It will be interesting to see what climate deniers make of this headline: New Study Shows Climate Change Largely Irreversible. If it is—irreversible, that is—then a reasonable response might be, so then why exactly am I being asked to conserve energy and buy a hybrid car and pay more for wind power . . . when we’re toast anyway?

The study, published in the current Proceedings of the National Academy of Sciences, comes from one of the nation’s leading climatologists, Susan Solomon of the National Oceanic and Atmospheric Administration (who also co-chaired the working group on the physical basis of climate change for the 2007 assessment of the Intergovernmental Panel on Climate Change). Solomon and her colleagues conclude that there’s no going back: changes in global temperature, rainfall, and sea level will be essentially irreversible for more than 1,000 years after we completely stop releasing carbon dioxide and other greenhouse gases into the atmosphere.

“It has long been known that some of the carbon dioxide emitted by human activities stays in the atmosphere for thousands of years,” Solomon said. “But the new study advances the understanding of how this affects the climate system.” Short answer: not for the better.

Current levels of CO² have reached 385 parts per million. Solomon’s study examined what happens if it reaches 450 to 600 ppm before emissions are halted. Chief among the lasting effects: irreversible decreases in dry-season rainfall in southern Europe, northern Africa, southwestern North America, southern Africa and western Australia, with results “comparable to those of the ‘dust bowl’ era,” write the scientists. The calculations also foresee “inexorable sea level rise” of up to 3 feet if we peak at 600 ppm and just under 6 feet if we go all the way to 1000 ppm. When rainfall decreases for periods not of decades but of centuries, human water supplies will fall, wildfires will become more frequent, ecosystems will change beyond recognition and deserts will expand.

The reason the climate impacts last so long is the world’s oceans. They act like an immense heat sink to “keep temperatures almost constant for more than a thousand years,” said Solomon.

The scientists conclude like this: “It is sometimes imagined that slow processes such as climate changes pose small risks, based upon the assumption that a choice can always be made to quickly reduce emissions and thereby reverse any harm within a few years or decades. We have shown that this assumption is incorrect for carbon dioxide emissions . . . . Irreversible climate changes due to carbon dioxide emissions have already taken place, and future carbon dioxide emissions would imply further irreversible effects on the planet.”

So, returning to the question about how society acts when experts conclude there’s no going back, I imagine this will undercut efforts to reduce greenhouse emissions, and perhaps increase support for geoengineering schemes that suck CO² out of the air, as I discussed in 2007. What may well get lost is this: although some changes are locked in, how bad those changes are depends on how many more millions of tons of greenhouse gases we load into the atmosphere.