The Intergovernmental Panel on Climate Change (IPCC) sent a strongly worded message to policymakers around the world early today: The new science of the past 6 years has only reinforced the already-confident conclusions of the 2007 IPCC assessment report. The world is warming, humans are behind most of the warming, and continued spewing of greenhouse gases would warm the world to dangerous levels by as early as midcentury, the report finds.
Climate contrarians seem to have scored no points with the panel, leaving climate science still squarely behind curbing greenhouse gas emissions if the most serious consequences of global warming are to be avoided. As Secretary-General Ban Ki-moon of the United Nations, a sponsor of the IPCC, put it at this morning’s press conference: “The heat is on; we must act.”
The IPCC’s Working Group I on the physical science of climate change found no evidence that greenhouse warming has stopped, as contrarians would have it. During the past decade, the global temperature may have “paused,” most likely due to natural climate fluctuations, the panel finds. But a single straight line still neatly fits the jagged upward trend of temperature of the past 40 to 50 years. The decade of 2002 to 2012 was the hottest decade ever recorded despite ocean cooling in the tropical Pacific Ocean. And the warming of the upper 2 kilometers of the world ocean—a huge heat sink relative to the atmosphere—continued apace through the 2000s. So “warming of the climate system is unequivocal,” the report concludes.
From all the signs, the panel concludes that it is “extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” That statement is strengthened from the “likely” characterization in the 2001 report and “very likely” in the 2007 report.
Given those findings and the rest of the improved understanding of the climate system, the IPCC projects that if carbon dioxide gas emissions—the primary cause of warming—continue to grow at the recent rate, the world would warm 2oC above 19th-century levels by the middle of this century. Two degrees of warming is a widely though informally recognized “danger level” above which society would suffer serious consequences. Only draconian emission reductions would avoid the danger level by 2100. And only such measures would avoid continued warming beyond 2100, the report says.
Of course, a strengthening greenhouse would drive more than warming. A continuation of high emissions that have already shrunk summer sea ice on the Arctic Ocean would eliminate it by midcentury, according to the report. Even draconian emission controls would only limit sea level rise to a serious 40-centimeter rise by 2100. Business-as-usual emissions would drive sea level up by a disastrous 80 centimeters, though the panel rejected suggestions of a truly catastrophic rise.
Extremes of weather and climate would also increase. The panel finds that humans have already contributed to an increase in the frequency and duration of heat waves and an intensification of heavy precipitation events. Further warming will not only “very likely” drive further such changes but also likely intensify droughts and tropical cyclones by late in the century, at least in the western North Pacific and North Atlantic.
The message for climate negotiators gathering in Warsaw in November was clear enough. Climate change “threatens our planet, our only hope,” said Thomas Stocker, co-chair of Working Group I. And that means, according to IPCC Chair Rajendra Pachauri, that “there is a need for us to reduce emissions of greenhouse gases if we really want to stabilize climate.”