994–2014) were built, including the quality control process. Appropriate corrections to the diffuse component were made when a shadowband was employed to make measurements. Analysis of the series reveals that annual mean global irradiance presents a statistically significant increase of 2.5 W m−2 (1.4 %) decade−1 (1988–2014 period), mainly due to what occurs in summer (5.6 W m−2 decade−1). These results constitute the first assessment of solar radiation trends for the northeastern region of the Iberian Peninsula and are consistent with trends observed in the regional surroundings and also by satellite platforms, in agreement with the global brightening phenomenon. Diffuse radiation has decreased at −1.3 W m−2 (−2 %) decade−1 (1994–2014 period), which is a further indication of the reduced cloudiness and/or aerosol load causing the changes.”