While greenhouse gases heat the Earth’s surface, they also cause rapid cooling far above us, at the edge of space.
In fact, the upper atmosphere, about 90 kilometers (56 miles) above Antarctica, cools 10 times faster than the average warming on the planet’s surface.
Our new research accurately measured this cooling rate and revealed an important discovery: a new four-year temperature cycle in a polar atmosphere. The results, based on measurements from 24 years of continuous Australian scientists in Antarctica, were published in two articles this month.
The findings show that the Earth’s upper atmosphere in a region called the “mesosphere” is particularly sensitive to rising concentrations of greenhouse gases. This provides a new opportunity to monitor how well government interventions are working to reduce emissions.
Our project also observes a spectacular natural phenomenon called noctilucent or night shining clouds. Although beautiful, more often these clouds are considered a bad sign of climate change.
Since the 1990s, scientists at the Davis Research Station in Australia have conducted measurements of more than 600,000 temperatures in the upper atmosphere above Antarctica. We did this using sensitive optical devices called spectrometers.
These devices analyze infrared light emitted by so-called hydroxyl molecules located in a thin layer about 87 km (54 miles) above the Earth’s surface. This “air flow” allows you to measure the temperature in this part of the atmosphere.
Our results show that at high atmospheres above Antarctica, carbon dioxide and other greenhouse gases do not have the warming effect they do in the lower atmosphere (in contact with other molecules). Instead, excess energy is radiated into space, causing a cooling effect.
Our new research more accurately determines this cooling rate. In 24 years, the upper atmospheric temperature has cooled by about 3 degrees C or 1.2 degrees C in a decade. This is about ten times more than the average warming in the lower atmosphere, about 1.3 degrees C over the last century.
Natural signals are not activated
Rising greenhouse gas emissions are contributing to the temperature changes we are recording, but many other factors are also important. This includes the seasonal cycle (warmer winters, colder summers) and the 11-year solar activity cycle (which includes quieter and more intense periods of the sun) in the mesosphere.
One of the challenges of the study was to disconnect all of these combined “signals” and to determine the extent to which each of them led to the changes we observed.
By observing this process, we discovered a new natural cycle not previously identified in the polar upper atmosphere. This four-year cycle, which we called quasi-quadrennial oscillation (QQO), has seen temperatures in the upper atmosphere vary by 3 to 4 degrees C.
Discovering this cycle was like tripping over a nugget of gold under a well-done claim. More needs to be done to determine its origins and full significance.
However, the findings have a significant impact on climate modeling. The physics driving this cycle are unlikely to be incorporated into the global models currently used to predict climate change. But a 3-4 degree C change every four years is a big signal to ignore.
We don’t know yet what drives the vibration. Despite the response, this also appears to affect wind, sea surface temperature, atmospheric pressure, and sea ice concentrations around Antarctica.
The ‘night shines’ clouds
Our study also observes how the cooling temperature influences the formation of night or “night-lit” clouds.
Night clouds are very rare – since 1998. At Australian Antarctic stations, we recorded about ten observations. They occur in the polar regions at an altitude of about 80 kilometers (50 miles) in summer. You can only see them from the ground when the sun is below the horizon at dusk, but still shines in a high atmosphere.
The clouds look like thin, light blue, wavy threads. They consist of ice crystals and require a temperature of approximately minus 130 degrees C (266 F). While unlit clouds are impressive, they are seen as a “channel for coal change” in climate change. As greenhouse gas emissions continue to cool the upper atmosphere, it is likely that dirty clouds will become more frequent.
There is already evidence that clouds are becoming brighter and more widespread in the northern hemisphere.
Measurement of change
Man-made climate change can radically change living conditions on our planet. Over the next few decades, in less than a lifetime, average global temperatures are expected to rise, leading to rising sea levels, changes in extreme weather conditions and changes in global ecosystems.
Long-term monitoring is important to assess change and to test and calibrate increasingly complex climate models. Our results contribute to a global observation network coordinated for this purpose by the Mesosphere Change Detection Network.
The accuracy of these models is crucial in determining whether governments and other interventions to curb climate change are actually effective.
John French, atmospheric physicist at the University of Tasmania; Andrew Klekociuk, Chief Research Fellow and Senior Lecturer, University of Tasmania, Australia, and Frank Mulligan, National University of Maynooth, Ireland.
This article has been republished in The Conversation under a Creative Commons license. Read the original article.