The heat is on!

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 ClimateChange 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."
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 difficult.

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.

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. 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.

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.

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."