Scientists now believe it is “extremely likely” that human activity is the dominant cause of global warming, a long-term trend that is clear despite a recent plateau in the temperatures, an international climate panel said Friday last week.
The Intergovernmental Panel on Climate Change (IPCC) used its strongest language yet in a report on the causes of climate change, prompting calls for global action to control emissions of carbon dixoide and other greenhouse gases.
“If this isn’t an alarm bell, then I don’t know what one is. If ever there were an issue that demanded greater cooperation, partnership, and committed diplomacy, this is it,” US Secretary of State John Kerry said.
The IPCC, which has 195 member countries, adopted the report on Friday after all-night talks at a meeting in Stockholm.
In its previous assessment, in 2007, the UN-sponsored panel said it was “very likely” that global warming was due to human activity, particularly the carbon dioxide emissions resulting from the burning of coal, oil and gas.
The change means that scientists have moved from being 90 percent sure to 95 percent — about the same degree of certainty they have that smoking kills.
“At 90 percent it means there is a 10 percent probability that it’s not entirely correct,” said Chris Field, Carnegie Institution scientist who is a leader in the IPCC, but was not involved in the report released on Friday. “And now that’s 5 percent. So it’s a doubling of our confidence. That’s actually a consequential change in our level of understanding.”
One of the most controversial subjects in the report was how to deal with what appears to be a slowdown in warming if you look at temperature data for the past 15 years. Climate skeptics say this “hiatus” casts doubt on the scientific consensus on climate change, even though the past decade was the warmest on record. 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 temperature data reflect 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 was not included in the summary.
Stocker said there was not 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.
“Our assessment of the science finds that the atmosphere and ocean have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased,” said Qin Dahe, the other co-chair of the working group.
The full 2,000-page report was not released until later, but the summary for policymakers with the key findings was published on Friday. It contained few surprises as many of the findings had been leaked in advance.