Mark V:

You’re right, we’re constantly getting bombarded by thermal radiation. If the earth had no way of cooling off, it would simply keep getting hotter and hotter. So the we must have some way of cooling off, some way of losing energy to space, because we can’t just sink it into the oceans, or ice caps.

When light shines on the earth, different places heat up by different amounts. Snow reflects most of the incoming light, so snowy areas tend to stay cool. Dark areas, like black parking lots, absorb incoming light, and so will warm up (urban heat island effect).

In order to keep at a relatively constant temperature, the earth must get rid of energy too. And it does: As emitted thermal energy (EM infrared), that is transmitted to space. Space is very cold. Only about 3 Kelvin (-454 F).

The hotter something is, the more infrared emitted. Clouds absorb or reflect the infrared from the surface and send it back into space. Deserts are hot during the day (well, what we usually think of as "deserts"), because they absorb a lot of incoming radiation, but at night they radiate energy back to space, so they get quite cool (one of the extreme dangers of trying to survive in a Sahara or Kalahari-style desert). As already stated, the "bright white" ice and snow reflects much of the thermal energy off of our planet (take a large piece of white paper out at a bright, sunny noon, and an equally large piece of darker paper, and sea which one is easier to look at – the white will probably make you squint).

So, to keep blabbering on for you…

The average albedo (reflectivity of the globe as a whole) of the Earth is about 0.3, which means that 30% of the solar energy is reflected back into space, while 70% is absorbed by the Earth and re-radiated as infrared (molecules will absorb energy, then they re-emit it; sounds funny, but that’s what happens). The planet’s albedo varies from month to month, but 0.3 is the average figure. It also varies very strongly around the glob: polar ice sheets have a high albedo, oceans low.

Here’s how it breaks down as far as thermal energy directly reflected.

6% reflected from the atmospheric gasses
20% reflected from clouds
4% reflected from the ground (including land, water and ice)

The remaining 70% of the thermal energy is absorbed:

51% absorbed by land and water, then back out again in the following sub-types:
– 23% transferred back into the atmosphere as latent heat by the evaporation of water
– 7% transferred back into the atmosphere by heated rising air
– 6% radiated directly into space
– 15% transferred into the atmosphere by radiation, then re-radiated into space

19% absorbed by the atmosphere and clouds, including:

– 16% reradiated back into space
– 3% transferred to clouds, from where it is radiated back into space

When the Earth is at thermal equilibrium – happy and healthy – the same 70% of sunlight as thermal energy that is absorbed is again re-radiated:

– 64% by the clouds and atmosphere
– 6% by the ground

However, we humans are throwing our happy planet off-kilter (depending on the scientists whom you want to believe). Global warming is a fact, but is greenhouse warming (i.e. human pollution) the problem? Maybe; decide for yourself. The largest and best-known apparent human impact is from the well-mixed greenhouse gases (CO2, CH4, halocarbons, etc.), totaling an increase in forcing of of the retainment of thermal energy. Currently, it represents less than 1% of the solar input, but may contribute to the observed increase in atmospheric and oceanic temperature. Considering that 1% is actually a lot of heat, that’s why it’s being addressed as a global problem.

Rachel:

Because a lot of it is reflected back into space by the polar ice caps. That’s one major reason why it is not good that they’re melting, among other obvious reasons.