Revisiting James Hanson's 1988 Global Warming Forecast to Congress

(Cross-posted from Climate Skeptic)

I want to briefly revisit Hansen's 1998 Congressional forecast.  Yes, I and many others have churned over this ground many times, but I think I now have a better approach.   The typical approach has been to overlay some actual temperature data set on top of Hansen's forecast (e.g. here).  The problem is that with revisions to all of these data sets, particularly the GISS reset in 1999, none of these data sets match what Hansen was using at the time.  So we often get into arguments on where the forecast and actuals should be centered, etc.

This might be a better approach.  First, let's start with Hansen's forecast chart (click to enlarge).

hansen forecast

Folks have argued for years over which CO2 scenario best matches history.  I would argue it is somewhere between A and B, but you will see in a moment that it almost does not matter.    It turns out that both A and B have nearly the same regressed slope.

The approach I took this time was not to worry about matching exact starting points or reconciling difference anomaly base periods.  I merely took the slope of the A and B forecasts and compared it to the slope over the last 30 years of a couple of different temeprature databases (Hadley CRUT4 and the UAH v6 satellite data).

The only real issue is the start year.  The analysis is not very sensitive to the year, but I tried to find a logical start.  Hansen's chart is frustrating because his forecasts never converge exactly, even 20 years in the past.  However, they are nearly identical in 1986, a logical base year if Hansen was giving the speech in 1988, so I started there.  I didn't do anything fancy on the trend lines, just let Excel calculate the least squares regression.  This is what we get (as usual, click to enlarge).

click to enlarge

I think that tells the tale  pretty clearly.   Versus the gold standard surface temperature measurement (vs. Hansen's thumb-on-the-scale GISS) his forecast was 2x too high.  Versus the satellite measurements it was 3x too high.

The least squares regression approach probably under-estimates that A scenario growth rate, but that is OK, that just makes the conclusion more robust.

By the way, I owe someone a thanks for the digitized numbers behind Hansen's chart but it has been so many years since I downloaded them I honestly forgot who they came from.


  1. Daublin:

    That's exactly right. We aren't talking about the projection merely being wrong. The overall shape of the graph doesn't match.

  2. kidmugsy:

    At what point did the climate alarmists slither from being incompetents into being crooks?

  3. Mercury:

    What is the significance/purpose of: "Estimated Temperatures During Altithermal and Eemian Times" ? Is that benchmark cherry picking?
    If we're concerned about anthropogenic global warming of the last 100 years or so why not look at recent changes in global temp etc vs. our current geological epoch only i.e. The Holocene which be began 11,700 years ago, right before which, much of N. America (for instance) was buried under a thick sheet of ice?

  4. Joe:

    As i recall, the warming rate since circa 1850 has been a fairly constant 1.0c per century (after adjusting for the AMO/PDO affect - fwiw, imo the current pause is most likely a result of the cooling side of the AMO and the overall long term trend has not changed),
    Secondly the cooling trend/LIA stopped circa 1750, took a hop warmer, then a short hop cool and finally emerging circa 1850 with the current warming trend. (note that most credit the end of the LIA circa 1850 - just noting that there was a short pause/warming trend circa 1750)

    My thoughts are that in order to understand the current warming trend, we have to understand why the cooling stopped circa 1750.

    Hard to fathom that going from the utopian 280ppm CO2 to 281ppm of CO2 was so powerful as to stop the cooling trend

  5. SFX:

    >>What is the significance/purpose of: "Estimated Temperatures During Altithermal and Eemian Times" ?

    Altithermal is a term for the Holocene Climatic Optimum, when temperatures were warmer than any time since. It's not an exact time, but it's believed to be 4000-6000 years ago. Oceans were also about three meters higher than present.

    Hansen knows this. Or knew it at the time he published.

  6. Q46:

    Unfortunately has nothing to do with fact, it is politics. More precisely global government.

  7. thesafesurfer:

    Can you imagine Hansen's claims of the effectiveness of counter warming policy if Gore had won the election and followed through with Draconian carbon controls. Hansen would have trotted out the real accurate info twenty years later and claim "look how our prudent steps worked, we need more."

    Really scary when scientists abandon science.

  8. joe:

    But we elected Obuma - and what happened
    The seas quit rising as he promised
    The earth's fever has "paused"

    He has been great


  9. joshv:

    BTW, How's your sine wave with a constant trend projection doing these days?