Surface air temperatures have been higher, on average, in the past 50 years than they were in the 50 years before that. Further complicating the picture, satellite measurements show that temperatures at higher altitudes have been falling since 1979. Second, even if there is a warming trend, we do not know if humans are causing it. Many scientists believe that the billions of tons of carbon dioxide, methane and other gases that humans are pumping into the atmosphere must be affecting the climate. Those gases, they say, strengthen the greenhouse effect, the process by which naturally occurring atmospheric gases trap solar energy radiated back upward from the Earth.
Many scientists disavow even that cautious assertion.
That is not what Mr. Schwanen argues. Though we don't know for sure whether global warming is occurring or whether humans are playing a part, we do know that the level of carbon dioxide in the atmosphere has risen from an estimated 280 per parts million per volume in 1800 to about 360 ppmv today. It is only sensible to worry about that. What is not sensible is to do something hasty. Environmentalists say that unless governments act now to cut emission levels dramatically, the disaster of global warming will overtake us.