Global cooling

Global temperatures in 2006 were the third coldest on record in the lower stratosphere, according to the National Climatic Data Center. Only 1997 and 2000 had colder temperatures since record keeping began in 1979 (Figure 1). Why is this important? Well, the stratosphere is that layer of the upper atmosphere approximately 14-22 km (9-14 miles) above the surface that contains our protective ozone layer. The main reason for the recent stratospheric cooling is due to the destruction of ozone by human-emitted CFC gases. Ozone absorbs solar UV radiation, which heats the surrounding air in the stratosphere. Loss of ozone means that less UV light gets absorbed, resulting in cooling of the stratosphere. Cooling of the stratosphere results in the formation of more polar stratospheric clouds, which require very cold temperatures to form. The presence of these clouds allows even more ozone destruction to occur, since the reactions responsible for ozone destruction occur much faster in clouds than in dry air. Thus, the recent cooling of the stratosphere allows high levels of harmful UV light to reach the surface. As CFC gases begin to decline in coming years thanks to banning of these substances in 1987, the stratosphere should start to warm, and ozone levels will recover.


Figure 1. Global lower stratospheric departure of temperature from average since 1979, as measured by satellites. The large spikes in 1982 and 1991 are due to the eruptions of El Chicon and Mt. Pinatubo, respectively. These volcanos ejected huge quantities of sulphuric acid dust into the stratosphere. This dust absorbed large quantities of solar radiation, heating the stratosphere. Image credit: National Climatic Data Center.

Greenhouse gases also cause stratospheric cooling
However, this recovery of the ozone layer is being delayed. A significant portion of the observed stratospheric cooling is also due to human-emitted greenhouse gases like carbon dioxide and methane. Climate models predict that if greenhouse gases are to blame for heating at the surface, compensating cooling must occur in the upper atmosphere. We need only look as far as our sister planet, Venus, to see the truth of this theory. Venus's atmosphere is 96.5% carbon dioxide, which has triggered a run-away greenhouse effect of truly hellish proportions. The average surface temperature on Venus is a very toasty 894°F! However, Venus's upper atmosphere is a startling 4-5 times colder than Earth's upper atmosphere. The explanation of this greenhouse gas-caused surface heating and upper air cooling is not simple, but good discussions can be found at Max Planck Institute for Chemistry and realclimate.org for those unafraid of radiative transfer theory. One way to think about the problem is that the amount of infrared heat energy radiated out to space by a planet is roughly equal to the amount of solar energy it receives from the sun. If the surface atmosphere warms, there must be compensating cooling elsewhere in the atmosphere in order to keep the amount of heat given off by the planet the same. As emissions of greenhouse gases continue to rise, their cooling effect on the stratosphere will increase. This will make recovery of the stratospheric ozone layer much slower.

Greenhouse gases cause cooling higher up, too
Greenhouse gases have also led to the cooling of the atmosphere at levels higher than the stratosphere. Over the past 30 years, the Earth's surface temperature has increased 0.2-0.4°C, while the temperature in the mesosphere, about 50-80 km above ground, has cooled 5-10°C (Beig et al., 2006). There is no appreciable cooling due to ozone destruction at these altitudes, so nearly all of this dramatic cooling is due to the addition of greenhouse gases to the atmosphere. Even greater cooling of 17°C per decade has been observed high in the ionosphere, at 350 km altitude. This has affected the orbits of orbiting satellites, due to decreased drag, since the upper atmosphere has shrunk and moved closer to the surface (Lastovicka et al., 2006). The density of the air has declined 2-3% per decade the past 30 years at 350 km altitude. So, in a sense, the sky IS falling!

What about global warming being caused by the sun?
Some scientists have theorized that increases in solar output are responsible for a significant portion of the observed global warming. For instance, Scafetta & West (2006) estimated that 25-35% of the global warming in the 1980-2000 period was attributable to solar variability. Other scientists disagree, finding no evidence of global warming due to solar activity changes since the 1940s. Since any increase in solar radiation would heat both the lower and upper atmosphere, the observed drop in upper atmospheric temperatures in the past 30 years argues against a large portion of the observed greenhouse effect being caused by solar variability. The observed cooling of the upper atmosphere in recent decades is strong evidence that the warming at Earth's surface is due to human-emitted greenhouse gases. It should also give us additional confidence in the climate models, since they predicted that this upper atmospheric cooling would occur.

I'll be at the annual meeting of the American Meteorological Society in San Antonio this week, and plan to post a blog from there Wednesday.

Jeff Masters