New climate target: Negative emissions growth

Just about every serious proposal to cap fossil-fuel emissions involves an 80 percent cut below 1990 levels by 2050. This might, if we're lucky, keep atmospheric CO2eq (a unit of measurement that expresses the total contribution of all greenhouse forcing gases as just carbon dioxide, for the sake simplicity) at 450 parts per million. But more and more we're hearing that that won't be enough to avoid exceeeding tipping points. More and more we're hearing that even 100% won't do the trick. Instead, say a growing chorus of scientists and analysts, we need to think about getting below current levels. Think about it for a minute.

Getting emissions to just one fifth of what they were in 1990 means aiming for a global average annual CO2 emissions allowance of roughly 1 tonne per person. We're now at close to 5 t of CO2eq globally, with Americans producing 20 tonnes each. So you can see the problem. Add to that the technological challenge of sucking existing CO2 out of the air, and it's enough to make one pack it in.

And yet, we've got Bill McKibben and his campaign. 350, in parts per million, is 37 ppm less than what's currently in there. He calls it the "safe line." But wait, he's not a climatologist.

James Hansen is, though. He's been saying that 350 ppm is the preferred target for a while now. He bases that on analyses of paleoclimatic data, specifically the estimate that it was when CO2 levels fell to 425 ppm +/- 75 ppm, 50 million years ago, that the ice started to build up on Antarctica. Therefore, goes the reasoning, if the CO2 rises above that level, the ice will start to melt, and the seas will rise. The low end of the uncertainty range is 350 ppm, so there you go.

Incidentally, I asked Hansen to comment on Lunt's paper. He said he hadn't read it yet, but that" both of those numbers are consistent with our "Target CO2" conclusion that the safe level of CO2 is no higher than 350 ppm." That paper, linked above, has now been accepted to publication, he added.

Others find this kind of reasoning a bit of a stretch, although fair enough given the lack of data we really need to nail down the danger zone. But Hansen's not the only one talking in such terms. Just the other week came a paper in Nature by Daniel J. Lunt at the University of Bristol, using a similar line of reasoning about Greenland's ice 3 million years ago when CO2 levels were around 400 ppm: is the fall from these mid-Pliocene values to lower values typical of the Quaternary that favoured the development of Northern Hemisphere glaciation and the growth of the Greenland ice sheet.

Last week we heard from John Schellnhuber, director of the Potsdam Institute for Climate Impact Research, who told the Guardian that only "a return to pre-industrial levels of CO2 would be enough to guarantee a safe future for the planet."

"It is a very sweeping argument, but nobody can say for sure that 330 ppm is safe," he said. "Perhaps it will not matter whether we have 270 ppm or 320 ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms."

How we get to 350 from 385 is an interesting thought experiment. Hansen says that agricultural practices ;;;; growing more forests and possibly burying carbon released from the burning other crops ;;;; might work, if we've already stopped burning coal and don't exploit unconventional petroleum reserves. Others talk about millions of giant carbon vacuums. Hideously expensive, but theoretically possible, I suppose. Schellnhuber says some of these ideas are "science fiction" at this point. But they seem more grounded in reality than the G8's most recent target of a 50% cut by 2050. I mean, what's the point of that?


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Then there are geo-engineering solutions, which might be able to alter the safe limit. For instance proposals for requiring white roofs, and light colored roadways etc, might be able to shift the temperature versus CO2 concentration by perhaps 10ppm. It is possible we might be able to find a few climate change BBs among the various shortwave radiation fixes.

This new paper is also consistent, noting that "even if atmospheric CO2 stabilizes at the current level of 380 ppm, fewer than half of existing coral reef will remain in such an environment":

Modest CO2 cutbacks may be too little, too late for coral reefs

Stanford, CAHow much carbon dioxide is too much? According to United Nations Framework Convention on Climate Change (UNFCCC) greenhouse gases in the atmosphere need to be stabilized at levels low enough to "prevent dangerous anthropogenic interference with the climate system." But scientists have come to realize that an even more acute danger than climate change is lurking in the world's oceansone that is likely to be triggered by CO2 levels that are modest by climate standards.

Ocean acidification could devastate coral reefs and other marine ecosystems even if atmospheric carbon dioxide stabilizes at 450 ppm, a level well below that of many climate change forecasts, report chemical oceanographers Long Cao and Ken Caldeira of the Carnegie Institution's Department of Global Ecology in the journal Geophysical Research Letters.

The researchers' conclusions are based on computer simulations of ocean chemistry stabilized at atmospheric CO2 levels ranging from 280 parts per million (pre-industrial levels) to 2000 ppm. Present levels are 380 ppm and rapidly rising due to accelerating emissions from human activities, primarily the burning of fossil fuels.

This study was initiated as a result of Caldeira's testimony before a Congressional subcommittee on Fisheries, Wildlife and Oceans in April of 2007. At that time he was asked what stabilization level would be needed to preserve the marine environment, but had to answer that no such study had yet addressed that question. Cao and Caldeira's study helps fill the gap.

Atmospheric CO2 absorbed by the oceans' surface water produces carbonic acid, the same acid that gives soft drinks their fizz, making certain carbonate minerals dissolve more readily in seawater. This is especially true for aragonite, the mineral used by corals and many other marine organisms to grow their skeletons. For corals to be able to build reefs, which requires rapid growth and strong skeletons, the surrounding water needs to be highly supersaturated with aragonite.

"Before the industrial revolution, over 98% of warm water coral reefs were surrounded by open ocean waters at least 3.5 times supersaturated with aragonite" says Cao. "But even if atmospheric CO2 stabilizes at the current level of 380 ppm, fewer than half of existing coral reef will remain in such an environment. If the levels stabilize at 450 ppm, fewer than 10% of reefs would be in waters with the kind of chemistry that has sustained coral reefs in the past."

For the ecologically productive cold waters near the poles, the prospects are equally grim, says Cao. "At atmospheric CO2 levels as low as 450 ppm, large parts of the Southern Ocean, the Arctic Ocean, and the North Pacific would experience a rise in acidity that would violate US Environmental Protection Agency water quality standards." Under those conditions the shells of many marine organisms would dissolve, including those at the base of the food chain.

"If current trends in CO2 emissions continue unabated," says Caldeira, "in the next few decades, we will produce chemical conditions in the oceans that have not been seen for tens of millions of years. We are doing something very profound to our oceans. Ecosystems like coral reefs that have been around for many millions of years just won't be able to cope with the change."

"When you go to the seashore, the oceans seem huge," he adds. "It's hard to imagine we could wreck it all. But if we want our children to enjoy a healthy ocean, we need to start cutting carbon emissions now."

By Steve Bloom (not verified) on 22 Sep 2008 #permalink

I know that this sounds like fatalism, but in my opinion, I don't think that there is any way to stop environmental degradation other than by greatly reducing the number of humans on Earth. If there were only 10% as many of us, our gas-guzzling, fossil-fuel-burning habits would not threaten the health of the world.

A Chinese-style 1 child policy for a couple of hundred years would do it. Or just a few 10-year periods of no new babies at all would do it, as well.