That the planet has warmed is a fact hardly anyone disputes — it has been measured with instruments on land and sea and in space. That humans have contributed to the warming through industrial activities is a theory supported by multiple scientific organizations, including the National Academy of Sciences, the American Association for the Advancement of Science, the U.S. Global Change Research Program, the National Oceanic and Atmospheric Administration, and NASA.
“Ultimately, we go back to physics. If you burn fossil fuel, you make CO2,” said Richard B. Alley, a geophysicist at Penn State University and author of “Earth: The Operator’s Manual.” “You can do this with bookkeeping. How much did we burn? How much CO2 does that make? Where is it? There it is.”
One of the twists in the debate is that the data that show the planet warming over the past century — data that skeptics often deride as untrustworthy — also show that the rate of warming has slowed in the past decade or so. “The warming has slowed since 1998,” said Tom Peterson, chief scientist for NOAA’s National Climatic Data Center.