A recent set of posts at Anthony Watt’s blog, particularly this one has sparked some interest over the internet as of late. From a quick glance, it looks like negative trends in specific humidity over the last half a century. Readers were quick to pick up on the connection to water vapor feedback, which is expected to at least double the sensitivity of climate to external perturbations (e.g., by human released CO2).
In the preceding post, on the greenhouse effect, I investigated the role of the greenhouse effect and its play in radiative balance, and how the presence of an atmosphere acts to raise planetary temperatures. The take home points should be that for a planet with no infrared absorbing layer above the surface, the fourth power of the surface temperature always approximates a value determined by the incoming solar radiation. The only way the surface temperatures can exceed this value is if there is an atmosphere which acts to be a blanket to outgoing radiation. A planet can also be heated by internal processes such as radioactive decay or rigorous convections from the mantle, but these are rather negligible on the terrestrial planets. Adding greenhouse gases to an atmosphere whose temperature decreases with height must act to warm the surface by making the net downward emission greater than zero. In this post, I will elaborate on specific greenhouse gases, the runaway greenhouse effect, and an antigreenhouse effect.
Jean Baptise-Fourier is generally credited with the discovery of a greenhouse effect, which involves the process by which the presence of an atmosphere acts to raise the surface temperature of a planet. This was extremely simplified at the time, and the term greenhouse never appears in his 1827 writing, but he did establish the effect of the atmosphere on incoming light versus outgoing infrared, and that some heat was absorbed by the atmosphere which was opaque in the infrared but transparent to incoming solar energy. A copy of his essay, translated by R.T. Pierrehumbert can be found here. We’ve made a lot of progress since then, as Arrhenius began to quantify the phenomenon nearly 75 years later, the work of Stefan and Boltzmann established the relationship between an object’s temperature and its outgoing radiation, where the role of convection and water vapor play, etc. Another pionerring paper, entitled On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground, which is the famous 1896 piece by Arrhenius who began to investigate what the effects of doubling the atmospheric CO2 content would be. At this time, most of the interest in the subject was in solving the mystery of the coming and going of ice ages. Like most pioneering efforts, Fourier or Arrhenius did not have the last word, and we still have much to learn today, but they proved to provide a big leap in how we understand planetary temperatures and the role of the atmosphere in radiation. Fourier was one of the first to speculate that human activities could influence climate, and such topics are rather important in modern times.
Greenhouse gases make up about a percent of all of the molecules in the atmosphere, and CO2 makes up about .038% by volume. That is an increase from .028% from pre-industrial time (fixed– comments). That means that today, if you went through the atmosphere sifting through molecules and collected one million of them, you should only find 380 that were CO2. There are often remarks which read like “how can such a small amount make such a large difference?”
From Part 1 we should be able to calculate the energy balance of a planet, and should be able to calculate the equilibrium blackbody temperature of an isothermal spherical zero-albedo planet, as a function of distance from a sun having a given photospheric temperature (the outer layer of the sun).
I thought I would work backwards a bit and go over some of the basics of radiative transfer/balance in the climate system, and how daily weather systems work. The post is rather broad, and (hopefully) easy-to-read version of what factors determine the Earth’s climate.
The Scientific Basis for Anthropogenic Climate Change
Note* The graphs that appear here can be clicked for enhanced versions
Climate Science can be a bit like detective work when dealing with an issue such as global warming. First of all, we have a “detection and attribution” process where we spot a problem, and then try to find a suspect. Like they do on CSI, you find a body, and then go and find evidence to put blame on someone or something. Right now, the problem is the globe is warming, and it appears that anthropogenic CO2 plays a large part in this trend.
