Friday, April 25, 2008

Ozone and Climate

Many people confuse the issue of climate change (or global warming) with the other major influence of humanity on the Earth's atmosphere: the hole in the ozone layer. These phenomena are actually caused by two very different aspects of atmospheric chemistry, and they have distinct consequences for the environment. But this week, climate scientists announced a surprising connection between the ozone layer and climate change: if we adopt one approach to cooling the planet, it could make the ozone problem worse.

Ozone is just another form of oxygen. The oxygen that we breath is really a molecule consisting of two oxygen atoms bound together. Ozone is similar, but it has three oxygen atoms. High in the Earth's atmosphere there is a relatively thin layer of ozone, and because of its specific structure it acts like sunblock -- filtering out much of the harmful ultraviolet radiation coming from the Sun before it ever reaches the ground. Back in the 1980's, scientists discovered that large holes were developing in the ozone layer, particularly over the polar regions of the planet. Further study linked this depletion to widespread use of chlorofluorocarbons (CFCs), particularly as refrigerants and as propellants in spray cans for consumer products. Alternative gases were quickly identified for these uses, and CFCs were phased out in most industrialized countries. The results were dramatic -- further monitoring of the ozone holes showed them begin to recover as the widespread use of CFCs was discontinued. It was a landmark achievement for climate science and its influence on public policy.

More recently it has become clear that the global climate is getting warmer and that, like ozone depletion, a substantial fraction of the effect can be attributed to human activities. In this case the underlying cause is the release of so-called greenhouse gases -- molecules like carbon dioxide (CO2) and methane (CH4) that trap the Sun's infrared radiation or heat from being released back into outer space. Large volumes of these gases are produced from burning fossil fuels like coal in power plants or gasoline in automobiles -- and as humanity releases more greenhouse gases than the environment can naturally absorb, it builds up in the atmosphere, traps more heat, and gradually warms the planet. This has been happening dramatically since the industrial revolution in the late 1800's, and unless we find suitable alternatives for our energy needs we risk a major disruption of our climate system later in this century.

Since this is a much bigger problem than finding substitute refrigerants and propellants -- and because the consequences are potentially much more severe -- some climate scientists are considering how to cool the planet without reducing greenhouse gas emissions. In the long term climate record that has been reconstructed from ice cores drilled in Antarctica, scientists noticed that historical periods of global cooling often follow major volcanic eruptions. The cooling was traced to large quantities of sulfates that were hurled into the Earth's upper atmosphere during the eruptions. The sulfates block some of the incoming sunlight from ever reaching the surface, which can counteract the warming effects of increased greenhouse gas concentrations (though other problems like ocean acidification still remain). So if global warming gets really out of control, the scientists reasoned that we could manually inject sulfates into the upper atmosphere like an artificial volcanic eruption to cool the planet.

Working from this premise, the study released this week examined the other possible consequences of engineering the climate with massive injections of sulfates -- and this is where ozone returns to the picture. It turns out that sulfates also deplete the ozone layer, and if enough of them were injected into the upper atmosphere to offset the expected warming in the coming decades it would destroy between 1/4 and 3/4 of the Arctic ozone layer and delay the expected recovery of the ozone hole over the Antarctic by between 30 and 70 years. So if we want a reasonable climate and an ozone layer, sulfates might not be the best answer.