Though scientists are adamant that human beings are causing climate change in general, they’re not certain that every change in climate is down to man-made greenhouse emissions. Now, though, researchers have taken a stab at distinguishing between climate events that are anthropogenic, and those that are more likely to be natural occurrences.
Their conclusion: about two-thirds of the 100 most serious changes listed in the most recent Intergovernmental Panel on Climate Change report–including melting glaciers in Europe and wildfires in Alaska–are the result of human activity.
Using sophisticated computer modeling, Gerrit Hansen from the Potsdam Institute for Climate Impact Research, in Germany, and Dáithí Stone from the Lawrence Berkeley National Laboratory, in California, analyzed 40 years of climate data between 1971-2010. For each climate change, they assessed the adequacy of available data, the relative resolution of their models, and then compared results with and without human emissions factored in. From that, they were able to grade each change on a confidence scale from “no confidence” to “very high” confidence.
See the map Hansen and Stone produced to illustrate the findings. You can see that melting glaciers in North America and South America, wild fires in southern Europe, and droughts in the South West U.S. are all “highly likely” caused by human activity. But there’s a low confidence surrounding the man-made nature of coastal erosion in northern Siberia, for example.
“Previous analyses linking observed impacts to climate change have been generic in nature, addressing whether there is an influence of human-related warming on impacts globally, without an inference to individual impacts,” says Hansen, in a press release. “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.”
The study is published in the journal of Nature Climate Change. Of course, a lack of confidence doesn’t mean humans aren’t to blame for an event. It just means the data isn’t there to support it. Over time, as they gather more data, it’s likely scientists will have a firmer grip of the relationship between what’s happening on the ground and what’s happening in the atmosphere.