In his Oct. 30 letter, "Appearances can be deceiving," Arlo A. Brown suggests that the discovery of ancient wetland-type moss in regions now covered by glaciers indicates that global warming is not anthropogenic.

This is a false deduction. The fact that the Earth was once warmer than it is now does not prove that the current rise in temperature has the same causes as prehistoric rises in temperature. Carbon dioxide is a greenhouse gas; therefore, if we put more carbon dioxide into the atmosphere, the greenhouse effect will increase. To disbelieve this is to ignore the basic laws of physics.

It is probable that global warming has both natural and man-made causes. Since we can do nothing about the former, we need to redouble our efforts to tackle the latter.