You probably remember your grade school science teachers explaining that energy can neither be created nor destroyed. That’s a fundamental property of the universe.
Energy can be transformed, however. When the Sun’s rays reach Earth, they are transformed into random motions of molecules that you feel as heat. At the same time, Earth and the atmosphere are sending radiation back into space. The balance between the incoming and outgoing energy is known as Earth’s “energy budget.”
Our climate is determined by these energy flows. When the amount of energy coming in is more than the energy going out, the planet warms up.
That can happen in a few ways, such as when sea ice that normally reflects solar radiation back into space disappears and the dark ocean absorbs that energy instead. It also happens when greenhouse gases build up in the atmosphere and trap some of the energy that otherwise would have radiated away.
Scientists like me have been measuring the Earth’s energy budget since the 1980s using instruments on satellites, in the air and oceans, and on the ground. It’s an important part of the new climate assessment from the United Nations Intergovernmental Panel on Climate Change report released Aug. 9, 2021.
Here’s a closer look at how energy flows and what the energy budget tells us about how and why the planet is warming.
Balancing energy from the Sun
Virtually all the energy in the Earth’s climate system comes from the Sun. Only a tiny fraction is conducted upward from the Earth’s interior.
On average, the planet receives 340.4 watts of sunshine per square meter. All sunshine falls on the daytime side, and the numbers are much higher at local noon.
Of that 340.4 watts per square meter:
- 99.9 watts are reflected back into space by clouds, dust, snow and the Earth’s surface.
- The remaining 240.5 watts are absorbed – about a quarter by the atmosphere and the rest by the surface of the planet. This radiation is transformed into thermal energy within the Earth system.
Almost all of the absorbed energy is matched by energy emitted back into space. However, a residual now accumulates as global warming. That residual has increased, from just under 0.6 watts per square meter at the end of the last century to 0.79 in 2006-2018, according to the latest data from the Intergovernmental Panel on Climate Change. The vast majority of that is now heating the oceans. While it might sound like a small number, that energy adds up.