Al Gore's new movie, An Inconvenient Truth is a powerful visual rendering of a variety of natural events that are claimed to prove that manmade global warming is upon us and rapidly getting worse. We all know that a well-done film can have a powerful emotional impact. This is a well-done film.

While you might have been under the mistaken impression that such things have happened before the modern global warming era, Gore associates severe weather events such as floods, droughts, and storms (for instance Hurricane Katrina) with mankind's use of fossil fuels. Other than the possibility that current tropical warmth could be making hurricanes a little stronger (which is debatable), there is little or no scientific evidence that global warming has caused more severe weather.

One of the more dramatic themes of the movie is the melting of the ice sheets -- Greenland and Antarctica -- and their contribution to sea level rise. Views in the movie of ice crashing into the ocean as it calves from glaciers will no doubt have the intended effect -- to scare people into believing that global warming is serious, ergo we must do something about it. Many people will come away from the film thinking, "global warming is obviously real because all that ice is falling into the ocean." But glaciers are continuously flowing entities. As long as snow continues to fall on them, they slowly spread outward like thick molasses, dramatically dumping their frozen cargo into the ocean.

Global warming is not required for this ice calving process to occur, any more than it is required for a river to flow toward the sea. It is part of the Earth's natural hydrologic cycle, whereby the ocean continuously evaporates water into the atmosphere, which is later returned back to the ocean again, either in liquid or frozen form.

Yet these movie visuals will appeal to our emotions, confusing our more rational thought processes. The intent is to associate in our minds these entirely natural processes with manmade pollution. Al Gore recently told Grist Magazine, "I believe it is appropriate to have an over-representation of factual presentations on how dangerous it (global warming) is..." I am reminded of Gore's angry, bellowing accusation of President Bush with respect to Iraq: "He played on our fears!" That is exactly what Mr. Gore is doing in this movie.

If we move beyond the emotional appeals, we find that the current science on the subject of whether the ice sheets are losing more ice than they are gaining from snowfall (the real issue), is somewhat mixed. Currently, the consensus of opinion is that the Greenland ice sheet has been experiencing a net loss in recent years, while the same conclusion about Antarctica is much less certain.

But a note of caution: our ability to actually measure this net loss is quite new and -- like all global warming related measurements -- subject to large uncertainties in both the measurements themselves and whether the measured changes can be attributed to mankind's activities. As an example, it has been fifteen years since John Christy and I started measuring global temperature trends from satellites. Yet we (and others) are still finding new corrections that need to be made to the data. The signals of global warming are so small compared to natural climate variability that it is usually difficult to measure them with any degree of certainty.

Another common mistake among scientists (and we never seem to learn our lesson on this one) is to infer some sort of long-term trend from an observed short-term change. If Greenland has lost ice in recent years, just the possibility that this could be part of a long term trend is sufficient to get lots of press claiming this conclusion as fact. But even if Greenland has been losing more ice than it has been gaining in recent years, this says nothing about whether the process is due to mankind.

But let's assume that the current period of global warmth is mostly due to mankind, and that this warming is indeed causing a net loss of ice from the ice sheets. What should we do about it? It's one thing to point out a problem, but another thing entirely to do something substantial to fix the problem. Therein lies the potential danger of movies such as "An Inconvenient Truth."

Al Gore's movie is reminiscent of Rachael Carson's 1962 book Silent Spring, which helped give birth to the modern environmental movement. The book provided scientific evidence, conveyed through eloquent prose, that our indiscriminant use of the pesticide DDT was having some negative environmental, and possibly human, side effects.

Unfortunately, it is often the case that policy decisions based upon emotion rather than facts can lead to unintended, negative consequences. As an eventual result of Carson's book, many countries of the world severely restricted the use of DDT. But without access to even very small amounts, some African countries now have a combined death toll of about 1-1.5 million people per year from malaria. Many millions more are permanently disabled.

Only relatively recently has the pendulum begun to swing with regard to DDT, and Africans are increasingly employing small amounts of the substance (despite international restrictions) and experiencing dramatic reductions in the incidence of malaria.

The dangers of misguided global warming policies resulting from activist movies like An Inconvenient Truth could have similar or worse effects on the world's poor than what has resulted from restrictions on DDT. While responsible DDT use has many more human benefits than dangers -- with virtually no negative effects on the environment -- the effect of DDT restrictions on our daily lives pales in comparison with the negative consequences that restricting access to affordable energy would bring.

Human health and longevity are directly related to wealth generation, which in turn requires abundant, inexpensive energy. Humans must alter their environment in order to thrive, and this takes energy.

I fear that Al Gore is becoming a modern day Rachel Carson. The New York Times book critic Michiko Kakutani favorably compared the book and film versions of An Inconvenient Truth with Silent Spring. The New Republic's Frank Foer called it a "cinematic Silent Spring." Emotional appeals will effect public policy changes, which will have widespread, unintended negative consequences.

On a positive note, Gore's movie provides an opportunity to spur public debate on the global warming issue. It is critically important for us to become better informed about global warming, how much of it is natural versus manmade, and especially what might be done from a policy standpoint about the manmade part.

We must be wary of letting emotional appeals govern policy decisions that have real world, life-or-death consequences, especially in the poorer countries of the world.

Dr. Roy Spencer is a principal research scientist for the University of Alabama in Huntsville and the U.S. Science Team Leader for the Advanced Microwave Scanning Radiometer (AMSR-E) on NASA's Aqua satellite.