The sensitivity of global temperature to changes in the Earth's radiation balance, known as "climate sensitivity," is a key parameter for understanding both natural climate change in Earth's past as well as future climate change resulting from greenhouse gas emissions.
An international team of scientists has developed a new approach for analyzing and synthesizing data from a variety of sources based on a detailed assessment of published paleoclimate (past climate) studies. James Zachos, professor of Earth and planetary sciences at UC Santa Cruz, and former UCSC graduate student Matthew Huber (now at Purdue University) were involved in the project because of their expertise on the extreme greenhouse climates of the past.
"The basic idea was to develop a quantitative assessment of climate sensitivity to greenhouse gas forcing from the past 65 million years to compare with theoretical results from climate modeling studies," Zachos said. "One benefit of this approach is that the paleorecord includes all the feedbacks, fast and slow, which can influence sensitivity to greenhouse gas forcing."
Numerous estimates of climate sensitivity have been made from studies of past climates, but researchers have used inconsistent definitions and varying time periods of analysis, resulting in a wide range of reported values for climate sensitivity. In the new study, led by Eelco Rohling of the University of Southampton, scientists developed a new approach, using a more consistent definition of climate sensitivity in prehistoric times. Their results, published in the November 29 issue of Nature, will help improve comparison with long-term climate projections developed by the International Panel on Climate Change (IPCC).
When the scientists evaluated previously published estimates for climate sensitivity from a variety of geological episodes over the past 65 million years, they found that the estimates varied over a very wide range of values, with some very high values among them. High values would imply a very strong temperature response to a change in radiative forcing, for example, due to increased carbon dioxide in the atmosphere.
The team found that this wide range was almost entirely due to the fact that different researchers used different definitions. Rohling's team needed to develop a definition and terminology that, applied consistently to estimates from the past, make those estimates compatible with estimates from climate models as used by the IPCC.
"Consistent intercomparison is a top priority, because it is central to using past climate sensitivity estimates in assessing the credibility of future climate projections," Rohling said. "Once we had developed the framework and we had elaborated all the different assumptions and uncertainties as well, we applied it to climate reconstruction data from the last 65 million years. This caused a much narrower range of estimates, and this range was now defined in such a way that we could directly compare it with estimates in the IPCC assessment for their longer-term (several centuries) outlook."
The scientists found that climate sensitivity consistently has been about 2.2 to 4.8 degrees Celsius per doubling of carbon dioxide, which closely agrees with the IPCC estimates. Currently, atmospheric carbon dioxide levels are around 392 parts per million (increasing by about 2.5 ppm per year). Pre-industrial values were around 280 ppm, so that a doubling would therefore imply 560 ppm. At the current rate of emissions increase, this would achieve a doubling roughly around 50 to 70 years from now, but this depends strongly on future emissions.
Zachos noted that the IPCC forecasts for the next several centuries fall within the low range of sensitivities observed in the paleoclimate records. In other words, they are conservative forecasts. "If there is any deviation from those forecasts, it is likely to be toward higher temperatures," he said.
This research stems from a three-day workshop, held last year at the Royal Netherlands Academy of Arts and Sciences in Amsterdam, that brought together about 40 internationally renowned specialists in past and present climate studies.
