Earth Image © NASA

SJUU: GLOBAL WARMING GROUP
Global Warming Science

GLOBAL WARMING SCIENCE
by Michael A. Pelizzari

Question:  What determine's the Earth's temperature?
Answer:  The principle of 'conservation of energy'.

1 PW = unit of power = 1 petawatt
= 1 quadrillion watts
= 1,000,000,000,000,000 watts
= output of one million large power stations

The amount of heat radiated by a planet depends only on its size and its temperature. For the Earth,

The Earth as a whole has to radiate only 120 PW to balance the influx of solar power. Therefore sunlight can only heat the Earth to 3 ° F below zero, which is how cold the planet feels from outer space.

Question: So why is the ground 59 ° F on average?

Answer: The atmosphere must be locking in heat from the ground (199-120=79 PW, to be exact), and not letting it escape to space. It's like a blanket whose inner surface is 59 ° F and outer surface is -3 ° F. This insulating property of the atmosphere is what we call the “greenhouse effect”.

Question: Why blame carbon dioxide for global warming? It's less than 0.04% of the atmosphere.

Answer: Just look at the atmosphere's absorption spectrum , which shows what fraction of light at each color (or more technically wavelength ), gets blocked by the atmosphere.

Most sunlight is in wavelengths shorter than 1 micron .* The sun is brightest at wavelengths the eye can see, from 0.4 microns (violet) to 0.74 microns (red), where the air is quite transparent.

* A micron is a millionth of a meter: human hairs are 100 microns thick.

Most heat radiates at wavelengths longer than 5 microns, where the air is practically opaque except for a transparent “infra-red window” between 8 and 13 microns. Below 8 microns, radiant heat is absorbed primarily by H2O (water vapor). Above 13 microns it's absorbed by CO2 (carbon dioxide). If the concentration of either gas increases, the air will absorb heat more efficiently and raise the global temperature.

Don't blame the H2O for global warming. Water vapor can only build up until the air becomes saturated with it. Any excess quickly condenses into clouds and raindrops.

But CO2 can keep building up and up without condensing, and its absorption by oceans, plants, soils and lakes is a centuries-long process. Since the start of the industrial revolution, CO2 has been added to the atmosphere faster than it's being absorbed. Its concentration in the air has therefore grown, from 275 ppm (parts per million) in 1800 to 380 ppm today, a 38% increase. Theoretically this should make the Earth warmer, and sure enough, Earth's average temperature has risen 1.4 ° F in that time.

If nothing is done to slow current trends, CO2 concentration by the year 2100 will exceed 550 ppm, or double the pre-industrial(1800) level. This is predicted to raise the global temperature by an additional 4° F, not only from the direct effects of adding CO2, but also from positive feedbacks triggered by the initial temperature increase. One such positive feedback is the increase in water vapor - the other major greenhouse gas - because hotter air can hold more water vapor before becoming saturated than colder air.

Picture the Earth today (left), and in a hypothetical future with much more CO2 in the atmosphere (right). Think of the heat radiating from the ground as passing through two heat vents to reach space. The small vent dumps heat directly (thin arrow), and the large vent dumps heat to the atmosphere (thick arrow) which in turn dumps heat to space (midsize arrow). Adding any more greenhouse gas to the atmosphere blocks the small vent a little more, forcing more of the heat through the large vent (raising the ground temperature as it goes).