Many of those skeptical about the causes of climate change suggest that the complex global climate models (GCMs) often used to make attribution statements are not trustworthy. Here I highlight that GCMs are not needed to roughly attribute nearly all of the observed warming (at least) to changes in greenhouse gases.
This simple attribution consists of a number of steps:
1) Well-mixed greenhouse gas (GHG) concentrations have increased since 1750 due to human activity. These GHG changes include CO2 (278 to 400ppm), CH4 (0.72 to 1.8ppm), NO2 (270 to 325ppb), as well as other gases such as CFCs. The uncertainties on these observations are very small compared to the changes.
2) The radiative forcings of these changes in GHG concentrations are calculated using ‘line-by-line’ radiation codes which examine the absorption of radiation by atmospheric constituents. These produce an overall change in radiative forcing of around 2.8 Wm-2 since 1750, or around 2.6 Wm-2 since 1850 [see Figure 8.6 and Table 8.6 of IPCC AR5]. The same radiation codes produce a value of around 3.7 Wm-2 for a doubling of CO2. The uncertainty on these values is around 10%.
3) The warming at the time of a doubling of CO2 is denoted by the transient climate response (TCR). Here we assume that all GHGs have the same climate sensitivity.
4) A simple model for the expected warming due to a change in greenhouse gas radiative forcing is:
ΔT = TCR × [ΔF(observed) / ΔF(2xCO2)]
5) Even assuming a low value of TCR (=1.0K), then a change in forcing of ΔF(observed)=2.6 Wm-2, when compared to a ΔF(2xCO2)=3.7 Wm-2, produces an expected warming of 0.7K since 1850. If TCR is relatively high (=2.0K), then the expected warming from GHGs alone is 1.4K.
6) The observed change in global temperatures is around 0.8K since 1850. So, even if TCR is at the very low end of the IPCC expectations, the changes in well-mixed greenhouse gases have produced a warming of comparable magnitude to that observed. If TCR is larger, then the fraction increases to be larger than the observed warming. And, this is all without using any GCMs.
7) Of course, other factors are important for how global temperature changes, but this calculation is independent of assumptions about those other factors. Changes in aerosols, volcanoes and land use act to cool the planet and offset the GHG induced warming & internal climate variability, ozone and solar changes are also important. These other forcings may even have a different ‘efficacy’, but this doesn’t affect the simple calculation above. These calculations also allow a direct comparison of the strength of the forcings. For example, net changes in solar forcing are around 0.05Wm-2 – much lower than that for GHGs.
In summary, changes in atmospheric greenhouse gases due to human activity have caused a large fraction of the observed warming. It is even likely that GHGs actually produced more warming than that observed (also see Figure below which is TS-10 from IPCC AR5 WG1), but this has been partially offset by other causes, such as changes in aerosols.