This post is about the motivation for this blog. So it's an important post. Perhaps this post should have come at the start. Anyway, diving in... what are the key facts of the matter about climate change?
Firstly, we know there is an important natural greenhouse effect on Earth. The sun, which is hot (at an effective temperature of 5000C) emits electromagnetic radiation at a high energy (high frequency; low wavelength - mostly in the visible and ultra-violet spectra). The Earth, which is warm (about 14C) re-emits electromagnetic radiation mostly at low energies (in the infrared spectra). Some of this infra red radiation is absorbed by certain gases in the atmosphere (water vapour and carbon dioxide) and then re-radiated. Half of this re-radiating heat radiation goes back down to earth, leading to a higher equilibrium surface temperature. We can easily calculate what the temperature of the earth would be without greenhouse gases - about minus 15Celsius. So we know that the temperature of the Earth is about 30Celsius higher than it would otherwise be, due to the effect of these gases. It seems that water vapour and carbon dioxide are the two most important of these gases, although other gases such as methane are also important.
Water vapour is the most important component of the greenhouse effect, but the concentration of water vapour in the atmosphere depends on temperature. The second most important gas carbon dioxide had an atmospheric concentration of about 280 parts per million by volume (ppmv) before human industrialisation (and after the ice ages where it dropped to 180 ppmv).
The most basic climate model w0uld suggest that a 100% effective greenhouse would raise (absolute) temperatures by 2^(1/4) or about 20%*250K=50 Celsius. This can be compared to the observed temperature increase of about 30 Celsius.
Simple models (similar to those of Arrhenius) suggest that the doubling of carbon dioxide concentrations from 280 to 560ppmv would raise global average temperatures by approximately 5C. Complex (General Circulation) models, and an analysis of the forcings in the ice ages (see previous post) suggest an average temperature rise of 3C for a doubling of CO2 concentrations, but with a probability range between about 1.2C - 6C (although there is non-zero chance of temperature rises above the maximum in this range).
Whilst there are arguments why the temperature might be less than the 3C, there are also arguments why it might be higher than this. How do observations of the whole world compare?
Next post I will discuss the observational record and what the estimates of climate sensitivity mean for us.