Hansen 1988 Revisited

Hansen’s 1988 temperature projections have recently received quite a bit of attention, e.g., RealClimate, WUWT and SkS. The pro-AGW sites state than Hansen has done very well, whereas the anti-AGW say that he hasn’t. Therefore, I thought that it would be a good time to revisit Hansen’s work to determine how well he did?

Temperature Sensitivity & What Can We Learn?

Dana1981 @ SkS states that:

“The observed temperature change has been closest to Scenario C, but actual emissions have been closer to Scenario B. This tells us that Hansen’s model was “wrong” in that it was too sensitive to greenhouse gas changes. However, it was not wrong by 150%, as Solheim claims. Compared to the actual radiative forcing change, Hansen’s model over-projected the 1984-2011 surface warming by about 40%, meaning its sensitivity (4.2°C for doubled CO2) was about 40% too high.

What this tells us is that real-world climate sensitivity is right around 3°C, which is also what all the other scientific evidence tells us. Of course, this is not a conclusion that climate denialists are willing to accept, or even allow for discussion.”

Perhaps. Climate sensitivity may be ≈ 3°C but we can also learn several other things as discussed below.

How Well Did Hansen Do?

Hansen Compared With the Real World

Figure 1 shows Hansen’s scenarios compared with the GISS Land-Ocean Index (LOTI). I have also added Dana1981’s data as Scenario D. This is the Scenario B data but with the temperature sensitivity reduced from 4.2°C to 2.7 °C. Dana did this by multiplying the Scenario B data by a factor of (0.9*3/4.2), which equates to temperature sensitivity of 2.7 °C (see SkS for the data). The SkS estimate for Scenario D appears to be based on Schmidt (2009).

Figure 1: Hansen’s 1988 Scenarios compared with Real-world Temperatures

It is evident from Figure 1 that the best fit for real world temperatures is Scenario C. However, the pro-AGW in SkS state that Scenario C is irrelevant because it uses the “wrong” sensitivity of 4.2°C and incorrect emissions. Therefore, perhaps I should modify my conclusion to real-world temperatures are following Scenario D, which has the “right” temperature sensitivity of 2.7°C and emissions that are close to actual emissions. It makes no difference; Scenarios C and D are similar, although Scenario D has tended to under-predict temperatures for the last 30 years or so.

2012 Projections

Hansen’s temperature projections for 2012 are compared with the LOTI data in Table 1. It should be noted that the 2012 LOTI temperature estimate is based on the 12-month running average from Jun-2011 to May 2012.

Scenario

2012 Anomaly (°C)

Comparison

With LOTI

(%)

Source

A

1.18

226%

Hansen (1988a)

B

1.77

205%

Hansen (1988a)

C

0.60

116%

Hansen (1988a)

D

0.67

128%

Dana (2011)

LOTI

0.52

100%

GISS LOTI

Note: The comparison with LOTI is based on Scenario/LOTI.

Table 1: Comparison of Hansen’s 1988 Temperature Projections for 2012

Comparing Hansen’s temperature projections with LOTI, it is evident that Hansen’ didn’t do very well.

Scenarios A and B overestimated real-world temperatures by a whopping 126% and 105% respectively. Scenario D over-predicts by 28% and even the no-increase-in-emissions Scenario C over-predicts real-world temperatures by 16%.

What do we learn? We could argue that climate sensitivity should be reduced to ≈ 2.1°C to correspond to the 28% over-prediction in Scenario D. However, I would suggest that we wait a few more years to determine the trend more accurately.

2019 Projections

The timeline for Hansen’s temperature projections for 2019 is presented in Table2. A summary of the comments made by different commentators are included to show how the favoured scenario/projection evolved with time.

Scenario

2019 Anomaly (°C)

Comparison

With

Scenario D (%)

Source

Comments
B

1.10

160%

Hansen (1988a)

In May 1988, Hansen states in AGU paper that, “Scenario A, assumes that growth rates of trace gas emissions typical of the 1970s and 1980s will continue indefinitely…[but]…since it is exponential, must eventually be on the high side of reality in view of finite resource constraints…Scenario B is perhaps the most plausible of the three cases.”
A

1.57

227%

Hansen (1988b)

In June 1988, Hansen states to US Congressional Committee that Scenario A was “business as usual.”
B

1.10

160%

Hansen (2005)

Hansen states that, “In my testimony in 1988, and in an attached scientific paper… Scenario A was described as “on the high side of reality”…The intermediate Scenario B was described as “the most plausible”… is so far turning out to be almost dead on the money.”
B

1.10

160%

Hansen (2006)

Hansen assesses the predictions and states that the close agreement, “for the most realistic climate forcing (scenario B) is accidental.” He states current estimate for sensitivity is 3 ± 1°C.
B-

1.00

144%

Schmidt (2007)

RealClimate blog, Schmidt states that forcings in Scenario B are “around 10% overestimate.”
B-

1.00

144%

Schmidt (2009)

RealClimate blog, Schmidt states that Scenario B, “is running a little high compared with the actual forcings growth (by about 10%)”
B

1.00

144%

Schmidt (2011)

RealClimate blog by Schmidt, “As stated last year, the Scenario B in that paper is running a little high compared with the actual forcings growth (by about 10%)”
D

0.69

100%

Dana (2011)

Skeptical Science blog, climate sensitivity reduced from 4.2 to 2.7°C for Scenario B. Use this as the benchmark for comparison.
?

?

?

Schmidt (2012)

RealClimate blog, Schmidt states that Scenario B, “is running warm compared to the real world (exactly how much warmer is unclear)”
C

0.61

88%

Hansen (1988a))

Hansen’s original Scenario C. This is the commitment scenario with emissions held at year 2000 levels. Include this as a measure of how well the other scenarios perform.

Note: The comparison with Scenario D is based on Scenario/Scenario D

Table 2: Evolution of Hansen’s 1988 Temperature Projections for 2019

It is evident from the timeline and narrative in Table 2 that the evolution in temperature is generally downwards; apart from the brief upwards spurt for US Congressional Committee presentation in June 1988 (more on this in unethical conduct later in this blog).

The following points are also evident:

  • There is a large reduction in the estimate for the 2019 temperature anomaly from Hansen’s estimate of 1.57°C in 1988 (as presented to the US Congress) to Dana’s estimate of 0.69°C in 2011.
  • Until recently (Schmidt, 2012) the overestimate in Scenario B was portrayed as ≈ 10% but Dana at SkS (2011) showed that the overestimate was ≈ 44%.

What do we learn? All of the pro-AGW blogs states that the Hansen Scenario B was pretty good estimate. I suggest that an error of ≈ 44% is pretty bad.

Unethical Behaviour

Hansen’s paper Hansen (1988a) was published in August 1988 but it is important to note that it was accepted for publication on 6 May 1988. This date is particularly relevant because Hansen stated on 6 May 1988 that:

Yet, one month later Hansen (1988b)
in his congressional testimony here he described Scenario A as “business as usual” (see below):


Notice that Scenario A is stressed to be “business as usual”. No mention to Congress that Scenario B was “most plausible” and that Scenario A was “on the high side of reality”.

Later (2006), Hansen re-worded his 1988 congressional testimony to be Scenario A, “was described as on the high side of reality”.


From the foregoing, it is evident that Hansen did not describe to Congress in 1988 that Scenario A was on the “high side of reality”. At best, he has been economical with the truth by re-writing history and (at worst) he has been unethical and totally unprofessional.

Conclusions

I offer the following conclusions regarding Hansen 1988:

  • Temperature forecasts (sorry, should I use the politically correct term projections?) for 2019 have plummeted from 1.57°C in 1988 to 0.69°C in 2011.
  • Estimates of temperature are in error by ≈ 60 for Scenario B and 127% for Scenario A.
  • Climate sensitivity has also fallen from ≈ 4.2°C to ≈ 2.1-2.7°C, i.e., it has fallen to 50-64% of Hansen’s 1988 estimates.

These sorts of errors do not represent pretty good estimates.

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