Monday, July 21, 2008

Greenhouse Earth

Temperature hit 100° in Atlanta today, the first time, I believe, this summer. During this time last year, while this blog was on hiatus, we had something like 20 consecutive days where the temps reached the 100s.

A substantial warming of the Earth occurred 55 million years ago. This warming event, known to geologists as the Paleocene/Eocene Thermal Maximum, was marked by a release of massive quantities of carbon dioxide to the ocean and atmosphere, resulting in elevated CO2 concentrations in the atmosphere and in ocean acidification.

When CO2 is released into the air, about a third ends up in the oceans. Like water, CO2 also dissolves in water and forms a weak acid, resulting in ocean acidification. A quarter is absorbed by terrestrial ecosystems, and the rest ends up in the atmosphere.

The CO2 of the Paleocene/Eocene Thermal Maximum may have resulted from volcanic activity in the western United States. Organic carbon released from marine sediments during the Himalayan Indian-Asian collision may also have contributed to the CO2 levels. The volumes of CO2 generated by these geologic processes could have markedly affected atmospheric concentrations.

Not surprisingly, the early Eocene also had the highest prolonged global temperatures of the past 65 million years. There was little or no polar ice and the globally averaged surface temperature was 2 - 4° warmer than present. Estimates of late Eocene atmospheric CO2 contents range from about twice to six times pre-industrial concentrations, with recent geologic evidence indicating an atmospheric CO2 concentration of 1,125 ppm or more.

The pre-industrial concentraion of CO2 in the atmosphere was about 275 ppm. Today, the concentration is about 385 ppm. However, the world will consume about 31 billion barrels of oil this year alone, six billion tons of coal, and a hundred trillion cubic feet of natural gas. The combustion of these fossil fuels will produce about 30 billion tons of CO2. Next year, global consumption of fossil fuels is expected to grow by 2 percent, meaning that emissions will rise by more than a half billion tons, and the following year consumption is expected to grow by yet another 2 percent. China alone is building roughly four new coal-fired power plants a month.

All available evidence confirms that the high concentrations of CO2 during the Eocene coincided with markedly increased warmth. If the current trends in emissions today were to continue, then sometime within the next 40 or 50 years the chemistry of the oceans will have been altered to such a degree that many marine organisms, including phytoplankton and reef-building coral, will become extinct. Meanwhile, atmospheric CO2 levels are projected to reach 550 ppm, or twice the pre-industrial levels and about 50% of the Early Eocene levels. This will virtually guarantee a significant increase in global temperature.

The extreme warmth of this Paleocene/Eocene Thermal Maximum, however, cannot be simulated with current climate models. A solution to this problem may be found in the recent recognition that biological activity controls the abundance of cloud condensation nuclei (CCN) in the unpolluted atmosphere. Water requires a particulate surface (or “seed”) to make the transition from vapor to liquid. CCNs are the small particles about which cloud droplets coalesce. When no CCNs are present, clouds will not form unless the air is supersaturated with moisture.

There are many different types of particulates that can act as CCN. Phytoplankton, for example, can produce CCN. Large algal blooms of phytoplankton occur in ocean surface waters over a wide range of latitudes. The phytoplankton produce large quantities of sulfate aerosols, which can act as significant CCNs.

As increasing CO2 is released to the atmosphere and ocean, the warming seas become more stratified, with most nutrients trapped in the cold bottom layers while most of the light needed for photosynthesis remains in the warm top layer. Under this scenario, deprived of nutrients, marine phytoplankton would decline, and the declining populations would be further challenged by the associated ocean acidification. This in turn, reduces the sulfate CCNs, which reduces the amount of cloud cover over the Earth, which reduces the amount of solar energy reflected back into space, which increases the amount of heat absorbed by the Earth. If the stress of elevated temperatures and the associated ocean acidification did indeed suppress marine and terrestrial ecosystems during the Eocene, the high temperatures can be reconciled with the CO2 levels.

A counter-hypothesis holds that an increase in global temperature would increase phytoplankton activity and therefore CCN numbers. Known as the CLAW hypothesis, named after Charlson, Lovelock, Andreae and Warren, the authors of a 1987 paper in the journal Nature, this was seen as a possible natural phenomenon that would counteract climate change. However, CLAW does not take into account ocean stratification and acidification, and no conclusive evidence to support this hypothesis has been reported.

Meanwhile, the forecast tomorrow is for a high of 98°.

No comments: