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