Whenever emailed a 'letter' from a 'scientist', fact check. Go to snopes.com and the always helpful CMD SourceWatch (Real Climate is another good source). At this point, they've stopped on whether or not warming is occuring. Now they're focus is on making it all a natural process (humans have no impact or at most a negligible impact). This is not entirely without merit but it is NOT a reason to sit on our hands. The extent to which we are impacting the atmosphere through gas and particulate pollution is the only thing that is in question. Still, as Carl Wunsch of MIT pointed out rather elegantly

Thus at bottom, it is very difficult to separate human induced change from natural change, certainly not with the confidence we all seek. In these circumstances, it is essential to remember that the inability to prove human-induced change is not the same thing as a demonstration of its absence. It is probably true that most scientists would assign a very high probability that human-induced change is already strongly present in the climate system, while at the same time agreeing that clear-cut proof is not now available and may not be available for a long-time to come, if ever. Public policy has to be made on the basis of probabilities, not firm proof.