Consistent and continuous observational records of air temperatures are critical for improving understanding of climate change and related impacts on our weather, as well as evaluating climate model simulations. However, obtaining consistent results from data records that span multiple types of observing systems has been challenging, limiting our ability to use the observations and realize their full value.
For the first time, researchers supported by CPO's Climate Observations and Monitoring Program and led by the University of Graz in Austria have been able to show consistently how temperatures in the troposphere and stratosphere have changed over the past four decades using the latest available observational records. Their findings are published in the current issue of the Journal of Climate.
“The troposphere, which is the layer nearest to Earth where weather conditions take place, has warmed by between 0.25 and 0.35 K every decade since the 2000s,” says Andrea Steiner from the Wegener Center for Climate and Global Change at the University of Graz, summarizing one of the findings. Going back to 1979, the troposphere has warmed a total of 0.6 to 0.8 K.
“The temperature trends in the upper troposphere, particularly around the tropics, are even more marked than at the Earth’s surface,” point out Steiner and her colleague Florian Ladstädter from University of Graz. At the Earth’s surface, trends are estimated to be increasing between 0.11 and 0.14 K per decade.
On the other hand, in the stratosphere, which lies above the troposphere, temperatures have been steadily falling by 1-3 K over the last 40 years. This cooling is caused by the rise in greenhouse gases which, in the stratosphere, re-emit thermal radiation from the Earth into space. Since the late 1990s, however, cooling of the lower stratosphere has slowed as the ozone layer has begun to repair itself. Ozone absorbs ultraviolet radiation and heats the stratosphere, counteracting the cooling effect from rising greenhouse gases.
The study findings have some far-reaching consequences. Changes in the temperature of these layers influence complex interactions with the Earth’s surface and the oceans, affecting large-scale circulation patterns that shape our global weather patterns. And since, according to the authors, the warming trends in the troposphere are associated with increased greenhouse gases, a significant reduction in carbon dioxide remains essential to prevent related impacts from these changes like extreme events.
Forming a consistent picture with a range of measurements
For their study, the research team evaluated and compared an extremely diverse range of data recorded at different altitudes from the late 1970s onwards. Sources included weather satellites, GPS radio measurements, and ground-based measurements.
“Instruments onboard NOAA polar-orbiting weather satellites played a key role in measuring global temperatures from the lower atmosphere to the upper stratosphere for trend estimates during the last four decades,” said Cheng-Zhi Zou, study co-author and scientist with NOAA’s Satellite and Information Service.
The fifth assessment from the Intergovernmental Panel on Climate Change noted that there was “only medium to low confidence in the rate of change of tropospheric warming...and low confidence in the rate...of stratospheric cooling.” Thanks to this study, the range of measurements came together to form a consistent picture, showing us that long-term observational data is essential for climate research. The findings fill an important gap in our understanding of air temperature trends, while advancing our ability to use atmospheric observations for climate applications.
This study is part of the activities from an international working group called ‘Atmospheric Temperature Changes and Their Drivers’ which is within the framework of the World Climate Research Program/Stratosphere-troposphere Processes and Their Role in Climate.