Global warming is "unequivocal." Sea levels are creeping up at the fastest rate in 2,000 years. Concentrations of CO2 in the atmosphere have reached "levels unprecedented in at least the last 800,000 years" (or before modern humans evolved). Most importantly "human influence on the climate system is clear" and "continued emissions of greenhouse gases will cause further warming." Those are some of the key messages in the "Summary for Policymakers" of the physical science of global warming from the Intergovernmental Panel on Climate Change released on September 27.
"The planet is red" in a global map of the change in average surface temperatures, noted Swiss climate scientist Thomas Stocker, co-chair of IPCC Working Group I responsible for this summary at a press conference. "The world is warming."
In the time since the 2007 version of this report, the human effect on the climate has grown more than 40 percent stronger, thanks to continued emissions of greenhouse gases and more precision in measurements, with carbon dioxide leading the charge. That molecule—released by the gigaton from human activities like fossil fuel burning and clearing forests—causes the bulk of global warming. The good news is that extreme global warming by century's end, anything above 3 degrees C or more, seems "extremely unlikely," in the words of the IPCC.
That's a fact likely to be seized on by those who wish to deny climate change. But, in some sense, this summary is aimed directly at countering some of the misinformation and misinterpretation around climate change. So the report notes that the current "pause" in new global average temperature records since 1998—a year that saw the second strongest El Nino on record and shattered warming records—does not reflect the long-term trend and may be explained by the oceans absorbing the majority of the extra heat trapped by greenhouse gases as well as the cooling contributions of volcanic eruptions. The Medieval Warm Period was only a regional anomaly, not the kind of global warming seen now. After all, 1983-2012 appears to have been the warmest period in at least the last 1400 years and the last decade alone is the warmest on record.
And the list of impacts just grows longer. Ice all over the world is melting, particularly in the Arctic, a trend that will continue unabated. Ocean circulation looks set to change, with unpredictable effects, and the oceans will become more acidic as well. Almost all of the world's coastlines will be affected by sea level rise. And developed countries and emerging economies have burned through more than half of the fossil fuels possible to keep total concentrations of CO2 in the atmosphere at a level that gives the world a chance to keep global warming below 2 degrees Celsius. (Interestingly, the IPCC has shifted from talking about concentrations in the atmosphere, like 400 parts-per-million, to total carbon budget in gigatons. Since 1880, 531 gigatons have been emitted and emissions should not exceed 800 gigatons of C for a better than 50-50 chance at keeping global temperature rise below 2 degree C.) "We cannot emit more than 1000 billion tons of carbon," Stocker says, noting that the IPCC numbers on which such regional and global climate projections are made will be available to anyone. "The higher the cumulative carbon emissions are, the warmer it gets."
Or, as his co-chair climate scientist Qin Dahe of the Chinese Academy of Sciences, put it via a translator in answer to a question about consumption in China: "If every Chinese has two or three cars like in the U.S., it will be a disaster for China as well as for the world." At present pace, the trillionth tonne would be emitted just before Christmas in 2040, according to calculations by Oxford physicist Myles Allen, and there's more than enough coal, oil and natural gas left in the ground to cook the climate. That's part of the reason why the IPCC included a final paragraph on geoengineering, or large-scale attempts to alter the climate by either blocking sunlight or removing CO2 (or other greenhouse gases) from the atmosphere. Of course, there's "limited evidence" and "insufficient knowledge" about whether such approaches could even work, particularly without their own side effects, the IPCC notes.
And keep this in mind, some of the greenhouse gases in the atmosphere today will persist for centuries. Even if CO2 emissions stopped tomorrow, climate change would continue. In other words, humanity is in the process of setting the Earth's thermostat. The world has already warmed by roughly 0.85 degree C since 1880 and further heat extremes are "virtually certain." So the question is: how much hotter can we stand? Or as United Nations Secretary General Ban-ki Moon put it in a video address to the IPCC press conference: "The heat is on. Now we must act."