Talk about bad timing. Last month, environmental activists launched a well-funded new attack on Republican "climate change deniers" in hopes of making global warming a big issue in 2014. But as the campaign gets underway, a new report from the world's leading climate scientists could leave environmentalists on the defensive, and the "deniers" more confident and assertive.

"HOLDING CLIMATE CHANGE DENIERS ACCOUNTABLE" read the headline of a League of Conservation Voters press release announcing a $2 million barrage of ads aimed at Republican Sen. Ron Johnson, as well as GOP Reps. Mike Coffman, Dan Benishek and Rodney Davis. "We're changing the terms of the climate change debate," said an LCV spokesman. "It's no longer acceptable to be a member of Congress and deny basic science."

Organizing for Action, the permanent arm of the Obama campaign, joined in, staging events and running an ad — "CALL OUT THE CLIMATE CHANGE DENIERS" — targeting House Speaker John Boehner and Sen. Marco Rubio, among others.

The goal is to place opposition to the global warming agenda — heavy environmental regulation, a cap-and-trade or carbon tax program, massive "green energy" expenditures, huge international wealth transfers — outside the realm of polite discussion. But the discussion is about to change.

On Sept. 27, the United Nations Intergovernmental Panel on Climate Change will release its fifth report on global warming. Earlier IPCC assessments — the most recent was in 2007 — were the foundation for reams of alarmist reporting. For example, after a 2009 update, the Washington Post ran a story headlined "New Analysis Brings Dire Forecast," reporting that a predicted 6.3-degree Fahrenheit increase in world temperatures "is nearly double what scientists and world policymakers have identified as the upper limit of warming the world can afford in order to avert catastrophic climate change."

That was then. Now, the new IPCC document will "dial back the alarm," in the words of a Wall Street Journal preview. According to the Journal, the report will state that "the temperature rise we can expect as a result of man-made emissions of carbon dioxide is lower than the IPCC thought in 2007." The computer forecasts used to produce those forecasts, it turns out, were wrong.

The effect could be enormous. If scientists now predict that the earth will warm less, and less quickly, than earlier thought — and also concede that the planet has not warmed at all in the last decade or so — the position of the environmental activists, and groups like Organizing for Action, will be significantly weaker. They'll have a harder time arguing for drastic and immediate action.

The downgrading of the warming threat, writes the Journal, "points to the very real possibility that, over the next several generations, the overall effect of climate change will be positive for humankind and the planet." It will be hard to argue for a doomsday scenario on the basis of that.

But after more than a decade of increasingly frantic predictions, the activists will not fold their tents and go home. "The climateers have been doing vigorous 'battle space preparation' ahead of the report," says Steven Hayward, a conservative scholar who writes frequently about the politics and science of global warming. "They're priming the media to say 'we're still doomed,' even though the case for doom has been badly eroded over the last couple of years."

Given how deeply the IPCC is invested in the issue — it shared the Nobel Peace Prize with Al Gore in 2007 — there's little doubt the report will give environmental activists at least something to work with. For example, it appears IPCC scientists will declare even more forcefully than before that they are absolutely certain human activity is causing warming. They will repeat previous calls for action against warming on a global scale. There will still be dire warnings.

But the political debate will change. There's no way Congress will consider upending the American economy with far-reaching tax or regulatory schemes on the basis of flawed computer projections about a phenomenon that may or may not require any action at all. The activists can produce as many ads as they want. They can call opponents "deniers" all they like. It just won't work.