Against all odds, the upper limit of our troposphere is still moving upwards.

Our planet is surrounded by a relatively thin layer of gases: the Earth’s atmosphere. And that atmosphere can again be divided into different layers. The bottom layer is the troposphere. It is bounded at the top – about 8 to 15 kilometers from the Earth’s surface – by the tropopause: an imaginary boundary that marks the transition between the troposphere and the stratosphere above it. New research now reveals that that tropopause has steadily increased since 2000. So the troposphere has grown.

weather balloon

Scientists reached this conclusion after using a weather balloon to determine the height of the tropopause (see box). The measurements indicate that the troposphere has become about 100 to 120 meters thicker between 2001 and 2020. And that can be traced back to climate change, the researchers say in the magazine Science Advances.

How do you determine where the tropopause is?
The tropopause is the boundary between the troposphere and the stratosphere above it. Those two layers in the atmosphere are clearly distinct from each other. “In the troposphere, the higher you go, the air temperature gets lower,” said Jane Liu, an atmospheric researcher and one of the authors of the study. Science Advances has appeared. In the stratosphere it is exactly the other way around: the higher you go in the stratosphere, the higher the temperatures are. “The tropopause is therefore the limit at which the temperature no longer decreases, but increases as you go up. In other words, it is the point at which air temperature reaches a minimum in both the troposphere and the stratosphere.”

Variations

Scientists have known for some time that the tropopause is not fixed. “When long-term temperatures in the troposphere and stratosphere are stable, the height of the tropopause is also more or less stable,” explains Liu. “The latter was the case before the 1960s.” But in the long run, temperature variations in the troposphere and stratosphere can cause the height of the tropopause to vary. “In addition, there are two ways in which human activities can influence temperature changes in the troposphere and stratosphere.” The first is through the release of ozone-destroying substances that – as the name suggests – do away with ozone in the stratosphere. “That leads to a decrease in temperature in the stratosphere.” In addition, we mess with the temperature of the troposphere and stratosphere due to the emission of greenhouse gases. “They cause warming in the troposphere and cooling in the stratosphere. And both tropospheric warming and stratospheric cooling can lead to an increase in tropopause.”

Unexpected result

Previous research has already shown that the tropopause has moved upwards between 1980 and 2000. What the study by Liu and colleagues now shows is that the tropopause has moved upwards at the same rate in the 20 years since (between 2000 and 2020). And that is quite unexpected. “With the implementation of the Montreal Protocol in the 1990s, the amount of ozone in the stratosphere started to recover slowly,” explains Liu. “As a result, the stratospheric cooling decreased after 2000. Some of the previous studies therefore predicted that the tropopause would continue to rise after 2000, but that the increase would be much slower than before 2000. We also expected that prior to our study. Therefore, it is quite surprising that the rate at which the tropopause has advanced after 2000 is comparable to the rate at which it advanced before 2000.”

Pre workout

Because stratospheric cooling has decreased after 2000, the continued climb of the tropopause after 2000 must be the result of tropospheric warming, the researchers say. “Assuming that the temperature in the stratosphere does not change and the rate at which the temperature in the troposphere decreases with height has remained the same (i.e. about 6.5 degrees per kilometer), in a warmer troposphere you need more height to reach the point. ranges where the lowest temperature in the troposphere corresponds to the lowest temperature in the stratosphere.”

As the temperature in the troposphere rises, the point at which the troposphere and stratosphere meet (which is also the point at which air temperatures in both the troposphere and stratosphere are lowest) will rise. Image: Jane Liu.

Weather

And so a higher tropopause is a sign of global warming. But does the expansion of the troposphere also have consequences, for example for the weather that takes place for the most part in this atmospheric layer? We asked Liu. “Research on the long-term impact of the increase in tropopause on climate and weather is very limited. We do see in the literature that links can be drawn between an increase in the tropopause and changes in weather, climate and atmospheric circulations. For example, it has already been suggested that the rise of the tropopause is the driving force behind the poleward and more powerful movements of the jet streams in the mid-latitudes.”

It will be clearer after this research how the tropopause will fare in the future. “Based on our study, as we continue to pump greenhouse gases into the atmosphere, the troposphere will continue to warm and the tropopause will continue to rise.” That is certainly not desirable. “If the height of the tropopause continues to increase, even if it all takes place at 10 to 20 kilometers altitude, this will eventually have consequences for the climate and again near the surface,” warns Liu.