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Possible causes to the recent Decrease in the Warming Rate

April 8, 2013; 8:34 PM ET

Dr. James Hansen, who has worked at NASA's Goddard Institute for Space Studies for 46 years and was director since 1981, has announced his retirement. Hansen wants to spend more time to focus on science and make it clearer to the public.

Dr. Hansen along with Pushker Kharecha and Makiko Sato recently issued a commentary titled 'Doubling Down on our Faustian Bargain'. You can read it in its entirety right here.

Why does the rate of global warming seem to be less this decade compared to what it was during the prior 25 years?

The surge of fossil fuel emissions, especially from coal burning along with the increasing atmospheric CO2 level is fertilizing the biosphere and thereby limiting the growth of atmospheric CO2. It also appears that the aerosol cooling effect is probably increasing.

Below are some of the more noteworthy excerpts made in the commentary in regards to CO2 growth and the impact of aerosols on the climate over the past decade.

Aerosol cooling probably reduced global warming by about half over the past century, but the amount is uncertain because global aerosols and their effect on clouds are not measured accurately.

Aerosol growth slowed in the 1970s with pollution controls in the U.S. and Europe, but accelerated again after 2000.

The rapid growth of fossil fuel CO2 emissions in the past decade is mainly from increased coal use mostly in China with little control of aerosol emissions. It is thus likely that there has been an increase in the negative (cooling) climate forcing by aerosols in the past decade.

An interesting point, however, is the failure of the observed increases in atmospheric CO2 to increase as rapidly as the fossil fuel source has increased. This fact is contrary to suggestions that terrestrial and ocean carbon sinks are tending to saturate as CO2 emissions continue.

Image courtesy of NASA GISS.

An informative presentation of CO2 observations is the ratio of annual CO2 increase in the air

divided by annual fossil fuel CO2 emissions, the "airborne fraction" (image above). This airborne fraction, clearly, is not increasing. Thus the net ocean plus terrestrial sink for carbon emissions has increased by a factor of 3 to 4 since 1958, accommodating the emissions increase by that factor.

We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large

increase of carbon uptake by the combined terrestrial and ocean carbon sinks.

Reduction of the net human-made climate forcing by aerosols has been described as a "Faustian bargain," because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO2 from fossil fuel burning remains in the surface climate system for millennia, the "devil's payment" will be extracted from humanity via increased global warming.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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