What #ExxonKnew Back When

November 25, 2015


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Working on a video about the recent revelations around Exxon’s climate science program, I came across a striking image yesterday that gives some indication of the skill of Exxon’s modeling, as far back as 1982.

Above, you’ll see a graph showing Exxon’s projections for global warming, published in a 1982 internal briefing document on the greenhouse effect. Fortuitously, one of the forecast years is 2015, so it gives us something we can immediately compare.  For this year, the graph projects an atmospheric CO2 level of 409 ppm, just a tad above our actual 400, and shows an “Average Temperature Increase” of .84 °C.  We’ll have to wait a few weeks for the final word on 2015, but I’m told .84 C is pretty close to what NASA GISS will show.

Stats gurus have already reminded me it’s not a straight up comparison.
NASA’s baseline for starting temps is the 1950 to 1980 global average, and Exxon uses the single year 1979 as a base.  Exxon’s co2 level is slightly higher.  Exxon assumes climate sensitivity as being 3.0°C (+/- 1.5°C), which is dead-on with the 1979 National Academy of Science estimate, and broadly consistent with today.

Not exactly “apples to apples”, – but I’ll call it “macintosh to honeycrisp”.
Exxon correctly predicted a warming, identified the mechanism, and got the time frame and magnitude basically correct.  Show this to your Uncle Dittohead when he tells you “the climate models don’t work.”

For comparison, we can look at contemporary projections from another mainstream climate scientist, for instance, Wallace Broeker from 1975.

Skeptical Science:

Wallace Broecker was among the first climate scientists to use simple climate models to predict future global temperature changes.  His 1975 paper Climatic Change: Are We on the Brink of a Pronounced Global Warming? is widely credited with coining the term “global warming”.

In that paper, Broecker modeled the effects of the expected future increase of CO2 due to humans burning fossil fuels, combined with a natural climate cycle which he estimated based on Greenland ice core records, and tweaked to match the observed temperature record at the time (Figure 1).


Figure 1:  Broecker’s global temperature prediction

This was a very simple model, excluding the effects of the sun, volcanoes, other greenhouse gases, aerosols, and so forth, which Broecker acknowledged:

“In this report only the interaction of the CO2 effect and natural climatic change is considered.  As other anthropogenic effects are shown to be significant and as means to quantitatively predict their future influence on global temperatures are developed, they can be included in models such as this.”

As it turns out, Broecker has been fortunate, because the cooling effects of human aerosol emissions have roughly cancelled out the warming effects of human non-CO2 greenhouse gas emissions since 1975, and solar activity has been flat over that period.  So the net effect of the factors which he did not take into account has been close to zero.  However, Broecker was also smart; the dominant effect on temperature since 1975 has been from CO2, as he expected.  It’s better to be lucky than good, but it’s best to be both.

Broecker anticipated the actual increase in CO2 very closely, predicting 373 ppm in 2000 and 403 ppm in 2010 (actual values were 369 and 390 ppm, respectively).  Broecker also used an equilibrium climate sensitivity of 3°C for doubled CO2; however, his model’s transient climate sensitivity worked out to be 2.4°C for doubled CO2.  Current climate models put equilibrium sensitivty at 1.5 times transient sensitivty, so Broecker effectively underestimated the thermal lag of the climate system, and the equilibrium sensitivity in his calculations was approximately equivalent to 3.6°C for doubled CO2 – a bit higher than today’s best estimates of 2°C transient sensitivity, 3°C equilibrium sensitivity.

We digitized Broecker’s prediction from Figure 1, and compared it to the observed global temperature change since 1975 (Figure 2).  We adjusted it slightly to reflect the current atmosperic CO2 concentration (390 ppm) as opposed to his predicted 403 ppm, because we’re interested in the accuracy of Broecker’s temperature predictions, not his CO2 predictions.

Broecker Prediction vs Observations

Figure 2: Broecker’s temperature prediction, adjusted to reflect measured CO2 changes, vs. GISTemp observed global surface temperature changes.

As you can see, Broecker’s prediction has matched the net global temperature change quite closely over the past 35 years.  His ‘natural cycle’ estimate held his prediction below the actual global temperature increase for most of the period, but  as illustrated in Figure 1, he predicted its effects would approach zero after 2000.  Not coincidentally, this is when his prediction most closely matches the observed global temperature.  Broecker overestimated the amount of global warming by 2010 slightly, by a bit less than 0.2°C.  This is probably mainly due to his slight overestimate of climate sensitivity, and potentially due to the increased cooling effects over the past decade.

It’s quite remarkable that a prediction made in 1975 using such a simple model of the climate system could so accurately match the observed global temperature change.  It’s a testament to the dominant effect of CO2, and the fact that we have had a solid understanding of the fundamental workings of the Earth’s climate for many decades.



One Response to “What #ExxonKnew Back When”

  1. […] plus or minus 1.5° C. Tables in Exxon’s 1982 Climate Change “Primer” for executives show predictions for 2015 markedly similar to contemporary estimates by NASA, and […]

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