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January-February 2009

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Weather Queries





I read in the Wall Street Journal that carbon dioxide (CO2) is not officially a greenhouse gas, and I have also heard it on the radio. But most Internet sources say that CO2 is a greenhouse gas. Can I get a definitive answer without global warming politics? 
Ed Marsh,
Fresno, California


I thank Charles A. Brock of NOAA's Earth Systems Research Laboratory for help in answering this question.

The ultimate source of energy for driving the atmospheric winds is the Sun. Of the energy from the Sun that reaches the top of the atmosphere, a little less than half is absorbed at the Earth’s surface, and another 30 percent is reflected back to space by the Earth’s surface, clouds, or air. Almost all of the Sun’s energy is transmitted at wavelengths less than 4.0 micrometers (μm). The Earth’s surface, warmed during the day by the Sun, is continually radiating energy upward, almost all of it at infrared wavelengths greater than 4.0 μm. For this reason, 4.0 μm is a convenient dividing line for separating solar radiation from terrestrial radiation. A greenhouse gas is one that is fairly transparent to solar radiation but efficient at absorbing infrared radiation. In other words, greenhouse gases let the Sun’s radiant energy pass through the atmosphere with little depletion (absorption), but they strongly absorb ground radiation trying to escape to outer space.

Different greenhouse gases absorb infrared radiation at different wavelengths, but all keep the Earth and its atmosphere considerably warmer than it would be without them. They are effective despite very low concentrations in the atmosphere. The most plentiful greenhouse gas by far, and also the most highly variable, is water vapor (H2O). Even in a warm and very moist atmosphere, water vapor comprises at most 3 percent of the mass of air. The other important greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbon gases, principally chloroflourocarbons, and ozone (O3).

Because your question referred specifically to carbon dioxide, we’ll concentrate on this gas. The diagram shows carbon dioxide measurements taken near the top of Mauna Loa in Hawaii. This site was chosen because it is far removed from sources of pollution, and the exceptionally clean air allows for accurate measurements of background values of atmospheric CO2. The undulating red curve shows seasonal variations of CO2 superposed upon the smoothed black curve. The steady rise in concentration is obvious, but if you put a ruler to the black curve, you will quickly be convinced that the increase is accelerating with each passing decade. In 2005, the concentration stood at 379 parts per million by volume (ppm). At the beginning of the industrial era (1750), it was only about 280 ppm.

Carbon dioxide has been a component gas in the atmosphere for much of the Earth’s geological history. Natural sources are volcanoes, decaying plant life, respiration by mammals, and hot springs and geysers. Anthropogenic (human-generated) sources—in particular, the burning of fossil fuels, which accounts for about two-thirds of the total—are the main cause of the increase in CO2 concentrations, especially in the past century. During the years 2000-2005, an average of 7 billion tons of carbon per year were emitted as a result of fossil fuel burning throughout the world. Since 1980, about half of the anthropogenic emissions have been taken up by the oceans, which are growing slowly more acidic, and by plant life.

The evidence is incontrovertible that the concentration of CO2 in the atmosphere is rising at an accelerating rate, and virtually no one questions any more whether the Earth’s surface and its atmosphere are warming. Only a small and shrinking minority are skeptical about the connection between the two. The Intergovernmental Panel on Climate Change (IPCC) issued a report last year titled “Climate Change 2007: The Physical Science Basis,” that, in my view, is an invaluable and authoritative compendium of what is known and not known about climate change.

This report was compiled by 152 lead authors from more than 30 countries and reviewed by more than 600 experts. A summary was approved by officials from 113 countries. A major conclusion of this report with regard to greenhouse gases is that “it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750.”

In the parlance of the IPCC reports, “extremely likely” means with a probability greater than 95 percent.

The recent wildfires in California had some interesting effects on the National Weather Service (NWS) forecasts of maximum temperature. According to the Sacramento, California, Area Forecast Discussion from August 11, 2008: “The fly in the ointment will be how much smoke from the wildfires that continue to burn in the mountains of far northern California gets trapped below the subsidence inversion and whether it inhibits [solar heating] much. This has been a significant factor during earlier heat episodes this summer.” Sacramento is 75 miles inland from the Pacific Ocean in the middle of the Central Valley, which traps smoke from the fires. It seemed that actual maximum temperatures were as much as 10°F below what the computer models predicted. The local forecasters did better, but their job was difficult because the thickness of the smoke varied widely across the area. My questions are: How much and for how long does the smoke affect temperatures or the weather? Does some of the smoke get as far as Denver?
Arnold Harding,
Elk Grove, California


I’ll provide a qualitative answer to your questions, because a quantitative answer would require some serious calculations. The amount of surface cooling caused by the smoke depends upon the thickness of the smoke, the fuel type (influencing the color of the smoke from sooty black to nearly white), the elevation angle of the Sun (a low Sun is less able to penetrate the smoke than a high Sun), and the albedo of the surface. Surface albedo measures how much sunlight the surface reflects or, equivalently, how bright it is. For example, fresh snow reflects most solar radiation, while a dark sea surface reflects very little.

Smoke is thickest near the source. If the lapse rate (decrease of temperature with altitude) is near 5.4°F per 1,000 ft, then vertical mixing is facilitated, and the smoke disperses quickly. On the other hand, if the lapse rate is stable—in particular, if there is an elevated inversion (a layer where the temperature increases with height)—the smoke may be trapped below the inversion and travel many tens of kilometers before dispersing much, either laterally or vertically. The subsidence inversion mentioned by the forecaster is one such example. Where the smoke is thick enough to dim or even block out the Sun, the maximum surface temperature could easily be depressed by 5 to 8°F.

The smoke particles (aerosols) scatter solar energy back toward the sky, thus diminishing the amount that reaches the ground. Some smoke particles like soot absorb sunlight effectively, thus further reducing the energy reaching the ground, but there are secondary effects on clouds. The absorption of solar energy by aerosols causes heating of the layer where the smoke resides. It is well documented that this heating suppresses the development of small cumulus clouds when conditions are otherwise ripe for their formation. Photographs over the Amazon Basin show plumes of smoke in cloud-free air surrounded on either side by cumulus clouds. Surface cooling combined with warming in the smoke layer leads to a decrease in the lapse rate and suppression of the convective updrafts that build cumulus clouds.

Clouds that do develop in air laden with aerosols contain cloud droplets that are smaller and more numerous than the droplets in clouds that develop in clean air. The former clouds are less likely to precipitate than the latter, and they reflect more sunlight. This is another way in which smoke can indirectly affect the weather.

For a given amount of smoke, its cooling effect at the surface diminishes as the surface albedo increases. For example, smoke depresses the maximum surface temperature less over a snow-covered surface than over a dark, plowed field. The effect on heating within the smoke layer is the opposite because solar energy reflected from the ground is absorbed by smoke particles as well as that coming directly from the Sun. Thus there is more heating in the smoke layer when the surface albedo is high than when it is low.

Readers who want more technical details should consult a paper by R. S. Stone and coauthors, “Radiative Impact of Boreal Smoke in the Arctic: Observed and Modeled,” published in 2008 in the Journal of Geophysical Research, Vol. 113, Paper D14S16.

Smoke plumes can easily travel 1,000 miles, producing a very noticeable haze, even at that distance. I remember seeing smoke from the great Yellowstone forest fires in the fall of 1988. On several days the plume passed over the Denver area, producing red, murky sunrises and sunsets.                                        

Weatherwise Contributing Editor THOMAS SCHLATTER is a meteorologist at NOAA’s Earth System Research Laboratory in Boulder, Colorado. He is also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado. Submit queries to the author in care of Weatherwise; 1319 18th St. NW; Washington, D.C. 20036; or by email to ww@heldref.org.

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