What does the fact behind the stratospheric cooling tell us?
Temperatures at the Earth’s surface have increased between 0.2 and 0.4 degrees Celsius over the past 30 years. The vast majority of scientists attribute this warming trend to higher concentrations of greenhouse gases (CO2, methane, CFCs and others) that trap heat, warming both the earth’s surface and the lower atmosphere.
But one of the other apparent paradoxes of greenhouse gas is that even though it seems to warm the earth’s surface, it also cools the higher layers of the atmosphere. For example, temperatures in the lower and middle mesosphere have dropped between 5 and 10 degrees Celsius in the last three decades.
Layers of Earth’s Atmosphere
exosphere — contains few particles that move into and from space.
exobase — the lower boundary of the exosphere.
thermosphere — temperature increases with height. The temperatures can rise to 1,500 degrees Celsius, but it would not feel warm because of the low air pressure in this layer.
The International Space Station orbits Earth in this layer.
mesopause — the boundary between the mesosphere and the thermosphere; the coldest place on Earth.
mesosphere — the layer in which most meteors burn up after entering Earth’s atmosphere and before reaching Earth’s surface.
stratopause — the boundary between the mesophere and the stratosphere.
stratosphere — contains the ozone layer; the layer where volcanic gases can affect the climate.
tropopause — the boundary between the stratosphere and troposphere.
troposphere — the layer closest to Earth’s surface in which all weather occurs.
http://ete.cet.edu/gcc/?/volcanoes_layers/
The science behind the observed stratospheric cooling is complex but important to understand. Some people cite this cooling as evidence that greenhouse gases are not causing warming and that human-induced climate change is not happening. But the result seems to be the opposite. In 1989, scientists predicted that increased greenhouse gas would cool the stratosphere.
Indeed, Venus’ atmosphere is 97% carbon dioxide, and temperatures on its surface can reach 482 degrees, but it has a stratosphere 4–5 times colder than ours.
It’s also worth remembering that the heat-trapping ability of greenhouse gases supports life on Earth’s surface. If greenhouse gases didn’t trap heat, Earth’s surface temperature would average around -18 degrees Celsius. Instead, it is around 15 degrees, which is more suitable for life. As a matter of fact, this is a very delicate balance, and the increase in greenhouse gases in the atmosphere disrupts this balance.
So why is our stratosphere cooling?
Human activities affect the stratosphere in two ways:
1. Depletion of the ozone layer
2. Increased carbon dioxide
The first of these two mechanisms are easier to explain. Stratospheric ozone absorbs the ultraviolet radiation emitted by the Sun. Once absorbed, it transfers the radiation energy to the ozone molecule, causing it to heat up. By accidentally depleting the ozone layer with chlorofluorocarbons (CFCs), we have reduced its ability to absorb this energy. Now ultraviolet radiation instead of ozone passes into the lower layers of the atmosphere or the surface of the Earth itself and is absorbed there. As this warms the Earth’s surface, the outer layer of the atmosphere cools as its ability to retain heat is reduced.
The second mechanism is a bit more complex and highlights how trace gases behave differently at different pressures and densities in the atmosphere.
In the troposphere, greenhouse gases slow the dispersal of energy emitted by Earth as infrared radiation into space. They do this by capturing the outgoing heat radiation and sending it back to earth.
But in the higher and thinner layers of the atmosphere, increased carbon dioxide has a cooling effect, improving the ability of these layers to radiate heat radiation into space.
In the stratosphere, heat is transferred between molecules mostly by radiation or conduction. Conduction refers to the exchange of energy made by the collision of molecules with each other. Just like in the lower atmospheric layers, the carbon dioxide molecules here can release energy as radiation that they absorb from the repulsion.
But at these heights, the photons released in this way have a chance to escape directly into space. There isn’t much to absorb them. Thus, the cooling ability of these higher layers increases with increasing carbon dioxide.
One of the important aspects of the observed stratospheric cooling is; It disproves the idea that the “Sun” is behind the warming of the earth’s surface in the last 30 years. If increased solar activity warmed the Earth, we would expect not only the troposphere to warm but the Earth’s stratosphere as well. However, greenhouse gases, which are behind the warming of the Earth’s surface, cause the stratosphere to cool as well.
Reference: Velasquez-Manoff, M. 2009. Why is Earth’s upper atmosphere cooling?