Deniers Fail to Blunt Hansen’s Accurate Climate Forecast

July 5, 2018

Since the 30th Anniversary of Jim Hansen’s 1988 climate predictions, the usual climate denial suspects have been furiously spinning the story to distort the record.
Famously, Hansen posited 3 scenarios, A, B, and C – with A assuming very high Greenhouse gas outputs, and very high temp rise – the scenario assumed, for instance, a high increase in chlorofluorocarbons, which were subsequently limited in an international treaty – so not as big a factor.
In addition, it is well understood that Hansen’s model had a somewhat high climate sensitivity,  4° C as opposed to the currently accepted 3°.
In the interviews I conducted last December – scientists conversant with the research discussed these factors, and point out that given our understanding of actual observations in the last 3 decades, Hansen’s model performed extraordinarily well.

Open Mind:

The forecast about global warming from James Hansen thirty years so starkly demonstrates that he’s been right all along about the effect of greenhouse gases on our climate, that there’s yet another attempt to make it look bad. The authors, Ross McKitrick and John Christy (“MC”), are so desperate that they resort to making a persuasive argument the only way possible: with bullshit. Shameless bullshit.

Observed temperature since the 1980s has landed between Hansen’s “Scenario B” (what was considered a middle-of-the-road greenhouse-gas future) and “Scenario C” (far less greenhouse gas emissions), but MC (McKitrick & Christy) try to make out that it should have been between “Scenario A” (rapid increase of greenhouse gases) and “Scenario B” because the amount of CO2 in the atmosphere turned out to be between those two scenarios. So, they claim, Hansen’s forecast was way off.

The “original” post by MC (McKitrick & Christy) contained this little gem:

“The whole debate has focused on comparisons of the 1988 and 2017 endpoints. Skeptical Science waived away the differences by arguing that if one adjusts for an overestimation in the rise of greenhouse gas (GHG) forcing, Hansen’s 2017 Scenario B prediction was not far off reality.”

As was pointed out at Skeptical Science, greenhouse gases and their total impact ended up nowhere near as high as scenario “A”, and even less than scenario “B”. Result: temperature increase should (according to Hansen) have been nowhere near as high as scenario “A” and even less than scenario “B”. Just like happened.

MC countered by looking at the rise of one and only one greenhouse gas: CO2. How do they justify ignoring methane (CH4) and N2O and chloroflourocarbons when Hansen had included them in his forecasts? By the following bullshit (emphasis mine):

“Note that Scenarios A and B also represent upper and lower bounds for non-CO2 forcing as well, since Scenario A contains all trace gas effects and Scenario B contains none.”

That’s shameless bullshit.

All three of Hansen’s scenarios include “trace gas effects” from multiple trace gases, not just CO2. Scenario “A” had them rising rapidly, scenario “B” had them rising more moderately, scenario “C” rising less rapidly still (in fact, after the year 2000 not rising at all). And scenario “A” includes even more trace gas effects than “B” and “C” — but the claim that scenario “B” (and “C”) contains none is bullshit.

Here’s what Hansen and his colleagues said in their published research at the time:

“… Scenario A assumes that growth rates of trace gas emissions typical of the 1970s and 1980s will continue indefinitely; the assumed annual growth averages about 1.5% of current emissions, so the net greenhouse forcing increases exponentially. Scenario B has decreasing trace gas growth rates, such the that annual increase of the greenhouse climate forcing remains approximately constant at the present level. Scenario C drastically reduces trace gas growth between 1990 and 2000 such that the greenhouse climate forcing ceases to increase after 2000. The range of climate forcings covered by the three scenarios is further increased by the fact that scenario A includes the effect of several hypothetical or crudely estimated trace gas trends (ozone, stratospheric water vapor, and minor chlorine and flourine compounds) which are not included in scenarios B and C.”

That’s clear. Scenarios “B” and “C” omit “several hypothetical or crudely estimated trace gas trends,” but don’t omit all non-CO2 trace gases. Claiming they do is bullshit.

Below, if you missed it, my overview of Hansen’s forecast at 30.


UPDATE: Real Climate:

The model predictions were skillful

Predictive skill is defined as the whether the model projection is better than you would have got assuming some reasonable null hypothesis. With respect to these projections, this was looked at by Hargreaves (2010) and can be updated here. The appropriate null hypothesis (which at the time would have been the most skillful over the historical record) would be a prediction of persistence of the 20 year mean, ie. the 1964-1983 mean anomaly. Whether you look at the trends or annual mean data, this gives positive skill for all the model projections regardless of the observational dataset used. i.e. all scenarios gave better predictions than a forecast based on persistence.




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