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Measurement
Matter emits thermal radiation in an amount that is directly proportional to the fourth power of its temperature. Some of the radiation emitted by the Earth's surface is absorbed by greenhouse gases and clouds. Without this absorption, Earth's surface would have an average temperature of −18 C. However, because some of the radiation is absorbed, Earth's average surface temperature is around 15 C. Thus, the Earth's greenhouse effect may be measured as a temperature change of 33 C-change.

Thermal radiation is characterized by how much energy it carries, typically in watts per square meter (W/m$2$). Scientists also measure the greenhouse effect based on how much more longwave thermal radiation leaves the Earth's surface than reaches space. Currently, longwave radiation leaves the surface at an average rate of 398 W/m$2$, but only 239 W/m$2$ reaches space. Thus, the Earth's greenhouse effect can also be measured as an energy flow change of 159 W/m$2$. The greenhouse effect can be expressed as a fraction (0.40) or percentage (40%) of the longwave thermal radiation that leaves Earth's surface but does not reach space.

Whether the greenhouse effect is expressed as a change in temperature or as a change in longwave thermal radiation, the same effect is being measured.