Climate change: detection and attribution of trends from long-term geologic data

Two questions about climate change remain open: detection and attribution. Detection of change for a complex phenomenon like climate is far from simple, because of the necessary averaging and correcting of the various data sources. Given that change over some period is detected, how do we attribute that change to natural versus anthropogenic causes? Historical data may provide key insights in these critical areas. If historical climate data exhibit regularities such as cycles, then these cycles may be considered to be the “normal” behavior of the system, in which case deviations from the “normal” pattern would be evidence for anthropogenic effects on climate. This study uses this approach to examine the global warming question. Two 3000-year temperature series with minimal dating error were analyzed. A total of seven time-series models were fit to the two temperature series and to an average of the two series. None of these models used 20th Century data. In all cases, a good to excellent fit was obtained. Of the seven models, six show a warming trend over the 20th Century similar in timing and magnitude to the Northern Hemisphere instrumental series. One of the models passes right through the 20th Century data. These results suggest that 20th Century warming trends are plausibly a continuation of past climate patterns. Results are not precise enough to solve the attribution problem by partitioning warming into natural versus human-induced components. However, anywhere from a major portion to all of the warming of the 20th Century could plausibly result from natural causes according to these results. Six of the models project a cooling trend (in the absence of other forcings) over the next 200 years of 0.2–1.4 °C.