Climate Change is Raising the Top of the Troposphere
(Inside Science) -- The troposphere, the bottom layer of Earth's atmosphere, contains most of the atmosphere's mass and clouds and is where most weather occurs. It stretches between Earth's surface and roughly 4 to 12 miles above sea level, depending on the location.
But a new paper shows that the top of the troposphere, called the tropopause, has risen about 50 to 60 meters per decade over the past 20 years, largely because of human emissions of greenhouse gases and ozone-depleting substances.
The troposphere is warmest near Earth's surface, and temperature decreases with height until the stratosphere. The stratosphere contains a layer of ozone, a gas that absorbs heat, so the air temperature starts to increase again. This turning point from decreasing to increasing temperatures is one measure of the height of the tropopause, said Pu Lin, an atmospheric scientist at Princeton University in New Jersey who did not contribute to the new paper.
The tropopause's height can be influenced by things like volcanic eruptions and typhoons, said Jane Liu, one of the new paper's authors and an atmospheric scientist at the University of Toronto. "But in the long run, the two most important factors are tropospheric temperature and stratospheric temperature," she said.
Human activities can affect these two temperatures. By extracting and burning fossil fuels, we release greenhouse gases, like carbon dioxide and methane, into the troposphere, where they trap heat and warm the air. Until the late 1980s, humans were also releasing more and more ozone-depleting chemicals, which had been commonly used in refrigerators and air conditioners. These chemicals cool the stratosphere by destroying the stratospheric ozone layer.
A warmer upper troposphere and cooler lower stratosphere cause their border, the tropopause, to rise. A new paper, published online Nov. 5 in the journal Science Advances, found that the tropopause rose about 50 meters per decade due to human activity between 1980 and 2000 and at a similar rate between 2000 and 2020. The authors estimated human activity accounted for roughly 80% of the total increase in tropopause height during that time.
Ben Santer, an atmospheric scientist at the Joint Institute for Regional Earth System Science and Engineering at the University of California, Los Angeles, said the paper is "another piece of the puzzle, if you will, pointing towards the reality of human-caused warming of the troposphere and cooling of the stratosphere." Santer did not contribute to the research.
Liu and her colleagues measured the height of the tropopause between 20 and 80 degrees north latitude, a band stretching from Cuba to Greenland, using temperature data from weather balloons. Scientists release these balloons twice a day from hundreds of locations worldwide. The balloons carry small boxes that measure temperature, pressure, humidity and wind, and they transmit this data back to Earth using radio signals.
The paper also used a GPS satellite dataset and other weather balloon datasets to measure the tropopause height increase. These measurements varied from 40 to 80 meters of increase per decade. While the measurements differ in total tropopause height increase, they all show a significant increase, a similar increase from 2000 to 2020 compared to 1980 to 2000, and that human activity accounted for most of the total height increase.
"All the observational data over this four-decade period of time tell us the same thing," said Liu. "This is a clear, robust and strong indication of the tropopause height increase."
The researchers also showed that tropospheric warming has become more important to the tropopause height increase over time. Greenhouse gas emissions rose between 1980 and 2020, and due to these additional emissions, the troposphere warmed at a faster rate between 2000 and 2020 than between 1980 and 2000. At the same time, emissions of ozone-depleting substances have plummeted, and the stratosphere's average cooling rate was significantly lower between 2000 and 2020 compared to between 1980 to 2000.
As a result, the researchers found that most of the tropopause height increase (about 70%) in the last 20 years could be attributed to the warming troposphere. This was a significant change from the period between 1980 and 2000, when tropospheric warming and stratospheric cooling contributed roughly equally to the tropopause height increase. "I personally find that the most interesting part [of the paper]," Lin said.
Humanity's impact on the tropopause can have consequences for the whole climate system. For example, Lin said, the tropopause temperature affects the amount of water vapor in the stratosphere, which in turn influences Earth's surface temperature.
Liu said climate change textbooks commonly include changes happening at Earth's surface, like sea level rise and glacial retreat, as indicators of human-caused global warming's effect. "The change in the tropopause height far away, at about a 10-kilometer distance, also can be included in those textbooks," she said.