A Trillion Tons: Stabilization of Carbon Dioxide (5)

A Trillion Tons: Stabilization of Carbon Dioxide (5)

I have gotten off of my planned series on open communities because of meetings and the end of the semester. I am at the American Geophysical Union Meeting in San Francisco. This is one of the biggest regular collections of geophysicists – perhaps even a meeting of ritual. I have been told that there are more than 19,000 registered this year. I’ve had a couple of talks (uncertainty, heat waves), and I have had students making presentations. In terms of presentations of ongoing research about the Earth, about climate, this meeting is overwhelming. You could take a bunch of reporters and send them to a 1000 talks and poster presentations and not cover it all. And, of course, it is hard to pick out what will prove to be important. The sessions I attended have focused on improving the evaluation of climate models, better use of climate data by practitioners, and linking global and local information.

There was a talk by Ray Pierrehumbert that has changed the way I think about carbon dioxide and managing the future heating of the planet’s surface. This is one of those interesting results that come from putting together basic information that has, honestly, been around a while. It’s a result that demonstrates the importance of synthesis in scientific investigation.

Some background (here is a longish list from this blog): Stabilization is the idea of controlling, stabilizing, the amount of carbon dioxide in the atmosphere at some concentration. Some commonly used numbers are that prior to the industrial revolution in parts per million (ppm) there were about 280 ppm. We are currently at about 390 ppm. Jim Hansen has argued we need to get back to 350 ppm. A number, more or less accepted as the lowest, reasonable target, is 450 ppm. And a number that has been used in many evaluations of the impact of global warming is doubled CO2, say, 560 ppm. Currently CO2 emissions are increasing at about 3% a year.

Pierrehumbert was giving his interpretation of a pre-publication report from the National Academy of Sciences, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia. These reports are written by expert panels, and Pierrehumbert was on the panel, which was chaired by Susan Solomon. (with an innocuous registration you can download a .pdf for free.) A take away message from the report is that stabilization of the climate requires us to consider the total accumulated amount of carbon dioxide that we have released. That is, CO2 does not really go away, and that to think about simply controlling emissions is not enough. It just keeps building up, and in the end, carbon dioxide wins.

In January of 2009 Susan Solomon and colleagues published a paper called Irreversible climate change due to carbon dioxide emissions. The article appeared in the Proceedings of the National Academy of Sciences. This paper focused specifically on carbon dioxide in the atmosphere and made the argument that the effects of man-made carbon dioxide on the climate would last more than 1000 years – their definition of irreversibility.

The lifetime of carbon dioxide in the climate system, specifically in the atmosphere is more difficult to calculate than for many greenhouse gases. This is because of the role of the oceans and the terrestrial ecosystems. A balance develops between the terrestrial carbon dioxide, the carbon dioxide in the ocean and in the atmosphere. While one can find citations that the lifetime of carbon dioxide in the atmosphere is on the order of 100 years, Solomon and her co-authors point out that when one considers how long today’s “excess” carbon dioxide from industry influences the climate, it is in excess of 1000 years. It for this reason that if we stopped burning fossil fuels immediately, that the Earth’s surface would continue to warm and remain warm.

Thinking about this, at least for any amount of time relevant to humans, this suggests that the carbon dioxide that we put in the atmosphere will just build up over time. Pierrehumbert’s exposition of this issue comes to the conclusion that given our stated desire to limit surface warming to only 2 degrees on average, we are allowed to put one trillion tons of carbon in the atmosphere (I have revised this to take care of the difference between unit of carbon dioxide or carbon equivalent. A ton of carbon equivalent is 3.667 tons of carbon dioxide, see EPA definitions). On time scales of 1000 to 10,000 years, it does not matter a whole lot if we put that trillion tons in over 20 years or over 100 years. From the point of view of an inhabitant for the next few years, I would take issue with it not mattering how fast it comes in – it would affect the peak amount and the amount of warming in the short term. But, as Pierrehumbert presented the argument, it makes sense, and the time scales are all close enough to be relevant and have important implications. Again, long-term effects are determined by carbon dioxide, and it matters what we do in the short term to limit these effects.

Pierrehumbert takes issue with Ramanathan’s proposal that we can manage the composition of black carbon and methane to buy time. The point being that it does not really buy time if we don’t actually control the amount of CO2 emission – because in the end what matters is the total amount of accumulation of CO2 in the atmosphere. Yes, the management of black carbon and methane makes a difference to temperature, but it is a relatively short blip even on a time scale of 100 years. It temporarily reduces the warming but does not exactly buy time, as CO2 continues to accumulate. Such a warming management tactic could be implemented at any time. (see Pierrehumbert on RealClimate, see also Hot the Brakes Hard).

This rethinking of the role of carbon dioxide challenges the standard stabilization curves that I and many others have used to discuss carbon dioxide balance. Here is that curve:




Figure 1: Stabilization of Atmospheric Carbon Dioxide as a Function of Emissions (from IPCC).

This curve suggests that if we start to reduce emissions at a certain rate in, for example, about 2040 and then follow a reduced emissions curve that the carbon dioxide in the atmosphere will decrease to some value. If you study the curve you will see the carbon dioxide decreasing significantly over 100 years. Pierrehumbert is essentially stating that this curve is not appropriate to reality, because CO2 is not leaving the atmosphere; it does NOT decrease significantly over 100 years. It, therefore, accumulates.

So I am going to try to read some graphs. According to this graph from Pierrehumbert’s blog on Realclimate, a total emission of one trillion tons of carbon would keep warming just below the two degree global average.



Figure 2: Warming based on accumulated carbon dioxide in the atmosphere (from RealClimate.org)

Then, according to Table 1 in Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia this trillion tons corresponds to about 430 ppm of carbon dioxide. This suggests that emissions reductions alone, no matter how drastic, will not get us back to 350 ppm. If this is the needed target, then we will have to learn how to remove carbon dioxide from the atmosphere.

So a question arises – over the past 150 years what is the accumulated carbon dioxide due to burning fossil fuels and how much do we have left? As I understand the emissions, we have used about half of our trillion tons, and we will get to our 1 trillion tons in 2040 with our current rate of emissions. Current reality suggests that we are increasing, not decreasing our emissions. It would be, therefore, an ambitious goal to limit our emissions to one trillion tons, and shift to non-carbon energy sources. We might buy some time by managing and reducing our carbon emissions, meaning to reduce the rate of emissions increase.

In a statement of what has been becoming more and more evident, the National Academy report "concludes that the world is entering a new geologic epoch, sometimes called the Anthropocene, in which human activities will largely control the evolution of Earth’s environment.” It will be ours to manage and engineer, either with knowledge and responsibility or without.

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Figure 3: A ton of carbon dioxide in Copenhagen.