A Musing Environment

The International Climate Change Taskforce warned a year or so ago that 400 ppm (parts per million) atmospheric carbon would be dangerous, as the chance of accelerated, runaway, or abrupt climate change becomes dangerously high if the temperature increase is 2 C (3.6 F) — at 400 ppm C, the chance is one in five that a 2 C increase would occur. There were several points of confusion for me, thanks to RealClimate for clarification. Go to their explanation or read my slightly simplified one, which also includes explanations from John Holdren’s US Climate Policy Post-Kyoto: Scientific Underpinnings, Policy History, and the Path Ahead (pdf), a 2003 report.

Where does 400 ppm come from? and other questions

Carbon dioxide is only one of the greenhouse gases, though the most important one being added to the atmosphere. There has been a steep increase in each of the major greenhouse gases normally in the atmosphere — carbon along with methane, nitrous oxide, and ground level ozone, as well as the addition of some not naturally present, such as CFCs. We are also adding pollution that cools the Earth, for example, aerosols –suspensions of tiny solid or liquid particles. All of the warming due to greenhouse gases besides carbon dioxide are currently being matched by the net cooling of what we have added to the atmosphere, so the net change over the past few hundred years is equal to the net change due to added carbon.

What level of carbon leads to what temperature increase? This isn’t known, because feedback mechanisms are only partially understood. Currently, northern forests are absorbing much of the carbon emitted, a negative feedback. There is increased release of carbon from the tundra, a positive feedback. Right now, the forests take out more carbon than the tundra adds, though there is presumably a time limit on how long the forest could continue growing. Another biggie: how quickly will highly reflective snow and ice be replaced in the Arctic with water and plants? What about methane release from ocean depths or permafrost or peat deposits? Currently, too little is known.

Most climatologists go with a temperature increase of anywhere from 1.5 C to 4.5 C if atmospheric carbon levels go to 550 ppm, double pre-industrial levels. That is, there is a range of estimates on climate sensitivity to disturbances. (Since it is known that the greenhouse gases added to date have increased the temperature 0.8 C and is estimated that if the atmospheric carbon remains constant, that the temperature will increase another 0.6 C, it is not clear why such a large range of climate sensitivities continues to be reported, explanations anyone?) If the climate sensitivity is 3 C, then increasing the carbon effective (the effect of changing carbon plus other greenhouse gases plus aerosols) level 2/3 of the way from 275 ppm to 550 ppm – to 460 ppm – will increase the Earth’s temperature by 2/3 of 3 C. Larger climate sensitivities will produce 2 C increases at lower greenhouse gas levels.

James Hansen in his talk, previous post, said that he doesn’t need to understand the theory, that the evidence over time has been sufficient to let us know what temperature increase goes with what increase in atmospheric carbon. Still there is a discussion, and models show that there is a 20% chance that a stable atmospheric carbon level of 400 ppm during this century may well lead to catastrophic changes. These catastrophic changes are on top of the regular really bad changes, such as droughts, the Eurasian heat wave of 2003, increased sea level and storms and fires, and dryer soil almost everywhere. These catastrophic changes would include rapidly increasing sea levels. Many of the changes cannot be predicted because it has been so long since the temperatures were this high.

We’re on track to reach 400 ppm in 2015. It’s pretty impossible to imagine any scenario that allows us to keep atmospheric carbon levels from going above 400 ppm.

So what level of carbon emissions will help us avoid 2 C warming?

It’s cumulative warming that matters, not the warming in one particular year. One possibility is to stabilize at 415 ppm soon, very soon. A possibly equivalent scenario is to allow atmospheric levels to reach 475 ppm by mid-century, and it will take major changes in our policies to limit carbon levels to that degree, and then radically reduce carbon emissions afterwards to decrease atmospheric levels down to 400 ppm. If we add less carbon than the ocean can absorb, atmospheric levels would come down, though it will take many tens or hundreds or thousands of years to get carbon levels down to what they would be without the industrial revolution.

To stop adding carbon to the atmosphere, we need to reduce carbon emissions some 60 – 80%, to what the ocean can absorb. To decrease atmospheric carbon levels, we would need to reduce carbon emissions even further.

Brief explanation of units: measurements include only the carbon portion of carbon dioxide, and are in the international system of billion metric tonnes carbon, or GtC.

In 2000, 1.6 GtC of the 6.4 GtC emitted was American, 25%. Deforestation added another 1.5 GtC and cement production another 0.2 GtC. We can reduce deforestation with better management, but the “middle of the road” scenario showed carbon emission reaching 14 GtC yearly by 2050, and more than 20 GtC yearly by 2100. This path obviously doesn’t get us to 60 – 80% reductions any time soon.

From Holdren:

The size of the CO2-emissions-reduction challenge becomes apparent when one recognizes that stabilizing the atmospheric concentration of CO2 requires not just leveling off emissions at a level not too much higher than today’s but subsequently bringing emissions down, over a period of many decades, to a fraction of today’s. For example, if one wished to stabilize the atmospheric concentration of CO2 at 550 parts per million by volume (ppmv) – that is, at about twice the pre-industrial concentration – and if one wanted to avoid stabilization trajectories that place too much of the burden of reductions in the early decades of the century, as well as avoiding those that involve extremely steep declines later, then one would want to level off emissions at about 11 GtC around the year 2035 (which is not so far above the total emissions of 8 GtC/year from fossil-fuel combustion, cement manufacture, and deforestation combined in 2000) and then begin gradually to decline, falling to about 6-7 GtC/year by 2100 and to 3-4 GtC/year by 2200. This particular target for stabilization of the atmospheric CO2 concentration, 550 ppmv, is an interesting one to consider further, in part because studies of the climate-change impacts likely to ensue at this concentration suggest that going further would be imprudent in the extreme, and in part because studies of the rate of transformation of the world’s energy-supply system needed to stabilize at any lower CO2 concentration than 550 ppmv makes this seem very difficult to achieve…

(T)he world would need to be obtaining from carbon-free sources by 2100 more than three times as much as energy as it was using altogether in 2000….As will be seen in what follows, there is as yet little sign of the sorts of policies and commitments that could yield the needed energy-intensity reductions and carbon-free-energy increases in any combination consistent with stabilizing atmospheric CO2 at 550 ppmv.

Now let’s not lose sight of the goal of stabilizing at a much lower level. To even get to 550 ppm carbon, Holdren recommends,

a doubling or tripling of US Federal support for research, development, and demonstration; a tripling or quadrupling of US assistance on low-emission energy technologies to developing countries; a prompt closing of the loophole for light trucks and SUVs in the US CAFÉ standards, followed by ramping up the fleet-average mileage requirement to 40 mpg by 2015; and, most important but also most difficult to achieve, either a gradually escalating carbon tax or a gradually tightening emissions cap implemented through tradable permits. I also believe that these measures are as likely to help as to harm the US economy, even before counting the climate damages averted.

Both the RealClimate piece and the Holdren piece contain more details, and the Holdren piece includes a look at the politics of Kyoto and both the Clinton and Bush administrations.

This entry was posted on Wednesday, February 8th, 2006 at 10:11 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.