What are the Greenhouse Gases?
Greenhouse Gases and the Energy Balance of the Earth
Solar energy heats the earth, and the earth cools to space by infrared radiation, which increases with temperature. So to reach balance, the earth warms up until it can radiate to space the energy that it gains from the sun. This is where the greenhouse gases (GHGs) are important. The GHGs are the gases that strongly absorb the earth’s thermal infrared radiation, so they effectively blanket the earth (by radiating energy back down to the surface) and reduce the cooling of the earth to space (because the radiation to space comes from higher in the atmosphere, where it is cooler). The primary GHGs are water vapor, carbon dioxide, methane, nitrous oxide, and ozone (H2O, CO2, CH4, N2O, O3), but there are many other industrially produced gases (like the hydrofluorocarbons, the HCFCs). These GHGs can rotate and vibrate and absorb the earth’s thermal radiation strongly. (Most of the atmosphere is nitrogen and oxygen (N2, O2), but these simple diatomic molecules are too tightly bound to absorb this thermal radiation.). The GHGs are critical to giving the earth the warm climate that supports life – without them the earth on average would be about 33◦C (59◦F) cooler - and the oceans would freeze.
The water vapor greenhouse:
Water vapor, which varies strongly across the earth with surface evaporation and atmospheric transports, is the most important GHG. The earth’s water cycle, is in balance on timescales of a few weeks, so atmospheric water vapor goes up with temperature on these short timescales. Essentially water, evaporated from the surface, rains out in a few weeks and the latent heating from the condensation of water in the atmosphere (which falls out as precipitation) balances the radiative cooling of the atmosphere. This means that the water vapor greenhouse effect has a rather large positive feedback to the climate system temperature, because if temperature rises, more evaporation gives more water vapor in the air, and this traps more of the earth’s thermal radiation.
The CO2 greenhouse:
CO2 is the second most important greenhouse gas and although there is much less CO2 than H2O in the air, it plays a critical role. Atmospheric CO2 is controlled by very different processes than is water vapor, and currently it is far out of balance. The amount of CO2 has increased 43 % in the past 100 years from about 280 to 400 parts per million (ppm). There are large exchanges of CO2 between land, atmosphere and oceans. In the natural earth system, before the human impact became large, these were close to being in balance over the global annual cycle. However the input of CO2 from burning fossil fuels and other industrial processes is so large than the biosphere and oceans can only take up about half of the human input – and this fraction appears to be decreasing with time (although with large variability). Consequently atmospheric CO2 is increasing quite rapidly (See Figure 1.) and the lifetime of this human waste product (coming from primarily the burning of fossil fuel) is a century or more. In fact some is dissolving in the oceans, which are becoming less alkaline, and some of this CO2 will be returned to the atmosphere centuries from now, once we have limited the use of or exhausted our fossil fuel reserves.
The Earth’s Energy Imbalance:
Because of increased GHGs in the atmosphere, the Earth at present has a net energy gain of about 1-2 Wm-2. A small part of this energy imbalance is melting ice, but the Earth must get warmer on average, until this imbalance can be radiated back to space. This warming of the Earth has a lag of 50-100 years because of the large thermal storage of the oceans. Furthermore, the climate system has instabilities and feedbacks that are only partially understood, so we cannot predict with certainty the future climate of the Earth. But unless atmospheric CO2 falls again, the future climate will be a warmer one.