An international group of climate scientists said on Friday it is “extremely likely” humans are the dominant cause of the global warming that has occurred since the mid-20th century. The announcement comes from the summary of a 2,500-page climate report from the UN Intergovernmental Panel on Climate Change (IPCC), due to be formally released on Monday.
The report is the fifth the IPCC has produced since 1990 and offers the strongest statement yet on mankind’s alleged role in global warming. The authors claim they have 95 percent certainty that humans are the primary cause, rather than natural variations—up from the 90 percent certainty claimed in the previous IPCC report, published in 2007. They said climate models have improved and that the evidence for humanity’s role in climate change has grown since the previous report. The IPCC reports are significant because world governments use them to justify policies such as caps on industrial carbon emissions.
“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, a co-chair of the scientific working group that produced the report.
Thomas Stocker, the other co-chair, predicted people would see the effects of climate change on a local level in coming years: “Heat waves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions.”
While the IPCC scientists raised their certainty of humanity’s role, they backed down on some other predictions, such as how sensitive the climate would be to a doubling of atmospheric carbon dioxide. While the 2007 report had estimated such a doubling would increase global temperatures by 3.6 to 8.1 degrees Fahrenheit, the new report lowers the possible range to 2.7 to 8.1 degrees.
The authors also tried to explain why global warming seems to have come to a standstill for the past 15 years. An El Niño event in the Pacific Ocean, airborne ash from volcanic eruptions, and reduced energy from the sun all seem to have contributed to the warming slowdown, they said. They expect warming to accelerate again in the coming years—warming the oceans, reducing global glacier volume, and raising sea level as much as 32 inches by the end of the century. They said the oceans have absorbed over 90 percent of the warmth accumulated between 1971 and 2010.
Not all scientists agree with the assessments of the IPCC. Last week WORLD reported the release of Climate Change Reconsidered II: Physical Science, an alternative climate report from the Nongovernmental International Panel on Climate Change. Its authors say humanity’s contribution to warming is probably small and modern climate models are unable to predict the future global climate with the accuracy needed to make significant policy decisions.
Some climate models have produced wildly exaggerated predictions—such as one in 2007 that predicted the North Pole would be ice-free by 2013. Although the summer Arctic ice cover saw a record low last year, it bounced back by 50 percent this year. In spite of global warming, Antarctic ice cover has been increasing since at least 1979, pointing to the overall complexity of the climate system.