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Stabilizing the Climate Requires Near-Zero Carbon Emissions
28 Feb, 2008 11:30 am
What warming is produced by an individual release of carbon dioxide to the atmosphere? This question was addressed by Damon Matthews (Concordia University, Montreal, Canada) and Ken Caldeira (Carnegie Institution, Stanford, USA) in a study titled "Stabilizing climate requires near-zero carbon emissions", scheduled to be published in Geophysical Research Letters on 7 March 2008.
Each increment of carbon dioxide emission contributes another increment to global warming. Therefore, to a close approximation, if we want to reduce emissions enough to stop global warming, we would need to eliminate all carbon dioxide emissions.
If the earth had no capacity to store heat, a puff of CO2 would very rapidly warm the Earth, and then the Earth would cool as the CO2 was absorbed by the oceans and land biosphere. But that is not what happens in the real world or in climate models.
The ocean is a huge reservoir of heat, although in this case it is better to think of it as a huge reservoir of cold. So, soon after the puff of CO2 is released, it is trapping a lot of heat radiation in the atmosphere, but most of this energy initially goes into heating the ocean and not the atmosphere. It takes a few decades before the upper ocean warms substantially, and it takes centuries to warm the entire ocean volume. Once the entire ocean is warm, it can take thousands of years to cool back down. So, after a release of CO2, most of the energy trapping initially goes into heating the ocean. Later, when CO2 concentrations diminish, the warm ocean helps keep the atmosphere warm. As a result, the climate signal from a CO2 emission does not look like the amount of CO2 in the atmosphere.
A puff of CO2 released to the atmosphere results in increasing global temperatures for a few decades as the ocean warms up, but then the climate remains warm for many centuries. Our simulations went out to 500 years, and there was very little reduction from peak temperatures even 500 years after the CO2 emission. It would take thousands of years for most of the peak warming to go away.
This study simplifies how we can think about climate effects of CO2 emission. Each CO2 emission results in increasing temperatures for a few decades, and then the Earth remains warm for many centuries. Thus, if we do not want the Earth to warm further and remain warm for many centuries, we would need to eliminate CO2 emissions.
Matthews, H. D., and K. Caldeira (2008), Stabilizing climate requires near-zero emissions, Geophysical Research Letters, doi:10.1029/2007GL032388, in press.