Greenhouse gas vs temperature

Roughly speaking, greenhouse gases have been continually increasing since 1900. They underwent a steady increase from 1900, with a sharp acceleration around 1950, followed by continued increase at this faster rate up until now. In contrast, global temperatures have oscillated. They showed a sharp increase from 1920, plateaued and then dropped around 1950, before picking up pace and increasing sharply again around 1970.

Recently I was asked by someone, if greenhouse gases drive global temperatures, why haven’t temperatures shown the same pattern of increase as greenhouse gases?

The work of Meehl et al. (2004, Journal of Climate), particularly the figures, provide a straightforward answer. The paper is a few years old (and doesn’t talk about the more complicated feedbacks present) however the same basic story is being told by the newer models (e.g. summary figures on p 40 of the IPCC report).

Figure 1e of Meehl et al (2004) basically reiterates the point. When greenhouse gases are used to predict the global temperature, they predict a general increase in temperature over the past ~100 years, particularly at the end of the last century. But they cannot account for the oscillation in the temperature around the middle of the century, nor much of the increase at the start of the last century when their concentrations were still very low.

The reason for this is that anthropogenic greenhouse gases are not the only thing influencing global temperature. Figures 1a to d show the temperature predictions from solar variation, ozone, volcanoes, and sulfate. These can explain some of the other variation. For example, solar forcing increases temperature at the start of the last century, which was a time when greenhouse gases were still relatively low.

However – and this is the important point – it is only when all forcings are combined, including the greenhouse gases, that both the oscillations and the recent increase is accounted for. A summary of all of this is shown in Figure 2d below:

Figure 2d from Meehl et al (2004)

The natural forcings explain some of the temperature record, but they cannot explain the mid 20th century warming. It is only when greenhouse gases are accounted for that the full pattern is explained.

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