Detection of the climate response to the solar cycle
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Optimal space-time signal processing is used to infer the amplitude of the large-scale, near-surface temperature response to the "11 year" solar cycle. The estimation procedure involves the following steps. 1) By correlating 14 years of monthly total solar irradiance measurements made by the Nimbus-7 satellite and monthly Wolf sunspot numbers, a monthly solar irradiance forcing function is constructed for the years 1894-1993. 2) Using this forcing function, a space-time waveform of the climate response for the same 100 years is generated from an energy balance climate model. 3) The space-time covariance statistics in the frequency band (16.67 yr)-1-(7.14 yr)-1 are calculated using control runs from two different coupled ocean-atmosphere global climate models. 4) Using the results from the last two steps, an optimal filter is constructed and applied to observed surface temperature data for the years 1894-1993. 5) An estimate of the ratio of the real climate response, contained in the observed data, and the model generated climate response from step 2 is given, as well as an estimate of its uncertainty. A number of consistency checks are presented, such as using data from different regions of the earth to calculate this ratio and using data lagged up to 5 yr. Our findings allow us to reject the null hypothesis, that no response to the solar cycle is present in the data, at a confidence level of 97.4%.