Despite serious scientific disagreement about the extent, causes and impact of global warming, climate alarmists have a pat answer: It doesn't matter. We know enough. We need to stop living the way we do in the United States.

"There will be deep uncertainty in the climatic future for a long time," one scientist told The New York Times. "But if you wait until it's diminished to some threshold that you assign and then learn that the problem is severe, it may be too late to do anything about it."

Fair enough. But have you ever noticed how quickly the tune changes when the issue is missile defense, rather than global warming?

Then, it seems, we never can have enough evidence. Despite eight successful intercepts since 1999 (versus two highly publicized flops), we're told missile defense won't work, so why waste the money?

The accusation can be turned on its head, of course. "You say the scientific evidence isn't strong enough to go forward with Kyoto," one reporter said to President Bush during a recent press conference in Europe. "So then how do you justify your missile-defense plan when there is even less scientific evidence that that will work?"

Good question -- but the questioner was flat wrong. We know with virtual certainty that we can knock missiles out of the air. The Navy alone has used a "theater" missile-defense system since the 1970s to protect its ships and planes from cruise missiles. The core technology behind it and other systems now in development (such as the Patriot PAC-3 missile, a successor to the one used against Iraqi Scud missiles during the Gulf War) has long been proven.

The challenge before us is not to determine if the technology can work, but to create a system that can protect as wide an area as possible at the lowest possible cost. As Frederick Seitz, a former president of the National Academy of Sciences, wrote in The Wall Street Journal, it's "an engineering challenge, not a scientific one."  

Pentagon officials already have confirmed that the Navy's sea-based system could be upgraded to provide a near-global defense, giving us the ability to position our ships wherever there is trouble and destroy threatening missiles before they hit their targets -- wherever those targets are.

No one claims this can happen overnight. We can't just snap our fingers and make it work. But to dismiss our capabilities as science fiction is worse than short-sighted. It's dishonest and -- considering the rapid spread of missile technology among "rogue" nations such as North Korea and Iran -- dangerous.

Contrast that with the widely reported connection between human activity and global warming, touted as high gospel by many of the same suspects.

Fact is, doubts about this connection aren't confined to some fringe element. You can find them sprinkled throughout a recent report from the National Academy of Sciences, which admits to "considerable uncertainty" on how much human activity affects warming patterns and states quite frankly that many climate changes are "not well understood."

And yet President Bush is supposed to apologize for not wanting to sign the Kyoto Protocol, a treaty that would force us to change our very way of life.

Environmental alarmists are loathe to admit it, but we're a lot closer to creating the perfect missile defense than we are to creating the perfect climate model. And that's a scientific fact.

Edwin Feulner is president of The Heritage Foundation (www.heritage.org), a Washington-based public policy research institute.