EE-Unit-II Earth’s Energy Balance

Essentially 100% of the energy that fuels the earth comes from the sun.  To maintain a constant global average temperature, all of the sun’s radiation that enters Earth’s atmosphere must eventually be sent back to space.  This is achieved through Earth’s energy balance.  Figure A depicts how the energy from the sun is absorbed, reflected, and emitted by the earth.

Figure A: Earth’s Energy Balance

100% of the energy entering earth’s atmosphere comes from the sun.

~50% of the incoming energy is absorbed by the earth’s surface i.e. the land and oceans.

~30% is directly reflected back to space by clouds, the earth’s surface and different gases and particles in the atmosphere (the earth’s albedo is 0.3 on average).

~20% is absorbed by the atmosphere and clouds.

The 70% of the sun’s energy that is absorbed by the earth’s surface, clouds, and atmosphere causes warming.  Any object or gas that has a temperature emits radiation outward, and this is ultimately re-radiated back into space.  This occurs 24 hours a day, and the energy is emitted as longwave radiation due to the characteristic temperatures of the earth and atmosphere.

Consider a stove, for example.  If you were cooking, you’d have the burner turned on so it would heat up.  The burner is like the earth and the heat source, be it gas or electric, is like the sun.  When you’re done cooking you turn off the burner, but it stays hot for a long while even after the heat source is gone.  Turning off the burner is like the sun going down.  Even though there is no more energy input, there is still energy output in the form of infrared radiation.  The burner stays hot because it’s still emitting the energy it absorbed earlier, just like the earth.  This time delay is sometimes called “thermal inertia.”

Most of the energy emitted from the earth’s surface does not go directly out to space.  This emitted energy is reabsorbed by clouds and by the gases in the atmosphere.  Some of it gets redistributed by convection.  Even more energy is released into the atmosphere through condensation.  The majority of the energy is reabsorbed by the greenhouse gases such as methane, nitrous oxide, ozone, carbon dioxide and water vapor.  These gases constantly emit the sun’s energy back into the atmosphere and keep the earth a habitable temperature.  Eventually, most of the energy makes its way back out to space and Earth’s energy balance is fairly well maintained.  The energy that doesn’t make its way out is responsible for global warming.

On a global scale, the atmosphere’s circulation and weather is an attempt to balance differences in solar energy that the earth receives across the globe.  Sunlight at the tropics is intense and direct and a lot of heating of land, atmosphere, and oceans occur there.  Sunlight in the polar regions is weak and indirect and does not do a good job of heating up the region.  Currents in wind and ocean water carry energy from the tropics toward the poles to help balance out the energy differences across the globe.