Satellite data confirms humans are the main cause of temperature rise
Compare satellite records of temperatures over the last few decades with multiple simulations from the latest generation of climate models, and one thing is clear – warming near the earth’s surface could not have occurred without the influence of human’s greenhouse gas emissions.
That’s the finding of a new study, just published in the Proceedings of the National Academy of Sciences identifies a unique human fingerprint in the 34-year record of temperature data collected by satellites. Other factors that can influence the climate, like the amount of sunlight reaching earth, can’t reproduce the warming patterns seen in recent years when simulated by climate models, the research finds.
The human fingerprint
Instruments mounted on satellites have been collecting data for over 30 years, creating a detailed record of how temperatures in different layers of the earth’s atmosphere are changing. According to the paper by a team of international scientists, that data reveals a distinct pattern, which can only be explained by rising levels of greenhouse gases.
The satellite data shows that in the atmospheric layer closest to earth’s surface – the troposphere – temperatures have risen steadily since satellite records began in 1979. But in the layer above – the stratosphere – temperatures have cooled. The scientists wanted to know what was causing the neighbouring layers of the atmosphere to behave differently.
Using 20 new generation climate models, the team tried to recreate the patterns seen in the atmospheric temperature record by looking at all of the various factors which could affect the climate.
But when they simulated all the natural factors, such as changes in the sun’s energy, volcanic eruptions and natural ocean-atmosphere cycles, they couldn’t recreate the same trends seen in the satellite data.
Only when the rise in greenhouse gases over recent decades was included in their climate model simulations could this unique pattern be achieved.
Watching climate change unfold
This revelation from the satellite data is “clear evidence for a discernible human influence on the thermal structure of the atmosphere”, say the authors. But satellites tell scientists about a lot more than just temperature rise.
As the image below shows, they help monitor changes across the entire climate system, from the surface of the land and seas to the upper limits of the atmosphere:
As another new study shows, satellite data offers a number of benefits over traditional ways of measuring the climate.
Since satellites are continuously orbiting the earth they offer much better spatial coverage than a network of weather stations, for example, and they are less likely to be biased by local factors which can affect things like temperature readings. They also allow scientists to measure changes in remote locations, such as the temperature of the ocean.
In recent years, satellites have been used in a number of different ways to observe how ice sheets, glaciers and sea ice are changing in response to rising temperatures. In 2012, satellites recorded melting over 97 per cent of the Greenland ice sheet, and witnessed the area of Arctic ocean covered in sea ice shrink to its lowest extent ever.
Satellites are also used to monitor important changes happening outside the climate system. They can measure phenomena such as the amount of sunlight reaching the earth’s surface, and the level of sunlight-reflecting aerosols in the atmosphere. Knowing how external factors like these are changing is important in weighing up the role humans’ role in the long term trend of global warming, relative to natural influences.
Confidence booster
The main downside of satellite data at the moment is that the record is quite short. Natural fluctuations in the climate can occur over decades, so separating out the human influence over such a short time isn’t easy. But as the new study today shows, it is possible – and as the satellite record continues to grow, that differentiation should become easier to make.
Even with this shortcoming, satellites have brought about major advances in climate science in the last 30 years. They help scientists monitor how that warming is affecting the earth as climate change unfolds – measuring changes in the size of the ice sheets for example. And they have helped scientists say with increasing certainty what was first suggested more than 75 years ago – that humans’ greenhouse gases are the main driver of global temperature rise.
Santer et al. (2013) Human and natural influences on the changing thermal structure of the atmosphere. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1305332110