A Succinct Explanation of Global Warming

CO2 is a greenhouse gas. An increase in CO2 in the atmosphere from the pre-industrial level C0 = 280 ppmv to a concentration C will increase thermal radiation by dF W/m2 where [1]

  dF = 5.35 * ln(C/C0)

To see how such a thermal radiation change dF affects the global mean temperature, one can look at changes in the past, for example volcanic eruptions, and see how thermal radiation and temperature correlated. These examinations, as well as others, point to a climate sensitivity such that a thermal radiation increase of 1 W/m2 will increase the temperature by around 0.8 °C. [2,3,4,5]

 A doubling of the CO2 concentration would mean a temperature increase of about 5.35*ln(560/280)*0.8 or about 3.0 °C from the pre-industrial level, which is often how climate sensitivity is presented. A tripling gives an increase of about 4.7 °C. Here are CO2 concentrations since 1780 and the resulting temperature change:

CO2 levels and resulting temperature increase 1780-2013
The blue line shows atmospheric CO2 concentrations. The red line shows the resulting temperature increase using a climate sensitivity of 0.8 °C per W/m2. The CO2 data comes from CDIAC and NOAA.


Following are atmospheric CO2 concentrations for the last 11,000 years:

CO2 levels over the last 11,000 years
The CO2 data comes from NOAA, CDIAC and NOAA.


Projected CO2 concentrations and resulting temperature change are shown below. The CO2 projections come from the RCP 8.5 "business as usual" scenario, which, among the more well-known scenarios, has matched actual CO2 levels the closest so far, although the scenario could very well deviate later on of course, particularly on coal use perhaps.

Projected CO2 levels and resulting temperature increase 1960-2060
The blue line shows atmospheric CO2 concentrations. The red line shows the resulting temperature increase using a climate sensitivity of 0.8 °C per W/m2. The CO2 data comes from NOAA and RCP 8.5.


The probability distribution function (PDF) for the climate sensitivity from the latest extensive IPCC report is shown below:
Climate Sensitivity PDF


References

[1] Gunnar Myhre et al., July 1998, "New estimates of radiative forcing due to well mixed greenhouse gases", Geophysical Research Letters 25 (14): 2715–2718, http://folk.uio.no/gunnarmy/paper/myhre_grl98.pdf

[2] http://www.skepticalscience.com/climate-sensitivity-advanced.htm, Dana Nuccitelli, "How sensitive is our climate?", Skeptical Science

[3] http://climatechangenationalforum.org/cause-and-effect-by-scott-denning, Scott Denning, "Cause and Effect", Climate Change National Forum, 2014-03-02

[4]
Reto Knutti and Gabriele C. Hegerl, 2008-10-26, "The equilibrium sensitivity of the Earth's temperature to radiation changes". Nature Geoscience 1 (11): 735–743, http://www.iac.ethz.ch/people/knuttir/papers/knutti08natgeo.pdf

[5] IPCC

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