Global Warming

An Succinct Explanation of Global Warming

2014

CO₂ is a greenhouse gas. An increase in CO₂ in the atmosphere means that heat will escape to space higher up, where it is colder, which leads to less heat escaping, and hence warming. [1] Specifically, a CO₂ increase from the pre-industrial level C₀ = 280 ppmv to a concentration C will increase thermal radiation by dF W/m² where [2]

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. [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, calculated using the above formula:

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:

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.

Estimates of climate sensitivity, for a doubling of atmospheric CO₂:

From [3], adapted from [5].

When should we have acted?

We could have been aware that there could be a CO₂ problem more than a century ago, arguably, and should have started to act half a century ago: Arrhenius laid down the basics in the late 19th century. [6,7,8] We then could have noted that CO₂ pollution was tied to GDP, maybe linearly (which Arrhenius maybe even did). So with exponential economic growth (which you get from yearly growth) there could be a problem, even though things were still unclear and there was no consensus at the time. Half a century ago things had became sufficiently clear [6,7,8] that we should have acted then.

References

[1] https://www.skepticalscience.com/saturated-co2-effect-basic.htm, Dana Nuccitelli, "Is the CO2 effect saturated?"

[2] 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

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

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

[5] 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, https://www.ethz.ch/content/dam/ethz/special-interest/usys/iac/iac-dam/documents/group/climphys/knutti/publications/knutti08natgeo.pdf

[6] http://www.skepticalscience.com/history-climate-science.html, John Mason, "The History of Climate Science"

[7] https://www.aip.org/history/climate/summary.htm, Spencer Weart, "The Discovery of Global Warming"

[8] https://www.aip.org/history/climate/index.htm, Spencer Weart, "The Discovery of Global Warming"

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