[aimless rant] SCIENCE! Here's a list of the things that (to my knowledge) have impact on global temperature. If I missed anything or am in error, please feel free to add or correct.
1: Solar irradiance
2: Earth's orbit (specifically, average distance from Sol)
3: Earth's albedo (% incoming light that is reflected back to space)
4: Atmospheric composition (notably greenhouse gases)
5: Aerosols/volcanoes… various random events such as meteor impacts
6: Ocean currents, ENSO
Now then…
1: The solar physicists say that the sun’s output is increasing, but on very long timescales, and thus is negligible in the short term. On the other hand there is the 11-year solar cycle which also has small effect on output – higher output during solar max, lower during solar min. The change is very small, though, so it is still being argued whether or not it has detectable contribution to global temperature change. The abstract I linked to earlier claims it does, but I’d like to see more data to support/reject that idea.
2: Earth’s orbit – doesn’t change significantly over human timescales, so can be disregarded when it comes to the current warming trend, if it exists.
3: Albedo. Oceans are very effective at absorbing heat, so is vegetation. Clouds and polar caps are very effective at reflecting heat. Clouds obviously are pretty variable, but consensus on snow/ice coverage is that it is decreasing, which thus decreases albedo and pushes toward a warming trend. This same principle, working both forward and reverse, is of major importance when considering the “snowball earth” hypothesis.
4: Ahh, the atmosphere. Finally something we know very well. And we know that human activity is increasing greenhouse gas concentrations. Let’s start with CO2 since everyone loves talking about it these days. CO2 levels are currently about 380 ppmv (about 0.04%) and is increasing by about 10 ppmv every decade. As for its greenhouse capability, to quote wikipedia: “Despite its relatively small concentration overall in the atmosphere, CO2 is an important component of Earth's atmosphere because it absorbs and emits infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode), thereby playing a role in the greenhouse effect.”
And of course CO2 isn’t the only greenhouse gas, nor is it the strongest. Water vapor by far is the strongest contributor, especially if one includes the effect of clouds. Whether or not we have an impact on water vapor concentrations in the atmosphere, I do not know, though from what I’ve read it is minimal/localized. That said, a warmer atmosphere will hold more water, and there’s another positive feedback effect.
As for other significant greenhouse gases, this is their change from pre-industrialization and the effect.
Gas Preindustrial Level Current Level Increase since 1750 Radiative forcing (W/m2)
Carbon dioxide
280 ppm 387ppm 104 ppm 1.46
Methane
700 ppb 1,745 ppb 1,045 ppb 0.48
Nitrous oxide
270 ppb 314 ppb 44 ppb 0.15
CFC-12
0 533 ppt 533 ppt 0.17
5: Mmm, volcanoes. They release CO2, yes, but their effect on climate is actually a brief cooling trend, because of ash and sulfur emissions. In recent years there have not been any (significant) eruptions.
6: Ocean currents, not my area of expertise. El Niño and La Niña obviously have short-term effects but maybe someone else can explain that?
So in summary, we have a combination of influences that effect the climate. When considering the current trends (hundreds of years), I’d say we can disregard number 2. Personally, I would argue that 3 and 4 are causing a warming trend, ala ‘global warming’. 5 causes short-term cooling trends, while 6 is also short term but possibly with various effects. As for 1, it is also short term, but the strength of the effect is in question.
[/aimless rant]
