As greenhouse gases warm the earth’s surface, they also cause rapid cooling far above us, at the end of space.
In fact, the upper atmosphere about 90 kilometers above Antarctica is cooling at a rate 10 times faster than the average warming of the planet’s surface.
Our new research accurately measures this cooling rate and reveals an important discovery: a new four-year temperature cycle in the polar atmosphere. The results, based on 24 years of continuous measurements by Australian scientists in Antarctica, were published in two papers this month.
The results show that the Earth’s upper atmosphere, in a region called the “mesosphere”, is extremely sensitive to rising concentrations of greenhouse gases. This provides a new opportunity to monitor how well government interventions to reduce emissions are working.
Our project also monitors the spectacular natural phenomenon known as “night lights”
Study of “air light”
Since the 1990s, scientists at Australia’s Davis Research Station have made more than 600,000 measurements of temperatures in the upper atmosphere over Antarctica. We did this with the help of sensitive optical instruments called spectrometers.
These instruments analyze the infrared glow emitted by so-called hydroxyl molecules, which exist in a thin layer about 87 kilometers (54 miles) above the earth’s surface. This “air radiation” allows us to measure the temperature in this part of the atmosphere.
Our results show that in the high atmosphere above Antarctica, carbon dioxide and other greenhouse gases do not have the warming effect they do in the lower atmosphere (by colliding with other molecules). Instead, excess energy is radiated to the space, which causes a cooling effect.
Our new research more accurately determines this cooling rate. In 24 years, the upper atmosphere has cooled by about 3 degrees C or 1.2 degrees C per decade. This is about ten times higher than the average warming in the lower atmosphere – about 1.3 degrees C in the last century.
Unraveling of natural signals
The increase in greenhouse gas emissions contributes to the temperature changes we have registered, but a number of other influences also play a role. These include the seasonal cycle (warmer in winter, colder in summer) and the 11-year cycle of the Sun (which includes quieter and more intense solar periods) in the mesosphere.
One challenge of the study was to unravel all of these combined “signals” to determine the extent to which each of them drove the changes we observed.
Surprisingly, in this process we found a new natural cycle that had not been previously identified in the polar upper atmosphere. In this four-year cycle, which we called the Quasi-Four-Year Oscillation (QQO), the saw’s temperatures vary by 3-4 degrees in the upper atmosphere.
The discovery of this cycle was like stumbling a gold nugget in a well-crafted claim. More work is needed to determine its origin and full meaning.
But the discovery is important for climate modeling. The physics that drive this cycle are unlikely to be included in the global models currently used to predict climate change. But a deviation of 3-4 degrees C every four years is a big signal to ignore.
We still don’t know what drives the oscillations. But whatever the answer, it also seems to affect winds, sea surface temperatures, atmospheric pressure and sea ice concentrations around Antarctica.
“Night glowing” clouds
Our study also monitors how cooling temperatures affect the appearance of night clouds or “night shining” clouds.
Concentrated clouds are very rare – we have recorded about ten observations from Australian Antarctic stations since 1998. They appear at an altitude of about 80 kilometers (50 miles) in the polar regions during the summer. You can see them from the ground only when the sun is below the horizon at dusk, but still shines in the high atmosphere.
Clouds appear as thin, pale blue, wavy threads. They consist of ice crystals and need temperatures around minus 130 degrees C (266 F) to form. Although impressive, night clouds are considered the “coal canary” of climate change. Further cooling of the upper atmosphere as a result of greenhouse gas emissions is likely to lead to more frequent night clouds.
There is already some evidence that clouds are becoming brighter and more widespread in the Northern Hemisphere.
Man-made climate change threatens to radically change the living conditions on our planet. Over the next few decades – less than a lifetime – the average global air temperature is expected to rise, which will lead to rising sea levels, extreme conditions and changes in ecosystems around the world.
Long-term monitoring is important for measuring change and testing and calibrating increasingly complex climate models. Our results contribute to a global network of observations coordinated by the Mesosphere Change Detection Network for this purpose.
The accuracy of these models is crucial in determining whether government and other interventions to limit climate change are truly effective.
John France, atmospheric physicist, University of Tasmania; Andrew Klekociuk, Principal Investigator, Australian Antarctic Division and Senior Lecturer, University of Tasmania, and Frank Mulligan, National University of Ireland, Maine.
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