Rising trends in atmospheric CO2

CO2 output is accelerating, the ocean and land sinks are getting less effective at absorbing it. So the rate of growth of carbon dioxide in the atmosphere is increasing. (Canadell, Le Quéré, and others, 'Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks', Proceedings of the National Academy of Sciences, 25 October 2007; URL: http://tinyurl.com/yqew8o [accessed 27 October 2007].)

The pre-industrial CO2 concentration in the atmosphere was about 280 parts per million. It was 381ppm in 2006. The growth rate between 2000 and 2006 was 1.93ppm, a significant increase on growth rates in earlier periods. Many policy-makers see it as vital to keep below concentrations of about 400ppm of CO2. The increase in the rate of rise of CO2 makes the achievement of this target more difficult.

Increases in the amount of CO2 in the atmosphere reflect the volume of global emissions and the effectiveness of the oceans and land mass in absorbing greenhouse gases. This paper contains evidence both that emissions growth is speeding up and that the greenhouse gas sinks are capturing less CO2.

The growth rate in emissions between 2000 and 2006 was 3.3% a year compared to 1.3% in the 1990s (please see the article on Chinese exports in this issue of Carbon Commentary for corroboration of this finding). This increase reflects fast economic growth, particularly in China and India and a worrying increase in the amount of CO2 produced per unit of global output. It cannot be stressed enough that this second cause of emissions growth is unexpected. We thought we were going to see energy use fall in relation to economic output.

By contrast, models have predicted a decline in the effectiveness of ocean CO2 ‘sinks’. This paper shows that we can have a strong suspicion (but not near certainty) that this process has started. The authors point to increasing wind speeds in the Southern Ocean as a primary cause. This turbulence ‘ventilates’ the carbon dioxide contained in the surface of the sea. Droughts in mid-latitude regions have contributed to the decreased efficiency of land absorption.

The paper concludes that – with large margins of error – economic growth generated 65% of the increase in atmospheric CO2; the decrease in the efficiency of the sinks generated another 18% and caused a rise in the carbon output required to generate a dollar of world GDP.

The authors summarise by saying that their results ‘characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing’.