First things first
This post should not be construed in any way, shape, or form to be comprehensive. Climate research spans more than a century and is a product of the work of thousands of scientists in dozens of fields of study. A post on an internet forum cannot possibly address all of it. This post should also not be construed as definitive proof of anything. The purpose is to establish that there is, despite what skeptics tell you, a body of empirical evidence that points to Anthropogenic Global Warming. (AGW) Lastly, it should be made very clear that I, Deuce, am not a climatologist and should not be considered an "expert" on this subject. My qualifications, if you can call them that, are a BS in Aeronautics, an IQ that internet tests claim is somewhat above average, and a lot of personal curiosity about, and interest in, the subject that has lead me to waste an awful lot of time at work reading about this stuff. So, read on if you're interested in some of the science behind the theory. Also, don't tell my boss, ok?
Some basic facts to establish
We need to address some fundamental facts in order to act as a base for this discussion. Although you wouldn't know it from spending time at these forums, agreeing on a base set of reality is important to debate!
- Mankind has caused an increase in Carbon Dioxide (CO2) levels in the atmosphere.
One would think that this point doesn't need to even be mentioned. After all, we burn rather staggering amounts of various fossil fuels, all of which emit CO2 when burned. Some skeptics, however, dispute that this leads to higher CO2 in the atmosphere, or that it leads to significant change. The usual arguments are that nature emits far more CO2 than we do, or that it absorbs what we put into the atmosphere. (or both) The people who argue this are full of hot air. (drumbeat) Actually, I'll refer to this as a mistake, rather than making the assumption that they are deliberately misleading people.
The reason this mistake is made is a lack of consideration for the other half of nature's carbon cycle. While nature does in fact emit a lot more CO2 than we do, (nearly 800 gigatons per year compared to our ~29) it also absorbs a metric assload every year as well. The net result (800Gt- 1 assload) is that nature actually acts as a net carbon sink. That is, nature pulls more carbon out of the air than it releases into the air.
Thanks, nature! Say, where does all this carbon go? Ahh, plants you say. Every year when plants grow, they do so by absorbing the carbon from carbon dioxide and spitting out the oxygen. Which is also swell for us. Also, the ocean absorbs CO2 as well, there's a tremendous amount of CO2 dissolved into the ocean. (a more pronounced example of this can be found in any soda bottle that hasn't been open for too long. the moral of the story is, don't shake the ocean)
As if the coffin needed an additional nail, we can also detect a difference between naturally-emitted CO2 and CO2 emitted by fossil fuels. While I don't have a great understanding of this research myself, the basics of it is that carbon has different isotopes that have a noticeable difference in ratio when comparing natural CO2 to fossil fuel CO2. C12 vs C13, or something to that effect.
This is simple, unassailable physics. We've known this for more than a century, but more recently fine-tuned the wavelengths when the Air Force needed to work on heat-seeking missiles. (apparently, knowing the absorption characteristics of atmospheric gases in the infrared spectrum is useful when you're trying to hit a fighter plan moving five hundred miles per hour by using that same spectrum. who knew.) So why is this all important?
- Carbon Dioxide absorbs radiation in the long-wave infrared spectrum
As you can see, the incoming solar radiation is primarily in the shortwave infrared spectrum. (as well as the visible spectrum, which is very convenient seeing as how visible light is... well, how we see) This radiation passes through the atmosphere more or less freely and hits the surface. The earth heats up and emits that radiation back. (think like an electric stove or hot pavement) The outgoing radiation is in the longwave infrared spectrum, and as we see from that chart, this radiation gets absorbed by several atmospheric gases: CO2, O2/O3 (ozone), H2O (water vapor), as well as some not-pictured gases like CFCs and methane. Oh no! The radiation comes in, but not all of it gets out! The greenhouse effect, we're doomed! Actually, for the most part this is a good thing. Without this effect, the earth would be about 33 C colder than it is now. Giant, lifeless ball of ice. Good for snowmen, bad for humans.
Ok, so why is THIS important? The fundamental fact is that CO2 is one of several greenhouse gases. It absorbs radiation that would otherwise have gone to space. This energy gets re-emitted in all directions, causing heating.
Papers on laboratory measurements of CO2 absorption properties « AGW Observer
A whole bunch of papers on the subject.
On to the empirical evidence
Phew, finally. So, we've got these theories about CO2 causing warming, but can we back that up with evidence? Why yes, we can! These days, it's actually quite easy. Remember that longwave infrared radiation that the earth radiates and CO2 absorbs? We can track that. What we monitor:
1) Outgoing radiation across the spectrum, via satellites.
2) Incoming radiation across the spectrum, via ground stations.
3) Temperature, of course
So, if the theory is that increases in greenhouse gases will cause the atmosphere to absorb more energy that would otherwise escape, causing heating, what can we expect to see?
1) Outgoing radiation in the spectrum absorbed by CO2 to decrease, relative to sun's output.
2) Incoming radiation in the spectrum absorbed by CO2 to increase, as some of this outgoing radiation is reflected back down to earth.
3) The changes in these two to correspond to the increase in CO2 in the atmosphere.
4) A rise in temperature
Hey, what do you know? This is exactly what we see! Outgoing radiation in CO2's absorption spectrum has decreased, the same radiation coming to the surface has increased. Same energy goes in, less energy goes out. Physics 101 tells us that this will cause heating, and we see a warming earth as well!
Papers on changes in OLR due to GHG’s « AGW Observer
Changes in outgoing radiation showing a decrease in the spectrum absorbed by CO2 and other greenhouse gases.
Increases in the radiation at the surface in those same wavelengths.
And, of course, a rise in temperature.
(this image from NASA GISS temperature data)
(last two images removed because the forums are jerks and I can only use 5 pictures)
Phew! That was a bit of work. I had typed up some more on the common skeptics' counter-arguments, but it got eaten by the computer and I don't feel like retyping it at this particular moment.
Now, I know there are skeptics out there who will just handwave this entire post with "CANT TRUST THE SCIENTISTS," but those people might very well be beyond help. The hope is that there are people out there who are more open-minded while being skeptical, who perhaps just haven't taken the time to learn more about the science behind it all. Maybe they'll read this and go "hmm, maybe there's more to this than I thought. I'll go read more and learn things!"
Hooray for learning.