Carbon emissions make satellites move faster
It's a non-intuitive consequence of CO2 emissions and climate change.
by Scott K. Johnson
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When you think about CO2 emissions from the use of fossil fuels, you probably think about climate change. Temperature. Maybe precipitation patterns. Storms. But do you think about managing the orbital mechanics of satellites?
As humans, we really only experience the lower atmosphere - the troposphere and stratosphere that extend about 30 miles from the surface. That's where nearly all of the gas in the atmosphere resides, and that's where weather happens. Even Felix Baumgartner, the daredevil skydiver, "only" jumped from the stratosphere. But technically, the atmosphere extends a whole lot higher than that. It's another 150 miles or so before we truly reach interplanetary space. A number of satellites, as well as the International Space Station, are actually whizzing around in a layer of the atmosphere called the thermosphere.
Down in the troposphere, CO2 is an important greenhouse gas. Add more CO2, and you trap more outgoing heat, warming the lower atmosphere. But up in the thermosphere, things are much different. Gas molecules are incredibly sparse - and increasingly so as you head outward from the Earth. Here, CO2 is actually a key coolant, as it absorbs energy from collisions with oxygen molecules, and then emits that energy as infrared radiation, sending much of it out into space.
And when the thermosphere cools, it contracts. That results in fewer molecules in the orbital paths of satellites, meaning the drag slowing their motion decreases. This phenomenon made news in 2010, when the thermosphere contracted to its smallest extent in 40 years of record. That was caused by low solar activity, but researchers noted that this didn't fully explain the low point, suggesting that rising CO2 concentrations may have been involved in the long-term trend.
One of those researchers (along with several of his colleagues) has now published the results of an analysis of CO2 in the thermosphere. It's a difficult thing to measure, but they used data the Canadian SCISAT-1 satellite collected by observing the setting Sun through the thermosphere above the horizon from 2004 to 2012. To untangle trends from variability caused by solar activity, they combined measurements of carbon monoxide and carbon dioxide. Ultraviolet light splits a portion of the CO2 into CO, which can be oxidized to turn back into CO2.
Added together, the trend becomes clear - CO2 in the thermosphere increased at a rate of 23.5 ± 6.3 parts per million each decade. That's much more than was previously thought, and it appears to account for the trend toward contraction of the thermosphere.
The researchers say that this unexpectedly large increase indicates that more mixing takes place between the thermosphere and the lower atmosphere (where anthropogenic CO2 is accumulating) than we realized. When they increased that vertical mixing in an atmospheric climate model, the behavior of the thermosphere matched the new observations.
This story illustrates some of the non-intuitive consequences of CO2 emissions and climate change. While politicians continue to debate the reality of climate change, the professionals who manage satellites show up to work and get down to the practical details of dealing with that reality. In this case, that means accounting for fine-scale modifications to the motion of satellites and space debris caused by a changing climate system. That will require continued monitoring of thermospheric CO2 to see if the upward trend observed over the past eight years continues smoothly or is altered by vertical mixing behavior or other processes.
Nature Geoscience, 2012. DOI: 10.1038/NGEO1626 (About DOIs).
Scott K. Johnson / Scott is a hydrogeologist and educator. He joined the Ars science crew in 2011.