Solar EUV Irradiance Variability Reflected in the Terrestrial Dayglow

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to calculate the aerosol optical depth because of the lack of direct quantitative space-based observations [Thomason and Pitts, 2009]. Solar irradiance, S, is estimated as the competing effects of sunspots and facular, identified in observations made by space-based radiometers [Lean et al., 2005]. The anthropogenic influence, A, is the net effect of eight different components, including greenhouse gases, land use and snow albedo changes, and (admittedly uncertain) tropospheric aerosols [Hansen et al., 2007]. The combination of natural and anthropogenic influences (at appropriate lags) in the empirical model captures 76% of the variance in the monthly global surface temperature record. Figure 1 illustrates that this statistical model tracks closely the observed global surface temperatures from 1980 to 2008, including the lack of overall warming during the past decade. The individual contributions to the net global surface temperatures of the natural and anthropogenic influences are also shown in Figure 1, including 0.2 o C warming during the 1997-98 ENSO, cooling approaching 0.3 o C in 1992 following Pinatubo, and ∼0.1 o C warming near peak solar cycle activity. To test our empirical approach, we determine the model parameters using observations from 1970 to 1999, and compare the observed surface temperature change with our empirical determination averaged over the subsequent five-year period from 2001 to 2005 (for direct comparison with Hansen et al., 2006, Figure 1B). Relative to the base period, observed global surface temperature increased 0.56±0.03 o C compared with 0.53±0.03 o C projected by our empirical model, where the uncertainties are obtained by combining the uncertainties of the means. Figure 2 shows that the observed and model-projected regional pattern change for 2001-2005 (relative to 1951-1980) and the (area-weighted) zonal surface temperatures are very similar. Note that our model is limited to those latitudes (approximately 60 o S to 70 o N, Figure 2) where actual observations are available for 50% of all months over the 30-year model regression period. Using global and regional surface temperature responses to the four individual influences parameterized by regression against the observations from 1980 to 2008, we forecast change from 2009 to 2030 by adopting the best estimate of how each influence will change in the future. The anthropogenic forcing in the past 40 years is well represented by a linear trend that we extrapolate into the future, as 4
shown in Figure 1. We assume that future solar irradiance cycles replicate cycle 23, with cycle 24 commencing at the beginning of 2009. Although solar activity (as indicted by sunspot numbers) was less in cycle 23 than in cycles 21 and 22, the total irradiance amplitude (near 0.1%) is similar in the three past cycles since it is the net effect of sunspot darkening and facular brightening, both of which are altered by solar activity. Since ENSO fluctuations and volcanic eruptions are not predictable on decadal time scales, we estimate their maximum likely future impact with a scenario that includes a Pinatubo-like eruption with peak impact in 2014 and a super ENSO with maximum impact in 2019, mimicking a similar sequence that occurred from 1992 to 1997 (Figure 1). 3. Results With the solar and anthropogenic scenarios in Figure 1, our empirical model predicts that global surface temperatures will increase at an average rate of 0.17±0.03 o C per decade in the next two decades. The uncertainty given in our prediction is the standard deviation of forecasts made with statistical models of three different epochs, the current epoch 1980-2008, our test epoch 1970-1999 and the historical record from 1890-2008. Within the uncertainties, the average warming rate in the next two decades is consistent with the 0.2 o C per decade warming forecast by IPCC [2007], also shown in Figure 1. The warming trend is opposite to recent projections of Keenlyside et al. [2008], who forecast an absence of warming in the next decade based on weakening of the Atlantic Ocean meridional overturning circulation (a change not explicitly included in our empirical model). The predicted warming rate will not, however, be constant on sub-decadal time scales over the next two decades. As both the anthropogenic influence continues and solar irradiance increases from the onset to the maximum of cycle 24, global surface temperature is projected to increase 0.15±0.03 o C in the five years from 2009 to 2014, with global annual temperatures 0.7 o C above the base period by 2014. However, our estimated annual temperate increase of 0.19±0.03 o C from 2004 to 2014 (Figure 1) is less than the 0.3 o C warming that Smith et al. [2007] predict over the same interval. From 2014 to 2019, global annual surface temperatures increase only minimally (0.03±0.01 o C), because declining solar irradiance is expected to cancel much of the anthropogenic warming. This lack of overall warming is analogous to the 5
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Page 9 and 10: Thomason, L. W., and M. C. Pitts (2

Solar EUV Irradiance Variability Reflected in the Terrestrial Dayglow

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