Tracing observed climate impacts to greenhouse gas emissions

01/28/2016 - Roughly two-thirds of observed climate change impacts related to atmospheric and ocean temperature over the past 40 years can be confidently attributed to human-generated greenhouse gas emissions, an international team of scientists found. For the impacts observed not just on regional but on continental scales, even three quarters are mainly due to our burning of burning fossil fuels. Evidence connecting changes in precipitation and their respective impacts to human influence is still weak, but is expected to grow.
Tracing observed climate impacts to greenhouse gas emissions
Map of observed climate impacts and the attribution to human-made greenhouse-gas emissions. Figure taken from Hansen/Stone (2015)

“Previous analyses linking observed impacts to climate change have been general in nature, addressing whether there is an influence of human-related warming on impacts globally, without an inference to individual impacts,” says Gerrit Hansen of the Potsdam Institute for Climate Impact Research, lead-author of the study published in Nature Climate Change. “Our analysis is the first to bridge these gaps for a large range of impacts, by assessing the role of human-related emissions in each impact individually, including impacts related to trends in precipitation and sea ice.” To this end, the scientists developed and applied a novel methodology.

Using a sophisticated algorithm, the study essentially required satisfaction of three distinct types of tests.  First, the algorithm assessed the adequacy of the available climate data—the so-called observational record—related to the particular regional impact over the 40-year period.  Was the data sufficient to provide a basis for understanding what actually had been taking place?  Next, the algorithm determined whether the climate models the researchers used provided sufficient resolution or detail concerning regional climate so as to be considered an appropriate source of information.  Finally, the researchers examined collections of model simulations with and without human emissions factored in to understand to what degree human emissions were responsible for a given impact, by comparing these simulations against observed trends.

The result of each test of data set quality or of observation-simulation agreement was expressed as a numerical score, and then these scores were merged into an overall measure of confidence in the hypothesis that human-generated emissions have affected the regional climate. “There are many ways we could combine the scores", says co-author Dáithí Stone of Lawrence Berkeley National Laboratory (Berkeley Lab), "but we found that it didn't matter which plausible method we used—the results all pointed to the same conclusions." According to Stone, cases where the link between human-generated greenhouse gas emissions and local warming trends were weak were often due to the fact that the climate observational record was insufficient in those regions to build a clear picture about what has been happening over the past several decades.

“In the future, human-made climate change – if unabated – will clearly cause more and more impacts,” says Hansen. “So the period we studied to pin down observed climate impacts is nothing is just a beginning.”

Article: Hansen, G., Stone, D. (2015): Assessing the observed impact of anthropogenic climate change. Nature Climate Change [DOI:10.1038/nclimate2896]

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