Satellite data show a cooling trend in the upper atmosphere. So much for global warming…. Right?
If you lived above 35,000 feet in the stratosphere, you would have experienced cooling temperatures over the past few decades. In fact, stratospheric temperatures reached a record low in 2016. Clearly there is no global warming, right? Well, it depends on your definition.
Global warming typically means long-term increases in temperatures near the surface of the earth, where we live. While stratospheric temperatures were at a minimum in 2016, the near-surface temperature reached a record high. So as it turns out, surface global warming is still occurring, but why is the upper atmosphere cooling? The answer is twofold.
First, stratospheric cooling over the past 40 years can be partially attributed to human emissions of ozone-depleting substances such as refrigerants. The ozone layer resides in the stratosphere and it absorbs solar UV radiation. This is important because it protects us from sun-related injuries, such as skin cancer. The Montreal Protocol was therefore signed in 1987 to require a phaseout of ozone-depleting substances. Although the production and consumption of such substances has now come to a halt, some damage has already been done and it will likely take decades for stratospheric ozone levels to return to normal.
Before ozone-depleting substances were regulated, the amount of ozone steadily decreased in the stratosphere. As the ozone decreased, so did the absorption of UV radiation, resulting in a cooling of the stratosphere. Now that the ozone layer is on the rebound, one might expect stratospheric temperatures to start increasing. Ozone depletion, however, is only part of the stratospheric cooling puzzle.
GREENHOUSE GAS EMISSIONS
Assuming you do not live on an airplane, you probably live in the troposphere. This is the lowermost layer of our atmosphere. It extends about 7 miles above the surface of the earth and it is where all the action happens since our weather is confined to this layer. In addition, about 80% of the mass in our atmosphere resides here, including most of the greenhouse gasses.
Much of the energy we receive from the sun passes right through the troposphere and hits the surface of the earth. The earth then warms up and emits energy back out into space. Without an atmosphere—or more specially, without greenhouse gases in our atmosphere—our average temperature at the surface would be well below freezing. Thankfully, we have a blanket of gases that keep some of this energy from escaping into space.
We can think of our troposphere as an insulating blanket that keeps energy close to the surface of the earth. However, by increasing the amount of greenhouse gases in our atmosphere, primarily in the form of CO2, our troposphere is quickly becoming a more efficient blanket. While the amount of energy that enters our climate system from the sun has remained relatively constant, the amount of energy that is allowed to escape to space is decreasing. The result? More energy is sticking around near the surface of the earth as shown by increasing temperatures in the lower part of the troposphere.
So why do scientist expect temperatures to cool in the upper part of the atmosphere when they are warming at the surface? Considering that about 80% of the atmosphere is located within the troposphere, we can think of the stratosphere as sitting on top of our atmospheric blanket.
Let's consider how temperatures might change below and above our atmospheric blanket as it becomes more efficient at trapping heat. To do this, think about covering yourself with two different blankets on a cold winter night:
If you use a very thin blanket, you would likely feel cold underneath and your body heat could easily be felt by someone placing a hand on the outside. This is because some of your body's heat energy is allowed to escape through the blanket.
If you use a thick blanket, you would feel warm underneath and your body heat might not be felt by someone placing a hand on the outside. This is because more of the energy remains near the source.
By adding greenhouse gasses to our atmosphere over time, we are making our blanket more efficient, resulting in lower-level warming and upper-level cooling.
Importantly, the cooling of our upper atmosphere is further evidence of the human fingerprint on our climate, particularly with respect to global warming. If surface warming were resulting from increasing solar energy, we would actually expect the upper atmosphere to be warming too!