STOCKHOLM — Scientists can now say with extreme confidence that human activity is the dominant cause of the global warming observed since the 1950s, a new report by an international scientific group said yesterday.
Calling man-made warming “extremely likely,” the Intergovernmental Panel on Climate Change used the strongest words yet on the issue as it adopted its assessment on the state of the climate system.
In its previous assessment, in 2007, the U.N.-sponsored panel said it was “very likely” that global warming was man-made.
One of the most controversial subjects in the report was how to deal with a purported slowdown in warming in the past 15 years. Climate skeptics say this “hiatus” casts doubt on the scientific consensus on climate change.
Many governments had objections over how the issue was treated in earlier drafts and some had called for it to be deleted altogether.
In the end, the IPCC made only a brief mention of the issue in the summary for policymakers, stressing that short-term records are sensitive to natural variability and don’t in general reflect long-term trends.
“An old rule says that climate-relevant trends should not be calculated for periods less than around 30 years,” said Thomas Stocker, co-chair of the group that wrote the report.
Many scientists say the purported slowdown reflects random climate fluctuations and an unusually hot year, 1998, picked as a starting point for charting temperatures. Another leading hypothesis is that heat is settling temporarily in the oceans, but that wasn’t included in the summary.
Stocker said there wasn’t enough literature on “this emerging question.”
The IPCC said the evidence of climate change has grown thanks to more and better observations, a clearer understanding of the climate system and improved models to analyze the impact of rising temperatures.