The debate over whether Earth's climate is changing and if humanity is responsible for that change closed in Paris on February 2. The Intergovernmental Panel on Climate Change (IPCC) released its summary for policymakers—a summation of the salient science in its much longer report due in May—in which it said that climate change is "unequivocal" and estimated the chances of humans being behind it at 90 percent, or "very likely."

New observations and new models contributed to this certainty, ranging from Antarctic ice cores to improved understanding of solar fluxes. Carbon dioxide (CO2) levels in the atmosphere—379 parts per million (ppm) in 2005—have reached levels not seen in the last 650,000 years, which have varied between 180 ppm and 300 ppm. The burning of fossil fuels is the main CO2 contributor to the atmosphere, followed by clearing land for agriculture.

A better understanding of the constituents of the atmosphere, as well as various natural processes on Earth and on the sun, has allowed scientists to sum the various forcings—factors that can increase or decrease the retention of heat on the planet—for the first time. [see graph above). While the sun is contributing an extra 0.12 W/m2 (watts per meter2)—and aerosols and cloud cover combine to cool Earth by 1.2 W/m2—CO2, methane and nitrous oxide, among other greenhouse gases, are warming the globe by 2.3 W/m2.

Such radiative forcing is easy to see in the recent climate record: 11 of the past dozen years "rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850)," the scientists note in the summary. The Arctic has led the way, its temperatures increasing at twice the global average, but worldwide days of extreme temperatures have become far more frequent over the past 50 years on every continent except Antarctica [see graph below].

The ocean absorbs most of the extra heat trapped by greenhouse gases—more than 80 percent—with temperatures rising up to 3,000 meters below the surface. Such warming provides stronger fuel for the furious storms called tropical cyclones that form over open waters (known in the Atlantic as hurricanes). It also causes the water itself to expand—so-called thermal expansion—contributing to a sea level rise of 0.17 meter (nearly seven inches) in the 20th century. And the rate of that expansion appears to be accelerating, averaging 3.1 millimeters (0.1 inch) per year between 1993 and 2003.

But, as the world's glaciers recede, melting ice is also contributing to the rise in sea levels. In fact, the last time the Arctic and Antarctic were three degrees Celsius (5.4 degrees Fahrenheit) warmer—roughly 125,000 years ago—sea levels rose by as much as six meters (20 feet) thanks to the melting of the Greenland ice sheet.

Such warming—three degrees C (5.4 degrees F)—is the scientists' best estimate of how much average temperatures would rise if greenhouse gas concentrations were allowed to double from preindustrial levels. Warming of 0.6 degree C (one degree F) is already guaranteed for this century, due to the CO2in place, which will remain there for centuries. That will have a host of impacts, from more severe droughts to more and stronger floods as a result of downpours.

A study of such impacts is now in the works by an assemblage of scientists under the auspices of the IPCC. That report is scheduled to be released in April, followed in May by another group's report on options for adaptation and mitigation. By November a complete synthesis of all three reports—totaling more than 1,600 pages—will be available. But the basic message is already clear: climate change is occurring and efforts must be made to minimize its dangerous consequences.