We know that humans are causing Earth’s climate to change. It used to be that “climate change” mostly referred to increasing temperatures near the Earth’s surface, but increasingly, climate change has come to mean so much more. It means warming oceans, melting ice, changing weather patterns, increased storms, and warming in other places.

A recent study has just been published that finds ‘fingerprints’ of human-caused warming someplace most of us don’t think about – in the higher atmosphere. Not only that, but these scientists have found changes to the seasonal climate – how much the temperature varies from winter to summer to winter – and the changes they found matched expectations.

The paper was authored by a top group of scientists including Benjamin Santer, Stephen Po-Chedley, Mark Zelinka, Ivana Cvijanovic, Celine Bonfils and Paul Durack from Lawrence Livermore National Laboratory; Carl Mears and Frank Wentz from Remote Sensing Systems; Qiang Fu from the University of Washington; Jeffrey Kiehl from the University of California, Santa Cruz; Susan Solomon from MIT; and Cheng-Zhi Zou from the University of Maryland. These are literally the best of the best climate scientists studying Earth’s atmosphere.

So, how did the scientists carry out their research? First of all, temperature measurements were made throughout the atmosphere using satellites. These satellites fly across the skies and take continuous measurements, so we get a picture of the whole planet. This is different from using a weather balloon that measures temperature only where the balloon flies. Satellites give a nearly continuous picture of what is happening everywhere.

While all that is great, satellites are prone to errors that have to be accounted for. For instance, their orbits change over time, which affects the accuracy of the measurements. They also have issues with a process called calibration. Finally, whenever a new satellite is launched, it may not behave the same as the satellite it replaced. While all of these issues can be dealt with, it’s a challenge to do so. But regardless, satellites are a very useful tool for understanding global changes in climate.

The authors compared these satellite temperature measurements with the “human influence” fingerprint patterns from computer models of the climate system. The fingerprints were based on computer calculations made with human-caused changes in greenhouse gas levels in the atmosphere.

The “human influence” fingerprints were also compared with results from “no human influence” calculations, which did not have any year-to-year changes in human factors. Comparing these two sets of calculations allowed the researchers to determine whether the “human influence” fingerprint matched the satellite data, and whether such a fingerprint match could be due to natural climate variability alone.

In particular, the researchers wanted to know whether they could detect a human fingerprint in the seasonal cycle of temperature changes in the atmosphere (as the atmosphere heats and cools during different seasons of the year).

They discovered that in the lower part of the atmosphere (the troposphere), the fingerprint of global warming can be found in the satellite measurements of the changing seasonal cycles of temperature. The “human influence” fingerprint matched the satellite patterns of seasonal cycle change. The match was significant – it couldn’t be explained by natural climate variability.

They discovered that in the Northern Hemisphere, the warming during the summertime is larger than the wintertime warming. So, in the north (between about 30 and 60 N), the seasonal temperature variations is increasing. In the Southern Hemisphere mid-latitudes, there is similar behavior, but the seasonal differences in warming are smaller. The reason the north and south behave differently is because north of the equator there is a lot more land and a lot less ocean compared to the Southern Hemisphere. Just take out a globe, find the equator, you will see for yourself.

But as you continue further toward the poles, say north or south of 60 degrees latitude, the trend reverses. The reversal is particularly evident in the north. The range of seasonal temperatures is becoming smaller. The main reason for this is the loss of ice. As the planet warms, ice melts and exposes more water and changes the reflectivity of the planet.

Much of the fingerprint work that has been done over the last 30 years looks at broad-brush changes in annual climate. The focus on climate fingerprinting with the changing seasons is relatively new and makes this paper a significant contribution.