Earth is constantly trying to balance the energy that comes from the Sun and the energy that our planet releases back into space. Since the industrial revolution, humanity has been throwing this energy budget off-balance, adding greenhouse gases that trap more radiation.
Adding these greenhouse gases leads to what is known as radiative forcing. Evidence for this phenomenon has been established by studying the greenhouse gases' concentrations and surface temperature changes over the last 150 years, but not directly. In a study published in Geophysical Research Letters, scientists report the first direct observations of the increasing global radiative forcing.
“This is the first calculation of the total radiative forcing of Earth using global observations, accounting for the effects of aerosols and greenhouse gases,” lead author Dr Ryan Kramer, from NASA’s Goddard Space Flight Center and the University of Maryland, said in a statement. “It’s direct evidence that human activities are causing changes to Earth’s energy budget.”
Since 1997, thanks to NASA’s Clouds and the Earth’s Radiant Energy System (CERES), scientists have been able to measure how much energy is coming from the Sun and how much energy Earth is reflecting back into space. This kind of data can track the changes to the energy budget, but not the cause of radiative forcing.
Other NASA satellites have helped with that thorny problem. They can be used to track changes in natural phenomena that can affect the energy budget. Clouds, the brightness of the surface, and water vapor are some examples of these. The Atmospheric Infrared Sounder instrument on NASA’s Aqua satellite measures the water vapor in our atmosphere, which absorbs heat – so depending on how much moisture there is in the atmosphere, the amount of energy that leaves the Earth will be different.
The team then created a new computational method to subtract all the variations from natural sources in the energy measurements taken by CERES. They showed that radiative forcing has increased by 0.5 Watts per square meter from 2003 to 2018. This is mostly due to increases in greenhouse gas emission, but the reduction of reflective aerosols also plays a part according to the researchers.
“Creating a direct record of radiative forcing calculated from observations will allow us to evaluate how well climate models can simulate these forcings,” explained Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies (GISS) in New York City. “This will allow us to make more confident projections about how the climate will change in the future.”
The new method is fast and can be used to track changes on a day-to-day basis, as well as being used as a testbed for models of what the future holds.