Growing concentrations of carbon dioxide (CO2) in the Earth’s upper atmosphere from human activities can be expected to induce a gradual contraction of the thermosphere, a change that will lower the drag on satellites but also diminish a natural destructive force for the re-entry of man-made orbital debris, according to a new study.
The study from a U.S. and Canadian science team led by the Naval Research Laboratory offers the first direct evidence that CO2 emissions from human activity are rising to the highest reaches of the atmosphere, including the thermosphere, or above 90 km (56 mi.) altitude.
The findings, published in Nature Geoscience on Nov. 11, are based on observations made with the Atmospheric Chemistry Experiment Fourier Transform Spectrometer aboard the Canadian SciSat-1 satellite between 2004 and 2012.
The observations confirm suspicions that concentrations of CO2 originating from human activity are increasing through the entire atmosphere, not just in the troposphere at 15 km and below. Prior to the study of SciSat-1 data, CO2 trends had not been measured above 35 km, the study team led by NRL’s John Emmert reports.
According to study estimates, concentrations of CO2 and carbon monoxide (CO) in the thermosphere are growing at a rate of 23.5 parts per million, plus or minus 6.3 parts ppm, each decade, which is about 10 ppm per decade faster than predicted by current simulation models of the upper atmosphere.
In the troposphere, concentrations of the greenhouse gas are increasing at a rate of about 20 ppm per decade. CO2 levels near the ground are about 390 ppm, and the study team believes that changes in the vertical mixing and transport of the atmosphere are responsible for the increased levels of carbon at higher attitude.
At lower altitudes, CO2 absorbs outbound infrared radiation (IR) and re-emits the heat toward the Earth’s surface. At higher altitudes, the much lower concentrations of carbon molecules are essentially transparent to IR. Instead, C02 and C0 react with atomic oxygen for thermal effect, releasing heat energy to space, cooling the thermosphere.