Editor,

David Powell has offered an argument against a significant human contribution to the global warming that we are observing today. The claim that human activity is a major contributor to global warming is “nonsense and a hoax,” he says, because there were warming periods long before there was any human activity to cause them.

There is, however, a fatal flaw in Mr. Powell’s reasoning. You can see it by means of this little parable. Suppose that a neighbor, a certain Mr. Smith, had a house fire 20 years ago. Fortunately, fire fighters were able to put the fire out before too much damage had been done. They investigated the cause and determined that it was an electrical fire — some old wiring had shorted out and started the blaze. Unfortunately, 10 years later Mr. Smith suffered a second fire in his home. This time it proved to be caused by some solvent-soaked rags that he had stored away in his attached garage.

Notice what happened. There were two fires resulting from two different causes. The fact that the first fire was due to an electrical short proved to have no bearing on the explanation for the second fire. The same logic applies to the argument advanced by Mr. Powell. We have had warming (and cooling) periods in the past, explainable by natural causes. This does not mean that the warming we are experiencing now is due exclusively to those same natural causes.

Climate scientists are keenly aware of past warming periods. After all, they are the ones who have provided the evidence for them. It is a genuine scientific challenge to tease out how much of the climate change we see today is due to human activity and how much would have occurred anyway. The evidence for a major human contribution is very strong, but is certainly open to discussion. The argument that Mr. Powell advances, however, is logically flawed and should be discarded. It only confuses the discussions that really need to take place.

JOHN PALKA

Langley