450 or 350 — Choose One and Go

by Bill Chameides | October 26th, 2009
posted by Erica Rowell (Editor)

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Thousands demonstrated around the globe on Saturday for 350 parts per million (ppm) while world leaders are still focused on 450 ppm.

Way back before the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere was about 287 ppm. Today, as a result of our burning fossil fuels (like oil, gas and coal) and clearing of forests, CO2 concentrations have crept up to around 387 ppm. (See related post.) If this trend continues, the 400 ppm threshold should be surpassed sometime in the next six to 10 years.

Such high concentrations have most climate scientists very concerned. Data from ice cores indicate that CO2 concentrations of this magnitude have not occurred for at least the last 800,000 years; one recent study suggests that such levels have not been reached during the past 15 million years.

At the same time, observational data collected over the last two to three decades provide very strong evidence that global warming trends since the 1980s and likely since the early 1900s was due in large part to the jump in CO2 concentrations.

CO2 Levels Higher, Now What

After decades of debate it appears that a consensus for action has been formed (but maybe not for long). Congress seems to be getting closer to passing climate legislation. Similarly, much of the global community has signaled intention to implement an international agreement to reduce global greenhouse gas emissions — although whether an agreement for how to do this will be reached at the upcoming Copenhagen meeting is very much in doubt.

The 450 CO2 Target

Agreeing that we need a climate policy is great, but any subsequent law or policy would be toothless without a specific goal. The U.N. Framework Convention on Climate Change (UNFCC), signed by 192 nations, including the United States, stated this important but hardly specific goal: “avoiding dangerous anthropogenic interference in the climate.”

Over the past five years or so, policy makers, American and otherwise, have closed in on a specific target: based on a fairly significant body of research (albeit with large levels of uncertainty), the line in the climate sand is two degrees Celsius. That is, global temperatures cannot increase by more than this if we are to avoid dangerous anthropogenic interference in climate.

Climate simulations indicate that if atmospheric CO2 concentrations reach 450 ppm there is a 50-50 chance that global temperatures will exceed the two-degree threshold. I suppose by splitting the difference between prudence and practicality, 450 ppm has been chosen as the target of stabilizing CO2 concentrations.

And Along Comes 350

While governments are tooling up to set the 450 ppm target for climate legislation and treaties, many scientists, engineers, and policy makers are scratching their heads trying to figure out whether a 450 ppm target is still possible. Some argue that if we’d started reining in CO2 emissions a decade ago, 450 ppm would be possible, but since we didn’t, keeping CO2 levels below 550 ppm is more realistic. Other studies suggest that 450 ppm is possible but will require strong action and soon.

Against this backdrop came a 2008 paper by James Hansen, a climate scientist at NASA, arguing that to avoid dangerous climate change, not only is 450 ppm too high, so is today’s concentration of 387 ppm. Avoiding dangerous climate change, argues Hansen, will require lowering CO2 concentrations to 350 ppm.

The scientific community has neither embraced nor rejected a 350 ppm target. Setting a value for the “safe level” of CO2 is a problematic (some may say poorly posed) task. The uncertainties are huge and the determination of what is meant by safe or dangerous is subjective.

Over the past year, Hansen’s 350 target has gotten some traction in the environmental community. Environmental luminary Bill McKibben formed the nonprofit, the group that staged the globally coordinated demonstration this past Saturday calling for adoption of the 350 ppm target.

So What Is the Bottom Line?

Is 450 ppm safe enough or should we shoot for 350 ppm? The answer depends to some extent (with apologies to our former president) on how dangerous you think dangerous is. It also depends on how you weigh practicality against prudence.

Keeping CO2 levels below 450 ppm is a major challenge, and we have no practical way to bring CO2 concentrations down from present-day levels on decadal timescales. On the other hand, major technological advances could radically change that assessment, and setting a very ambitious goal could hasten the development of those advances.

For me, choosing 450 or 350 is less important than getting started at lowering CO2 emissions as much — and as fast — as we can. If technologies improve sufficiently or the science demands it, we can readjust the target appropriately. I think Gavin Schmidt, a climate scientist from the Goddard Institute for Space Studies, put it best:

“If you ask a scientist how much more CO2 do you think we should add to the atmosphere, the answer is going to be none.”

filed under: carbon dioxide emissions, climate change, faculty, global warming, policy, temperatures
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