University of Chicago law professor Cass Sunstein thinks so. He makes the case in today's Washington Post:

For the United States, the cost of the Iraq war will soon exceed the anticipated cost of the Kyoto Protocol, the international agreement designed to control greenhouse gases. For both, the cost is somewhere in excess of $300 billion. These numbers show that the Bush administration was unrealistically optimistic in its prewar prediction that the total cost would be about $50 billion. And the same numbers raise questions about the Bush administration's claim that the cost of the Kyoto Protocol would be prohibitive, causing (in President Bush's own words) "serious harm to the U.S. economy."

The incidence of the two costs is not equivalent. Iraq is funded from general tax revenues while any Kyoto-style policy would be likely to increase energy prices. For some, these distributional differences may matter. Moreover, Kyoto represents but a tiny downpayment toward an emission stabilization policy. (Of course, one might say the same about Iraq and its relation to the overall war on terror.)

Meanwhile, on the other side of the Atlantic, the EU's fledgling carbon trading market has experienced tremendous price volatility, due in part to an over-allocation of emission credits. Europe has yet to impose real limits on its own carbon emissions, and questions remain how effectively its carbon trading market will reduce the cost of curbing greenhouse gas emissions.

[NOTE: Post edited to fix typos.]

UPDATE: As some commenters have noted, the comparison of the costs of Iraq and Kyoto is misleading — but this is not necesarily a point in Professor Sunstein's favor. As a former colleague points out,

Wartime spending is primarily a wealth transfer from taxpayers to soldiers, defense contractors, and Pentagon bureaucrats. Although defense spending may reduce GDP (because taxes divert resources from more highly valued uses), the GPD loss is usually less than the amount of wealth transferred, which after all boosts investment, employment, and profits in defense-related industries. The Energy Information Administration estimated Kyoto would reduce U.S. GDP by $100-$400 billion in 2010. Is Sunstein prepared to assert that the Iraq War reduces U.S. GPD by $100 billion annually? I doubt it, because the economy is growing robustly. Sunstein not only fails to compare apples (GDP losses) to apples, he also fails to compare oranges (wealth transfers) to oranges.

I am also told that Sunstein wrote in his 2005 book Laws of Fear that "the Kyoto Protocol appears to impose costs in excess of benefits - and this is so even if improbable catastrophic risks are taken into account." (P. 171)