Is human activity the cause of global warming?

Climate panel issues strong warning

Associated Press

STOCKHOLM

Scientists now believe it’s “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.

The Intergovernmental Panel on Climate Change used its strongest language yet in a report on the causes of climate change, prompting calls for global action to control emissions of CO2 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,” said U.S. Secretary of State John Kerry.

The IPCC, which has 195 member countries, adopted the report Friday after all-night talks at a meeting in Stockholm.

In its previous assessment, in 2007, the U.N.-sponsored panel said it was “very likely” that global warming was due to human activity, particularly the CO2 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 wasn’t involved in the report released 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 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.

The full 2,000-page report isn’t going to be released until Monday, but the summary for policymakers with the key findings was published Friday. It contained few surprises as many of the findings had been leaked in advance.