Last week’s blog looked at various methods of distinguishing “natural” vs. “artificial” vanilla. I used this as a jumping off point to facilitate answering the much more important question of how we distinguish anthropogenic climate change from “natural” climate change (i.e. that which took place way before humans had the capacity to inflict any changes on the physical environment of the planet).
Deniers’ most common argument is that they agree that the climate is changing but that it has been doing so since long before humans were around. More specifically, they claim that carbon dioxide couldn’t be a cause of such changes in the here and now because it is a “natural,” “harmless” compound.
Tom Curtis summarized the issue and enumerated the problems with such reasoning in his July 25, 2012 post on Skeptical Science. Skeptical Science is a blog for which I have the utmost respect (July 13, 2013) and which published one of my own guest posts. Here is a condensed outline of the data Curtis provided:
There are ten main lines of evidence to be considered:
1. The start of the growth in CO2 concentration coincides with the start of the industrial revolution, hence anthropogenic;
2. Increase in CO2 concentration over the long term almost exactly correlates with cumulative anthropogenic emissions, hence anthropogenic;
3. Annual CO2 concentration growth is less than Annual CO2 emissions, hence anthropogenic;
4. Declining C14 ratio indicates the source is very old, hence fossil fuel or volcanic (ie, not oceanic outgassing or a recent biological source);
5. Declining C13 ratio indicates a biological source, hence not volcanic;
6. Declining O2 concentration indicate combustion, hence not volcanic;
7. Partial pressure of CO2 in the ocean is increasing, hence not oceanic outgassing;
8. Measured CO2 emissions from all (surface and beneath the sea) volcanoes are one-hundredth of anthropogenic CO2 emissions; hence not volcanic;
9. Known changes in biomass too small by a factor of 10, hence not deforestation; and
10. Known changes of CO2 concentration with temperature are too small by a factor of 10, hence not ocean outgassing.
Figure 1 – Anthropogenic and total atmospheric carbon dioxide concentrations and the 14C isotopic decline (for more details see my Oct 3, 2017 blog on attributions)
Figure 1 demonstrates the changes in the first four items on this list, which I have also described in earlier blogs. The decline of 14C is the common denominator with the characterization of “natural” vanilla from last week’s blog. This important metric, however, becomes far less useful after the end of WWII because of the atmospheric contamination from the nuclear testing that took place at that time (As stated in the caption for Figure 1: “After 1955 the decreasing 14C trend ends due to the overwhelming effect of bomb 14C input into the atmosphere”).
Yet even the limited decline of 14C from 1900 to 1950 is telling: that period constitutes the start of the significant anthropogenic contributions to the atmospheric concentrations of carbon dioxide.
These important results present a strong argument that most of the increase in the atmospheric concentrations of carbon dioxide comes from humans burning fossil fuels. Deniers say that this is not sufficient grounds for associating the increase carbon dioxide concentration with climate change.
To make the argument that carbon dioxide is the main greenhouse gas responsible for climate change, I will quote one of the most famous scientists of the 20th century. Far from being thought of as a climate change–centered scientist, Edward Teller is instead known for creating the hydrogen bomb after the Second World War.
But Teller associated carbon dioxide with the global climate when he made a speech at the celebration of the centennial of the American oil industry in 1959:
Ladies and gentlemen, I am to talk to you about energy in the future. I will start by telling you why I believe that the energy resources of the past must be supplemented. First of all, these energy resources will run short as we use more and more of the fossil fuels. But I would […] like to mention another reason why we probably have to look for additional fuel supplies. And this, strangely, is the question of contaminating the atmosphere. [….] Whenever you burn conventional fuel, you create carbon dioxide. [….] The carbon dioxide is invisible, it is transparent, you can’t smell it, it is not dangerous to health, so why should one worry about it?
Carbon dioxide has a strange property. It transmits visible light but it absorbs the infrared radiation which is emitted from the earth. Its presence in the atmosphere causes a greenhouse effect [….] It has been calculated that a temperature rise corresponding to a 10 per cent increase in carbon dioxide will be sufficient to melt the icecap and submerge New York. All the coastal cities would be covered, and since a considerable percentage of the human race lives in coastal regions, I think that this chemical contamination is more serious than most people tend to believe.
This connection is a simple physical property of carbon dioxide that falls under the scientific discipline called “spectroscopy” (December 10, 2012). The connection is one of the most important parameters that characterizes climate change, as shown in Figure 2; we call it “climate sensitivity.”
Figure 2 – Projected temperature increase as a function of projected carbon dioxide increase (from the 4th IPCC report – AR4) December 10, 2012 blog
Radiative forcing due to doubled CO2
CO2 climate sensitivity has a component directly due to radiative forcing by CO2, and a further contribution arising from climate feedbacks, both positive and negative. “Without any feedbacks, a doubling of CO2 (which amounts to a forcing of 3.7 W/m2) would result in 1 °C global warming, which is easy to calculate and is undisputed. The remaining uncertainty is due entirely to feedbacks in the system, namely, the water vapor feedback, the ice-albedo feedback, the cloud feedback, and the lapse rate feedback”; addition of these feedbacks leads to a value of the sensitivity to CO2 doubling of approximately 3 °C ± 1.5 °C, which corresponds to a value of λ of 0.8 K/(W/m2).
In the next blog I will look at how climate change deniers self-justify labelling this as “fake news,” thus relieving themselves of the burden of engaging with (or even considering) the associated science.