Tuesday, December 22, 2009

Carbon Dioxide Lessons

With the Copenhagen meeting in the news, I wanted to give my talented students an up-to-date picture of climate change science. The best available evidence makes deciding what to do more rationale and less emotional.

Examining climate change means understanding the greenhouse effect, and the atmosphere's key greenhouse gas, carbon dioxide (CO2). A greenhouse gas has two essential characteristics. First, it lets short-wavelength sunlight pass through the atmosphere.

This sunlight heats up the Earth's surface, which then sends out its own light energy, in the form of longer wavelength infrared light.

Greenhouse gas characteristic #2 - it absorbs the infrared light from the Earth, preventing this heat energy from passing through the atmosphere, away from the planet. When carbon dioxide absorbs this infrared light, it heats up and emits its own infrared light, most of which heads back towards the Earth. Infrared heat bounces back and forth between the surface and the atmosphere, warming the planet.

Infrared light eventually leaks through the atmosphere back to space. But the more greenhouse gases there are, the longer this takes and the warmer we get.

We depend on the greenhouse effect to keep Earth's temperatures livable. Take away all greenhouse gases and the average temperature would be 60°F colder. But it is the increased greenhouse effect caused by higher than usual carbon dioxide levels and other greenhouse gases that drives our climate change concerns.

Why is CO2 increasing, and by how much? When we burn coal, oil, gasoline, natural gas and even wood, carbon dioxide is a by-product.

Before people started burning fossil fuels there were about 280 parts of CO2 per million parts of other gases in the atmosphere (280 ppm). Today the CO2 concentration is around 387 ppm, higher than it has been for at least 650,000 years, and it continues to rise at an ever-increasing rate, now nearly 2 ppm per year.
The Keeling Curve, showing the record of atmospheric carbon dioxide measurements from the observatory atop the Mauna Loa volcano in Hawaii

Do we know that this atmospheric CO2 comes from burning fossil fuels? Yes. As John Rennie recently pointed out in Scientific American (11/30/2009), human activities now release over 8 billion tons of CO2 each year, while the nearest "natural" source, volcanoes, average less than one-third of a billion tons per year. Moreover, the amounts of different isotopes of carbon atoms found in the atmosphere confirm that the vast majority of this CO2 comes from burning fossil fuels.

Carbon dioxide is not the only greenhouse gas. Water vapor is also transparent to sunlight and absorbs infrared light. As some climate skeptics point out, there is more water vapor in the atmosphere than CO2, so why worry about CO2?

Carbon dioxide absorbs a portion of Earth's infrared light that water vapor cannot absorb, and water vapor cycles in and out of the atmosphere in days, while CO2 molecules can stick around for 100 years. Also, as temperature rises due to carbon dioxide's greenhouse impact, more water evaporates into the air, further increasing the greenhouse effect. In this way water evaporation makes carbon dioxide's global warming impact even greater.

There are still other greenhouse gases - including methane and ozone air pollution. To compare the relative warming caused by the human-generated portion of these gases, imagine Earth covered with football fields, each with an overhead array of light bulb sockets filled with 100 Watt lightbulbs. Carbon dioxide's warming effect would be like turning on 74 such lightbulbs over each football field. Methane adds 21, nitrous oxides and hydrocarbons add 22, and ozone air pollution adds 16.

Accumulated over the planet, the energy increases produced by adding all these 100 Watt lightbulbs warms the Earth significantly more than the background greenhouse effect we depend on for a comfortable life.

How much warmer?

Long-frozen mountain glaciers and the giant ice sheets atop Greenland and Antarctica are melting at increasingly faster rates. Since 1974, 95% of 829 documented physical changes on the planet have been in directions consistent with warming (see Nature, http://www.nature.com/nature/journal/v453/n7193/abs/nature06937.html).

The same study found that in 90% of over 29,500 cases of plants or animals altering the timing of their seasonal cycles or shifting their home ranges, the changes were consistent with warming.

This article appeared in the Raleigh News and Observer on Thursday, December 17, 2009.

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