by Judith Curry

The scientific debate is now over; the moment of closure has arrived. – Shaun Lovejoy

EOS has published an opinion piece by Shaun Lovejoy: Climate Closure. Below are excerpts; see the paper (open access) for the figures and methodological details.

In the battle of public opinion over climate change, we can play to science’s strengths by shifting tactics: Instead of struggling to prove humans are to blame, let’s prove denialist fantasies wrong.

A straightforward line of reasoning demonstrates that the only viable explanation of postindustrial warming is an anthropogenic source. This explanation is compatible with the “pause” in the warming since 1998, and it demonstrates that, in a statistical sense, such a pause is extremely likely.

Global warming science has concentrated on proving the theory that the postindustrial warming is largely caused by human activities. Yet no scientific theory can be proved beyond all doubt, and our attempts to convince people of the science are entering a period of diminishing returns.

Below, we summarize a straightforward disproof that achieves this closure so that the only viable explanation of the warming is anthropogenic. The same methodology also shows how the anthropogenic theory is compatible with the “pause” in the warming since 1998 and, indeed, in a statistical sense, that such a pause is extremely likely.

Figure 1a shows the global annual temperature plotted not as a function of the date, but rather as a function of the CO2 forcing. Even without fancy statistics or special knowledge, it is easy to see that the temperature (plotted in green) increases very nearly linearly with some additional fluctuations; these represent the natural variability. The slope (black), 2.33°C per CO2 doubling, is the actual historical increase in temperature due to the observed increase in CO2: the “effective climate sensitivity.”

The difference (residues) between the actual temperature and the anthropogenic part is the natural variability. So knowing only the slope of Figure 1a and the global annual CO2, we could predict the global temperature for the next year to this accuracy. Clearly, this residue must be close to the true natural variability.

The range of the straight line in Figure 1a is an estimate of the total anthropogenic warming since 1880—about 1°C. What is the probability that the denialists are right and that this is simply a giant natural fluctuation? This would be a rare event but how rare?

To check that comparisons of the current period against the historical record are valid, L1 reconstructed records of volcanic and solar activity. That study concluded that the statistics of the industrial epoch variations are no different from the preindustrial ones. Volcanic activity was highly intermittent but no more so than usual; solar activity, which denialists often blame for the observed warming, has, if anything, diminished over the last 50 years [Foukal et al., 2006].

Then, L1 used preindustrial temperature series drawing on several sources to estimate the likelihood of a given amount of natural temperature change. Applying the usual statistical approach—the bell curve—to these data leads to the conclusion that the chance of a 1°C fluctuation over 125 years being natural is in the range of 1 in 100,000 to 1 in 3,000,000. This is a rough estimation: for long periods, the standard deviation of temperature differences is twice the 0.1°C value. Hence a 1°C fluctuation is about five standard deviations, or a 1 in 3,000,000 chance.

However, nonlinear geophysics tells us that the extremes should be far stronger than the usual bell curve allows. L1 shows that 1°C, century-long global-scale fluctuations are more than 100 times more likely than the bell curve would predict. This gives a probability of at most 1 in 1000, which is still small enough to confidently reject this possibility.

One can apply the same type of analysis to the hiatus in the warming. Figure 1b shows that it is actually a natural cooling event sufficiently large (≈0.3°C) that it has masked the more or less equal anthropogenic warming over the period.

Although this cooling is somewhat unusual, it is not rare: statistical analysis shows that similar 15-year coolings have a natural return period of 20–50 years. Additionally, in this case, the cooling immediately follows the even larger prepause warming event (1992–1998). That is, the pause is no more than a return to the mean; it can be accurately hindcast.

In any case, far from supporting denialist claims that the warming is over, this return is a necessary consequence of the theory of anthropogenic warming that predicts that the natural variability will cause fluctuations to stay near the long-term anthropogenic trend.

The scientific method is much more effective at rejecting false hypotheses than in proving true ones. By estimating the probabilities of centennial-scale preindustrial temperature changes, with 99.9% confidence we are able to reject the denialist hypothesis that the industrial age warming was from solar, volcanic, or other natural causes, leaving anthropogenic origin as the only alternative.

The scientific debate is now over; the moment of closure has arrived. Although climate scientists must move on to pressing scientific questions such as regional climate projections and the space–time variability, our species must tackle the urgent issue of reducing emissions and mitigating the consequences of the warming.

JC comments

Ok ‘denialists’, this is your opportunity to poke holes in Lovejoy’s argument. Its pretty easy, actually.

First point: The IPCC attributes the warming since 1950 as due to human greenhouse gas emissions. Lovejoy discusses the warming since 1880. About 40% of the warming since 1880 occurred prior to 1950, and is not attributed to human greenhouse gas emissions. Further, according to the IPCC paleo analysis, the globe has been warming for the past 400 years, which also cannot be attributed to human greenhouse gas emissions. The statistics of Lovejoy’s analysis are entirely different if you are looking at a warming period of 65 years rather than 125 years.

Second point: After finally getting the ‘mainstream’ climate scientists to pay attention to the AMO and PDO as possibly being important in the attribution of warming (or lack thereof) in the 20th and 21st century, we need to be reminded that there are centennial scale and even millennial scale internal variations in ocean circulation, that have an unknown impact on global temperatures.

Third point: Dismissing multidecadal to century scale variations in solar radiation is frankly ludicrous. You can argue about the magnitude of the impact of these variations on global climate, but dismissing the existence of these variations is completely unjustified.

Fourth point: Major volcanic eruptions do not occur uniformly in time. E.g. the early part of the 19th century had a relatively large number of strong volcanic eruptions.

Finally, I am appalled that EOS has published an article that liberally uses the word ‘denialist’ to refer to scientists (such as myself) that consider the possibility that ocean oscillations and solar variability play significant roles in determining the 20th and 21st century climate variability.

Lovejoy’s agenda is clear, and it isn’t a scientific one:

our species must tackle the urgent issue of reducing emissions and mitigating the consequences of the warming.

Fuzzy reasoning by the proponents of human caused warming and urgent emissions reductions do not help their ’cause’.

I can only wonder what the EOS editors were thinking when they published this.