Rise of Atmospheric Carbon is Accelerating

This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

Last month, Michael explained why we need to cut emissions as soon as possible. A new paper published in the Proceedings of the National Academy of Sciences adds even more urgency. It says that:

  • Carbon dioxide (CO2) emissions from fossil fuel use have accelerated since 2000, and
  • Earth’s oceans may be taking up an increasingly smaller fraction of the extra CO2.

This paper received coverage even before it was published. Now that it’s out, let’s take a closer look.

The study looks at three components of the carbon cycle: CO2 levels in the atmosphere, CO2 emissions from human activities, and CO2 uptake by plants and oceans (called the "carbon sink").

In the natural carbon cycle, carbon dioxide is released into the air by decomposing biomass and animal respiration. Living plants pull CO2 out of the atmosphere as fuel for growth, and oceans absorb it. Then plants and animals die and the cycle continues.

Barring extreme events like huge volcanic eruptions, until humans started adding to atmospheric CO2 by burning fossil fuels, the amount of carbon emitted and absorbed were in balance, so atmospheric CO2 stayed fairly constant at 280 parts per million (ppm).

Last year, CO2 concentration hit 381 ppm – the highest it’s been in 650,000 years – and the growth rate is accelerating. In the 1990s, CO2 concentration went up 1.49 ppm per year. Between 2000 and 2006, it increased at a rate of 1.93 ppm per year. This is the highest growth rate since scientists started taking direct measurements of CO2 levels in 1959.

Is the accelerated increase caused by accelerating emissions or increasingly lower uptake by plants and oceans? The evidence is that it’s due to both.

The growth rate in emissions from fossil fuels has more than doubled in recent years. In the 1990s, emissions grew an average of 1.3 percent per year. Since 2000, the growth rate has averaged 3.3 percent per year. That said, it should be borne in mind that six years is a very short time. An apparent trend like this may not be sustained if, for example, the global economy slows or carbon efficiency improves.

And the carbon sink? The absolute amount of CO2 taken up by the sinks has increased, but the "airborne fraction" – the fraction of CO2 emissions that are not absorbed and stay in the atmosphere – has increased slightly, from 40 to 45 percent, since measurements began in 1959. The year-to-year variation is very large, but over the long term, it appears that the carbon sink, particularly in the oceans, is taking up a smaller fraction of the CO2 emissions.

Scientists have warned for a long time that the ability of Earth’s oceans and/or land ecosystems to offset humans’ CO2 emissions could decline over time.  Although we cannot be sure that these trends will continue (let’s hope they don’t!), this paper suggests that such a process may be underway. If these results give you a "sinking" feeling, that’s understandable, but I hope the main effect will be to increase motivation for change.

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One Comment

  1. Posted November 6, 2007 at 4:41 pm | Permalink

    This is the most important issue that faces our generation and our societies and governments cannot waste another second without reducing the emission of carbon ie reducing the amount of coal, oil, and carbon that we burn for fuel. I am working to pass the energy bill that the US Congress is currently debating. If passed in its entirety it would be the first and therefore largest step the US has taken toward reducing its dependence on oil. If passed in its entirety it would create a standard of 35 mpg in all new cars by 2020 and a standard that 15% of electricity come from renewable sources by 2020. It is a conservative bill in many respects, yet it is very important as it would put the US on the green path to carbon emission reduction for the first time. One step is needed for more to come. I urge you to sign the petition at http://www.energybill2007.org in order to tell our representatives how important the global warming crisis is.