duce warm and cool regions that interchange over decade to century time scales ( 32 , 36 ), but whose effects largely cancel in hemispheric averages. Third, while there is reasonable evidence for greater climate variability during the Holocene than has been observed during the period where instrumental data are available ( 37 , 38 ), there is no evidence in the statistics that a major unidentified source of natural variation is present during the instrumental record. Such a source would have to mimic, perversely, either solar irradiance changes or the changes in atmospheric CO2 to cause the observed temperature changes and to be mistaken for them. Similarly, while mindful of the many caveats on data quality, spatial coverage, etc. given in ref. 1, the appearance of possible leap-year artifacts at a level below 10 mK in the residuals suggests that the data cannot be as untrustworthy as is occasionally implied. The residual temperature variation remaining once the known effects of precession, solar irradiance changes, and atmospheric CO2 concentration are removed bound unknown effects to about 200 mK peak-to peak in the hemispheric average series during the last century.

Consider the null hypothesis that the observed temperature fluctuations and atmospheric CO2 levels are independent: The probability that the hemispheric temperatures would fluctuate purely by chance in such a way to produce the observed coherences with CO2 is exceedingly low. Given that the records encompass more than a century, the probability is so low that one would not expect to see such an event by chance during the age of the earth. The probability of the observed coherence between atmospheric CO2 and changes in the timing of the seasons shown in figure 13 of ref. 2 without a causal connection is similarly low. Consequently one must strongly reject the hypothesis of independence between atmospheric CO2 and temperature. The alternative hypothesis, that increasing levels of atmospheric CO2 plus a slight change in solar irradiance are causally responsible for the observed changes in temperature, in contrast, results in test statistics that are ordinary in every way. Because major changes in climate as a response to human use of fossil fuels have been predicted for more than a century ( 39 , 40 ), their detection can hardly be considered surprising.

From examining the data records I conclude: Changes in solar irradiance explain perhaps one-quarter of the increase in temperature during the last century. The changes in atmospheric CO2 concentration resulting from human consumption of fossil fuels cause most of both the temperature increase and the changes in the seasonal cycle.

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