Most climatologists agree that world temperatures have, on average, risen somewhat over the past century. The Intergovernmental Panel on Climate Change (IPCC) estimates that temperatures have increased by 0. 6° C (or 1. 1° F). Yet there are some troubling problems even with that figure. And there is a lot of uncertainty about whether that increase was caused by humans' production of greenhouse gases.

A temperature measurement sounds like a straightforward, reliable piece of scientific data, but global temperature records are averages of huge collections of numbers obtained from all over the globe. For instance, in the United States, areas of the Southwest show an increase as high as 1. 5° to 2° F (or about 8° to 1. 1° C) per decade since 1966.

For the same period, other regions in the South and far northeastern states show more than 0. 5° F...