The scientific research into global warming goes back quite a few years. A key paper on the working of CO2 in the atmosphere (referred to then as carbonic acid) is by Svante Arrhenius (Nobel Laureate in Chemistry, 1906) in April, 1896. It is here (click on figures below to enlarge):
It is quite readable, give it a look. It references research by Joseph Fourier published in 1827 (done in 1824) establishing the existence of the Greenhouse effect, Pouillet in 1838, Roentgen (recipient of first Nobel prize in Physics in 1901) and Angstrom (Rumford medal in 1872 - died before the Noble prize was endowed). The heat-trapping abilities of greenhouse gases were corroborated by Irish scientist John Tyndall with experiments beginning in 1859.
|From page 798 of the NCA 2014|
Here is how it works. CO2 absorbs and emits infra-red radiation which is an electromagnetic radiation like light, microwaves, radio waves, and X-rays. Heat is transmitted from the earth as infra-red radiation (invisible to humans). The CO2 in the atmosphere absorbs some of this heat energy and then shortly after emits it in a random direction. Some infra-red radiation (heat) is radiated to earth and some is radiated into space. Without CO2 and other greenhouse gases, all the heat generated on earth would radiate into space and the earth would be colder. The heat originally comes from a variety of sources including the Sun, volcanic activity, burning of fossil fuels, forest fires, etc. The problem comes when too much CO2 (and other greenhouse gases like methane - CH4, and H2O) gets into the atmosphere. Then the greenhouse effect traps more heat. Plants absorb CO2 but only up to a point. If there are fewer plants or more CO2 emitted, the amount of CO2 in the atmosphere increases. This is not disputed.
I mentioned H2O. Water vapor is also necessary for life but it also acts like a greenhouse gas. Oxygen, O2, and Nitrogen, N2, are the main gases in our atmosphere but they do not absorb or emit energy in the infra-red region (no dipole moment). We live on a knife edge of not-too-much/not-too-little of many things including H2O and CO2. Warmer oceans evaporate more water further adding to the greenhouse effect so it becomes self-reinforcing. None of this is at all controversial. No scientist disputes that CO2, methane (CH4) and H2O (water vapor) absorb and emit in the infra-red region of the electromagnetic spectrum. The amount of CO2 in the atmosphere has significantly increased. Disputation revolves around measuring the *effects* of that increase in CO2 and other gases.
|From Page 23 of the National Climate Assessment 2014|
For more on the chemistry, c.f., http://en.wikipedia.org/wiki/Greenhouse_gas
To test any scientific hypothesis we look for what it can predict and see if that happens. The more that can be predicted and then verified as happening, the stronger the support for the theory. There are quite a few predictions global warming has made which have become true. One obvious thought is that if there is more CO2 (and other greenhouse gases) radiating *more* heat back to earth, then the atmosphere above the CO2 layer should get *less* heat. In fact, that is the case, the troposphere closer to earth has measurably warmed while the stratosphere above it has cooled down. If global warming were due to changes in solar radiation it would have affected all parts of the earth and its atmosphere.
Above chart from http://www.skepticalscience.com/its-not-us-intermediate.htm
Another obvious test of the hypothesis is that if the lower atmosphere is warming, a warmer surface on oceans and lakes should evaporate more water and the warmer air should be able to hold more water. This could result in either more frequent rains or heavier rains of the same frequency. Computer models of the 1980's predicted heavier rains, not more of them. This is what is happening. Weather is very complex and air can retain its moisture for a while but when conditions are right for rain, it comes down in much greater quantities.
|From page 36 of the National Climate Assessment 2014|
In fact the vapor pressure of air (amount of water it can hold) rises *exponentially* with temperature. A 10 degree Celsius change from 20 deg. Celsius (68 F) to 30 deg. Celsius(86 F) causes water vapor pressure to *double*. From 30 C to 40 C (86-104 F) it *doubles* again. This is an experiment that can be done in any lab and is not disputed.
Above graph from:
This shows up in the world (including the US) as more frequent heavy rains - see http://nca2014.globalchange.gov/highlights/report-findings/extreme-weather#tab1-images - and more flooding.
|From page 260 - NCA 2014|
At the same time you get more flooding, the obvious effect of a warmer climate is that it should dry out more quickly as well so you will get more droughts. This is also the case.
|Temperature and rainfall from 1919 to 2012|
"Tree ring data suggests that the drought over the last decade in the western U.S. represents the driest conditions in 800 years." page 38 of the National Climate Change Assessment pdf form available here (also a very slick web version):
NY Times items here:
The American Institute of Physics (AIP.org) has a very good article on the evolution of the science of climate change here: http://www.aip.org/history/climate/co2.htm
I very highly recommend that AIP article for more depth. Climate change was not initially understood very well and there were many mistakes made along the way. It is fascinating to see the fits and starts with which the scientific understanding matured.
- Michael Goldman