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

What the science says...

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Hansen's 1988 projections were too high mainly because the climate sensitivity in his climate model was high. But his results are evidence that the actual climate sensitivity is about 3°C for a doubling of atmospheric CO2.

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)

In 1988, James Hansen projected future temperature trends using 3 different emissions scenarios identified as A, B, and C.  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 (Hansen 1988).  As shown in Figure 1, the actual increase in global surface temperatures has been less than any of Hansen's projected scenarios.

Figure 1: Global surface temperature computed for scenarios A, B, and C, compared with two analyses of observational data (Schmidt 2009)

As climate scientist John Christy noted, "this demonstrates that the old NASA [global climate model] was considerably more sensitive to GHGs than is the real atmosphere."  Unfortunately, Dr. Christy decided not to investigate why the NASA climate model was too sensitive, or what that tells us.  In fact there are two contributing factors.

Radiative Forcing 

A radiative forcing is basically an energy imbalance which causes changes at the Earth's surface and atmosphere, resulting in a global temperature change until a new equilibrium is reached.  Hansen translated the projected changes in greenhouse gases and other factors in his three scenarios into radiative forcings, and in turn into surface air temperature changes.  Scenario B projected the actual changes we've seen in these forcings most closely.  As discussed by Gavin Schmidt and shown in the Advanced version of this rebuttal, the radiative forcing in Scenario B was too high by about 5-10%.  Thus in order to assess the accuracy of Hansen's projections, we need to adjust the radiative forcing and surface temperature change accordingly.

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 (radiative forcings).  Hansen's climate model had a global mean surface air equilibrium sensitivity of 4.2°C warming for a doubling of atmospheric CO2 [2xCO2].  This is on the high end of the likely range of climate sensitivity values, listed as 2-4.5°C for 2xCO2 by the IPCC, with the most likely value currently widely accepted as 3°C.

Since the Scenario B forcing was about 5-10% too high, its projected global surface air temperature trend was 0.26°C per decade, and the actual surface air temperature trend has been about 0.2°C per decade (NASA GISS), Hansen's climate model's sensitivity was about 25% too high.  Thus the real-world climate sensitivity would be approximately 3.4°C to in order for Hansen's climate model to correctly project the ensuing global surface air warming trend.  This climate sensitivity value is well within the IPCC range.

In other words, the reason Hansen's global temperature projections were too high was primarily because his climate model had a high climate sensitivity parameter.  Had the sensitivity been approximately 3.4°C for a 2xCO2, and had Hansen decreased the radiative forcing in Scenario B slightly to better reflect reality, he would have correctly projected the ensuing global surface air temperature increase.

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 3) has been less than projected in Scenario B (Figure 2), 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 2: Scenario B decadal mean surface air temperature change map (Hansen 1988)


Figure 3: Global surface temperature anomaly in 2005-2009 as compared to 1951-1980 (NASA GISS)

Hansen's Accuracy

The appropriate conclusion to draw from these results is not simply that the projections were 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 20 September 2010 by dana1981.

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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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