Radiated heat from land, oceans and low clouds is held near the earth’s surface by greenhouse gases but must eventually escape to space, as shown above. Areas with more cirrus cloud cover tend to emit less heat to space.
Reflected sunlight from low clouds, ice and other bright surfaces returns some of the sun’s energy back into space. The oceans and other dark surfaces reflect less light and are warmed by absorbing sunlight.
Incoming sunlight varies with location and with the seasons. The above map shows the average sunlight falling on the earth’s surface in March, as measured in watts per square meter.
Cirrus clouds are high, thin and cold. Many of these wispy clouds allow sunlight to pass through easily, but they tend to trap rising heat. A future increase in cirrus clouds would probably warm the planet, while a decrease would most likely cool it.
0
213
425
W/m2
85
218
350
W/m2
0%
15%
30%
cirrus
cover
0
275
550
W/m2
EARTH
LOW CLOUDS
Outgoing light
reflected by
clouds and
surface
Light absorbed
by surface and
radiated as heat
Incoming
sunlight
Outgoing
heat
Heat trapped
by cirrus clouds
Light absorbed
by atmosphere
CIRRUS CLOUDS
Global Energy Balance To maintain a stable temperature, the earth must balance the incoming energy of sunlight with the outgoing energy of reflected light and radiated heat. Clouds alter the balance in several ways, with an overall cooling effect in today’s climate. Humans are adding greenhouse gases to the atmosphere, causing warming near the surface, and a big question is how clouds will react.