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What do we learn from James Hansen's 1988 prediction?

What the science says...

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Although Hansen's projected global temperature increase has been higher than the actual global warming, this is because his climate model used a high climate sensitivity parameter. Had he used the currently accepted value of approximately 3°C warming for a doubling of atmospheric CO2, Hansen would have correctly projected the ensuing global warming.

Climate Myth...

Hansen's 1988 prediction was wrong
'On June 23, 1988, NASA scientist James Hansen testified before the House of Representatives that there was a strong "cause and effect relationship" between observed temperatures and human emissions into the atmosphere. At that time, Hansen also produced a model of the future behavior of the globe’s temperature, which he had turned into a video movie that was heavily shopped in Congress. That model predicted that global temperature between 1988 and 1997 would rise by 0.45°C (Figure 1). Ground-based temperatures from the IPCC show a rise of 0.11°C, or more than four times less than Hansen predicted. The forecast made in 1988 was an astounding failure, and IPCC’s 1990 statement about the realistic nature of these projections was simply wrong.' (Pat Michaels)

Hansen et al. (1988) used a global climate model to simulate the impact of variations in atmospheric greenhouse gases and aerosols on the global climate.  Unable to predict future human greenhouse gas emissions or model every single possibility, Hansen chose 3 scenarios to model.  Scenario A assumed continued exponential greenhouse gas growth.  Scenario B assumed a reduced linear rate of growth, and Scenario C assumed a rapid decline in greenhouse gas emissions around the year 2000.

Misrepresentations of Hansen's Projections

The 'Hansen was wrong' myth originated from testimony by scientist Pat Michaels before US House of Representatives in which he claimed "Ground-based temperatures from the IPCC show a rise of 0.11°C, or more than four times less than Hansen predicted....The forecast made in 1988 was an astounding failure." 

This is an astonishingly false statement to make, particularly before the US Congress.  It was also reproduced in Michael Crichton's science fiction novel State of Fear, which featured a scientist claiming that Hansen's 1988 projections were "overestimated by 300 percent."  Moreover, Michaels has continued to defend this indefensible distortion.

Compare the figure Michaels produced to make this claim (Figure 1) to the corresponding figure taken directly out of Hansen's 1988 study (Figure 2).

 

Figure 1: Pat Michaels' presentation of Hansen's projections before US Congress

 

Figure 2: Projected global surface air temperature changes in Scenarios A, B, and C (Hansen 1988)

Notice that Michaels erased Hansen's Scenarios B and C despite the fact that as discussed above, Scenario A assumed continued exponential greenhouse gas growth, which did not occur.  In other words, to support the claim that Hansen's projections were "an astounding failure," Michaels only showed the projection which was based on the emissions scenario which was furthest from reality. 

Gavin Schmidt provides a comparison between all three scenarios and actual global surface temperature changes in Figure 3.

 

Figure 3: Hansen's projected vs. observed global temperature changes (Schmidt 2009)

As you can see, Hansen's projections showed slightly more warming than reality, but clearly they were neither off by a factor of 4, nor were they "an astounding failure" by any reasonably honest assessment.  Yet a common reaction to Hansen's 1988 projections is "he overestimated the rate of warming, therefore Hansen was wrong."  

In fact, when skeptical climate scientist John Christy blogged about Hansen's 1988 study, his entire conclusion was "The result suggests the old NASA GCM was considerably more sensitive to GHGs than is the real atmosphere."  Christy didn't even bother to examine why the global climate model was too sensitive or what that tells us.  If the model was too sensitive, then what was its climate sensitivity?

This is obviously an oversimplified conclusion, and it's important to examine why Hansen's projections didn't match up with the actual surface temperature change.  That's what we'll do here.

Hansen's Assumptions

Greenhouse Gas Changes and Radiative Forcing

So which scenario was the most accurate representation?  Figures 4 below provides the answer.  The radiative forcings for Hansen's three scenarios were estimated using the simplified radiative forcing expressions from the 2001 IPCC report, based on the projected greenhouse gas atmospheric concentrations for Hansen's scenarios.  The actual radiative forcing estimates are taken from Skeie et al. (2011).

Hansen et al. only modeled the temperature response to greenhouse gas changes (and a few simulated volcanic eruptions).  So in his simulations, the greenhouse gas (GHG)-only forcing and 'all forcings' are the same.  In reality, they are not, with the main non-GHG forcing involving human aerosol emissions, whose effects remain one of the biggest uncertainties in climate science.

In our analysis here, we're interested in the changes since 1988, particularly through 1998.  The radiative forcing changes since 1988 are shown in Figure 4.

Forcings since 1988

Figure 4: Radiative forcing changes (1988 to 2010) for the three emissions scenarios in Hansen et al. 1988 (dark blue [A], red [B], and green [C]) vs. Skeie et al. (2011) GHG-only (light blue) and all anthropogenic forcings (purple).

Both the GHG-only and net anthropogenic forcing changes between 1988 and 1998 were very close to Hansen's Scenario C, consistent with Figure 1 above, primarily due to the CFC emissions reductions as a result of the Montreal Protocol.

Recreating Michaels' Congressional Testimony Graphic

As Figure 4 shows, Hasen's Scenario B is currently closest to the actual forcing (according to Skeie et al.), but running about 16% too high (since 1988).  Figure 5 reproduces Hansen's Scenario B with a 16% reduction in the warming trend, to crudely correct for the discrepancy between it and the actual radiative forcing.  This might be what Michaels' graphic would look like if he were to give an accurate version of his presentation today:

Hansen Actual Prediction

Figure 3: Observed temperature change (GISTEMP, blue) and with solar, volcanic and El Niño Southern Oscillation effects removed by Foster and Rahmstorf (green) vs. Hansen Scenario B trend adjusted downward 16% to reflect the observed changes in radiative forcings since 1988, using a 1986 to 1990 baseline.

In Figure 3 we've included both GISTEMP data, and GISTEMP with solar, volcanic, and El Niño Southern Oscillations removed by Foster and Rahmstorf (2011).  The 1988 to 2010 trends are similar, 0.20°C per decade with the natural effects, 0.18°C per decade without.  Scenario B has a 0.23°C per decade trend, but when removing a simulated volcanic eruption in 1996, the trend decreases to about 0.22°C per decade.

As the figure above shows, Hansen's 1988 model overpredicted the ensuing global warming.  However, it only overpredicted the warming by approximately 15 to 25%, which is a far cry from the 300% overprediction claimed by Michaels in his 1998 congressional testimony.

Climate Sensitivity

Climate sensitivity describes how sensitive the global climate is to a change in the amount of energy reaching the Earth's surface and lower atmosphere (a.k.a. a radiative forcing).  Hansen's climate model had a global mean surface air equilibrium sensitivity of 4.2°C warming for a doubling of atmospheric CO2 [2xCO2].  The relationship between a change in global surface temperature (dT), climate sensitivity (λ), and radiative forcing (dF), is

dT = λ*dF

Knowing that the actual radiative forcing was slightly lower than Hansen's Scenario B, and knowing the subsequent global surface temperature change, we can estimate what the actual climate sensitivity value would have to be for Hansen's climate model to accurately project the average temperature change.

What we find is that Hansen's results add to the long list of evidence that climate sensitivity is not low.  As noted above, Hansen's model overpredicted the ensuing global warming thus far by approximately 15 to 25%.  Thus if we estimate that the sensitivity of his model was 15 to 25% too high (which is an oversimplification, but will give us a reasonably accurate back-of-the-envelope estimate), this suggests the actual climate sensitivity is approximately 3.4 to 3.6°C for doubled CO2, which is close to the IPCC best estimate of 3°C.

The argument "Hansen's projections were too high" is thus not an argument against anthropogenic global warming or the accuracy of climate models, but rather an argument against climate sensitivity being as high as 4.2°C for 2xCO2, but it's also an argument for climate sensitivity being around 3°C for 2xCO2, which is consistent with the range of climate sensitivity values in the IPCC report.

Spatial Distribution of Warming

Hansen's study also produced a map of the projected spatial distribution of the surface air temperature change in Scenario B for the 1980s, 1990s, and 2010s.  Although the decade of the 2010s has just begun, we can compare recent global temperature maps to Hansen's maps to evaluate their accuracy.

Although the actual amount of warming (Figure 5) has been less than projected in Scenario B (Figure 4), this is due to the fact that as discussed above, we're not yet in the decade of the 2010s (which will almost certainly be warmer than the 2000s), and Hansen's climate model projected a higher rate of warming due to a high climate sensitivity.  However, as you can see, Hansen's model correctly projected amplified warming in the Arctic, as well as hot spots in northern and southern Africa, west Antarctica, more pronounced warming over the land masses of the northern hemisphere, etc.  The spatial distribution of the warming is very close to his projections.

 

Figure 4: Scenario B decadal mean surface air temperature change map (Hansen 1988)

 

Figure 5: Global surface temperature anomaly in 2005-2009 as compared to 1951-1980 (NASA GISS)

Hansen's Accuracy

Had Hansen used a climate model with a climate sensitivity of approximate 3°C for 2xCO2 (at least in the short-term, it's likely larger in the long-term due to slow-acting feedbacks),  he would have projected the ensuing rate of global surface temperature change accurately.  Not only that, but he projected the spatial distribution of the warming with a high level of accuracy.  The take-home message should not be "Hansen was wrong therefore climate models and the anthropogenic global warming theory are wrong;" the correct conclusion is that Hansen's study is another piece of evidence that climate sensitivity is in the  IPCC stated range of 2-4.5°C for 2xCO2.

Last updated on 22 January 2012 by dana1981. View Archives

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Comments

Comments 1 to 19:

  1. I have updated the chart with rolling 12 month values for the latest GISS data (to June 2010).



    It is of possible interest to those looking at "record" temperatures that the current 12 month average is the highest on record (both for station data and Land plus Ocean data).
  2. Hi there - I'm relatively new to commenting here so apologies if I'm missing something. I've read through dana1981's Advanced and Basic versions of this rebuttal, and something important appears to be omitted from this Basic version - namely that Pat Michaels was misleading in saying that "That model predicted that global temperature between 1988 and 1997 would rise by 0.45°C." Together with Peter Hogarth's updated chart (above), it appears that even though Hansen overestimated the sensitivity parameter, his Scenario C projection is not far off from the GISS measured temperatures. I'm not sure if it's too late to make any updates to the rebuttal, but the key conclusion here might be that Hansen's 1988 projections - even though based on far less data than we have now - were within the range of what has actually been observed. Furthermore, the measured warming provides support that Hansen had the fundamentals of climate science correct, namely that human factors are driving GHG emissions and causing global warming that is significant enough that it can be directly measured over just a few decades - not centuries from now.
  3. Also while actual temps are in the range of Scenario C, greenhouse gas emissions have not followed those in that particular projection. It makes more sense to focus on Scenario B, which has been very close to actual emissions, and then determine why the actual temp change has been lower (mainly the climate sensitivity factor difference).
  4. Ok thanks for clarifying about Scenario C. It still might not hurt to explain in the "Basic" version that: 1) Michaels was misleading by focusing on Scenario A and ignoring Scenario B, and 2) Hansen had less data in 1988 and got the sensitivity wrong, but his overall theory (GHG and temp increases) has been borne out by observations in the last decades. Thanks again for the great post here!
  5. Another misleading analysis of Hansen's 1988 scenarios

    this time bob carter and david evans getting it hideously wrong.

    Note the substitution of tropospheric temperatures when the projections were for surface temperatures.

    Note the complete disregard of non CO2 greenhouse gases in order to claim scenario A best fits reality. Check the comments. There is a quote mine of Hansen 1988 to support that disregard.

    Worse of all a complete lack of research. It's like they haven't even bothered reading any analyses of the 1988 scenarios, including in some of hansen's later papers. Like they didn't even use google.

    There's enough wrong in that article for a skepticalscience correction imo. If you google some of the text in the article you will find it's been copy pasted around over the years.

    Another bad thing is how none of the commenter seem to know it's wrong.
  6. Doesn't the link from RealClimate show the projected CO2 for 2010 was 392ppmv? And wasn't the actual level in 2010 389ppmv? It's a minor point, but it looks like you have that reversed on your chart.
  7. Disregard my above post, I misread the column from the Realclimate link. :)
  8. What are the basic reasons why Hansen et al chose a climate sensitivity of 4.2C as input to their model?
  9. Climate sensitivity isn't an input, it's built into the model based on how various feedbacks react to a given forcing. I think understanding ocean interactions was one of the big challenges that took a while, perhaps the amount of CO2 uptake by the oceans.
  10. The article says "we find that in order to accurately predict the global warming of the past 22 years, Hansen's climate model would have needed a climate sensitivity of about 3.4°C for a doubling of atmospheric CO2."
    Can you show me those results. I'd love to see just how well the model worked for the 3.4 degree forcing.
  11. dging, see the final figure in this post (although it uses 3°C sensitivity, not 3.4°C. As you can see, the result is that the projection is a bit low).
  12. Is this a reasonable way to look at Hansen's prediction?

    It shows the slope at the time Hansen made the prediction against the slope from the same point till now.

    He was wrong about some of the numbers but his claim that the rate of warming would considerably increase certainly bore fruit.
  13. RealClimate has a new post describing an article from Hansen 1981 and its prediction of future warming. Hansen was about 30% lower than observed warming for this 30 year validation. Perhaps a review of this article could be added to the predictions link.

    Unfortunately, that prediction calls for a rapid increase in global warming in the near future.
  14. What was Hansen using as the climate sensitivity to doubling CO2 back in 1988? Perhaps it is mentioned somewhere but I'm failing to see it.
  15. balanceact - See the intermediate version of this post - his sensitivity estimate was 4.2°C/doubling.
  16. "Forecast temperature trends for time scales of a few decades or less are not very sensitive to the model’s equilibrium climate sensitivity. Therefore climate sensitivity would have to be much smaller than 4.2ºC, say 1.5-2ºC, in order to modify our conclusions significantly." Hansen (1988)
  17. Russ - I guess that depends on what's considered 'significant'. Transient climate response tends to vary fairly proportionately to equilibrium sensitivity, so a lower sensitivity also means a lower transient response, and a smaller short-term warming. Not a huge difference, but like I said, it depends what you consider 'significant'.
  18. A minor note, inspired by re-reading Myhre et al 1998 on the radiative forcing of various greenhouse gases:

    The forcing from a change in CO2 is estimated as F = α * ln(C/C0) - this is a shorthand fit to what is calculated from a number of line-by-line radiative calculations. 

    The 1990 constant, which is what I presume Hansen used in the 1988 model, had a constant α = 6.3, while Myhre et al 1998, using better radiative estimates, has α = 5.35. And that value has been used ever since in modeling estimations. 

    I suspect that difference in estimating radiative forcing may be responsible for much of the 4.2°C/doubling sensitivity Hansen 1988 (over)estimated, as opposed to the roughly 3°C/doubling value used now. 

  19. Tamino has updated Hansen's 1988 prediction by swapping in actual values of forcings (except volcanic) more recently than was done by RealClimate seven years ago.  The forcings are closest to Hansen's Scenario C forcings.  So actual temperatures should have been closest to Hanson's Scenario C model projection.  Guess what?

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