Ms. Rabett told Eli to get off his tired old well the bunnies know what, and go where the sun don't shine. So the Rabett dialed up the US EPA responses to challenges to its Endangerment Finding for increasing CO2 concentrations and considered the matter of solar influences
Comment (3-35):Ho Ho Hummmm.
A number of commenters (e.g., 0670) argue that the sun is the primary driver of global temperature changes. Several commenters (3323.1, 4003, 4041.1, 4932.1, and 5158) referred to a new 2009 paper by Scafetta and Willson suggesting that the IPCC used faulty solar data in dismissing the direct effect of solar variability on global temperatures. Commenters also cite other research by Scafetta and others that suggests that solar variability could account for up to 68% of the increase in Earth’s global temperatures. One commenter (1616.1) attributes 0.14°C of the warming since 1950 to increased solar irradiance, and another 25% of warming since 1979, as in Scafetta and West (2006) (3596.1). Another commenter (7031) states that the correlation between solar variations such as sunspots and global climate has been pointed out by several scientists, such as Scafetta and West (2008). A number of specific climate-related regional phenomena have been related by commenters (e.g., 3596.1) to solar variability, such as sea surface temperature, floods, droughts, monsoons, and North Atlantic drift ice.
We have reviewed the comments and the literature submitted and have determined that changes in solar irradiance are not a sufficient explanation for recent climate change. The contention that direct solar variability can explain recent warming is not supported by the bulk of the scientific literature. As the TSD notes, the IPCC Fourth Assessment Report estimates that changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12 (+0.06 to +0.30) W/m2, or approximately 5% of the combined radiative forcing due to the cumulative (1750–2005) increase in atmospheric concentrations of CO2, CH4, and N2O (2.30 W/m2 with an uncertainty range of +2.07 to +2.53 W/m2). The natural 11-year cycle of solar irradiance has a magnitude of less than 2 W/m2 at the distance of the Earth—which, once corrected for albedo and distribution over the surface area of the planet, is a magnitude of less than 0.35 W/m2.
In addition, Karl et al. (2009) state that “if most of the observed temperature change had been due to an increase in solar output rather than an increase in GHGs, Earth’s atmosphere would have warmed throughout its full vertical extent, including the stratosphere. The observed pattern of atmospheric temperature changes, with its pronounced cooling in the stratosphere, is therefore inconsistent with the hypothesis that changes in the Sun can explain the warming of recent decades. Moreover, direct satellite measurements of solar output show slight decreases during the recent period of warming.” A number of other recent studies also show results that contrast with the interpretation that solar variability is driving recent warming. Both Lockwood and Fröhlich (2008) and Lean and Rind (2009) show that the solar contribution to warming in recent decades has been small or negative, consistent with the IPCC attribution of most of the warming in recent decades to anthropogenic GHGs.
The attribution of components historical climate change to solar activity involves a number of issues. The first is the actual reconstruction of historical solar activity: even for the last three decades there is some controversy, as is evident in the differences between Scafetta and Willson (2009), which uses a total solar irradiance composite from the Active Cavity Radiometer Irradiance Monitor (ACRIM) analysis of satellite data, and Lockwood and Fröhlich (2008), which uses a composite based on the Physikalisch- Meteorologisches Observatorium Davos (PMOD) analysis of satellite data. These two composites don’t even agree on the sign of the solar irradiance trend over this time period.
Lockwood and Frolich analyze both datasets and find that the ACRIM dataset is inconsistent with methods of historical reconstructions that have shown correlations between historical solar activity and climate. Krivova, Solanki, and Wenzler (2009) also find no evidence of an increase in total solar irradiance (TSI) from 1986 and 1996 using an analysis based on magnetograms. Scafetta and Willson, on the other hand, claim that the PMOD approach requires a correction of the data from the earth radiation budget (ERB) system on the NIMBUS7 satellite, and this correction has been rejected by one of the scientists on the NIMBUS team (D.V. Hoyt, personal communication to Scafetta, 2008). Neither dataset shows an increase of solar irradiance between the minima of 1986 and 2008, which would be required in order to explain warming over that period.
Therefore, reconstructions of recent solar variability do not agree, but in one case show no trend, and in the case of the Lockwood and Fröhlich reconstruction the solar contribution during this period would have been a cooling, not warming, influence.
The second issue is that in order for solar irradiance to be a major driver of recent warming, there must be an amplification effect that is active for solar irradiance that is not active for forcing due to GHGs. Studies such as Scafetta (2009) often rely on a significantly different factor for solar irradiance than is used for GHG climate sensitivity. Additionally, the Scafetta study relies on a “slow lag” solar response and the timescale chosen has itself been the subject of dispute. The climate sensitivity for this slow lag response used by Scafetta is 0.46° K/Wm-2. Note that this is compared to the total solar irradiance: therefore, the effective sensitivity to the average solar irradiance according to Scafetta would be (4*0.46)/0.7 = 2.6° K/Wm-2. This can be compared to a climate sensitivity range of 2 to 4.5, or about 0.5° K/Wm-2 to 1.2° K/Wm-2. Additionally, Scafetta claims that solar variability accounts for most of the recent warming and that GHG sensitivity is on the low end of the range: this means that Scafetta is effectively claiming that sensitivity to solar variability is on the order of five times the sensitivity to forcing by GHGs, without a good mechanism to explain this extreme difference. Although it is not impossible that there are differences between solar and GHG induced changes, the evidence for an amplification of the magnitude needed to explain recent warming is weak. For example, while Meehl et al. (2009) find an amplification of the solar cycle variability is needed to explain certain patterns of tropical Pacific climate response, the authors note: “This response also cannot be used to explain recent global warming because the 11-year solar cycle has not shown a measurable trend over the past 30 years.”
Moreover, the sensitivity needed is nowhere near as large as the Scafetta sensitivity, and the behavior explained is geographically localized, which is different from a global increase in sensitivity. Therefore, the evidence for an amplification of the magnitude needed to explain recent warming is weak.
Some other authors also show some correlations between solar variability and regional trends. Eichler et al. (2009) find a strong correlation between solar activity (as reconstructed by carbon-14 and beryllium-10 proxies) and temperatures in the Siberian Altai region. However, the authors note that “underlying physical processes are still not yet understood” in terms of amplifying a weak solar signal (in terms of radiative forcing) in order to see larger effects, and also that “[i]n large spatial scale hemispheric or global reconstructions the solar signal may therefore even vanish” because the “main effect of solar forcing is presumably on location, routes, and stability of atmospheric pressure systems, which all act on regional scales.” The conclusion of the Eichler work is that while solar activity was a main driver for temperature variations in the Altai region preindustrially, during the industrial period they found that only CO2 concentrations show a significant correlation with the temperature record. They did find agreement with the northern hemisphere (NH) temperature reconstruction of Scafetta and West (2007) in that they found that only up to approximately 50% of the observed global warming in the last 100 years can be explained by the sun. Note that this conclusion provides 50% as an upper limit to the explanatory power of solar variability, and this is for the full century. Therefore, for the last 50 years, this conclusion is still consistent with the IPCC (2007b) statement that “[m]ost of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
Therefore, to summarize: attempting to attribute late-20th century temperature change mainly to solar variability requires choosing a specific solar dataset, assuming a simplified model with different “fast” and “slow lag” responses based on timescales from a controversial paper, and assuming that the climate system is several times more sensitive to changes in solar irradiance (or other, non-radiative changes in the sun) than it is to changes in GHG forcing. All three of these assumptions are counter to the conclusions of the IPCC and CCSP assessments and not viewed as established conclusions in the literature. While science in this area will continue to evolve, our review did not uncover any compelling alternatives to the science represented in the assessment literature, and summarized in the TSD.