Human evidence: Cooling in the higher atmosphere
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Measurements show that the stratosphere and some higher atmospheric levels have cooled while the surface and lower atmosphere have warmed. This vertical pattern of contrasting temperature trends is consistent with what scientists expect from global warming caused by increasing levels of greenhouse gases such as carbon dioxide in the atmosphere.
Scientists measure temperature changes in the stratosphere using data from instruments carried on board satellites and weather balloons. Unfortunately, there are some uncertainties regarding the size of the temperature changes they’ve managed to observe because of the difficulties involved in measuring temperatures in the higher levels of the atmosphere. However, it is clear that both satellite and weather balloon measurements show warming trends in the lower atmosphere (troposphere) and cooling trends in the stratosphere. This is consistent with what scientists expected from global warming caused by increased levels of greenhouse gases in the atmosphere.
Scientists have carried out studies using climate models to simulate the physical principles governing the behaviour of the atmosphere. The results predict that higher concentrations of atmospheric greenhouse gases will cause more heat to be trapped near the Earth’s surface, which in turn will boost the natural greenhouse effect. The increased total greenhouse effect causes higher levels of the atmosphere – such as the stratosphere – to cool while the lower atmosphere and the surface grow warmer. This is a ‘signature’ of global warming driven by greenhouse gases. If the Sun were simply giving off more energy, we’d expected to see warming in the stratosphere as well as the troposphere, but all the evidence suggests that this is not happening.
The troposphere is the lowest atmospheric layer, where most weather occurs. The top of this is some 9–18 km above our heads, depending on latitude, and is far colder than the Earth’s surface because, on average, temperatures decrease with altitude in the troposphere. Above this level is the stratosphere, in which it gets warmer with height. This is because the incoming ultraviolet component of solar energy sets off chemical reactions involving the continual destruction and recreation of ozone, releasing heat in the stratosphere. Another important process for stratospheric temperature is that it is also affected from below by energy given off by the Earth’s surface.