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.