2010-09-20 04:41:54Basic Rebuttal 89: Hansen's 1988 projections were wrong
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215

I've also put together an Intermediate and Advanced version and would appreciate any feedback on those.  But here's the basic version.

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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.
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.  There are two main reasons for Hansen's warming overestimates:

  1. Scenario B, which was the closest to reality, slightly overestimated how much the atmospheric concentrations of greenhouse gases would increase (particularly methane and chlorofluorocarbons [CFCs]).
  2. Hansen's climate model had a rather high climate sensitivity parameter.  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).
If we take into account the slightly lower atmospheric greenhouse gas increases and compare the observed versus projected global temperature warming rates, as shown in the Advanced version of this rebuttal, 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.  This is within the likely range of climate sensitivity values listed as 2-4.5°C by the IPCC for a doubling of CO2, and even a bit higher than the most likely value currently widely accepted as 3°C.
 
In short, the main reason Hansen's 1988 warming projections were too high is that he used a climate model with a high climate sensitivity, and his results are actually evidence that the true climate sensitivity parameter is within the range accepted by the IPCC.
2010-09-20 05:04:50Comment
Robert Way

robert_way19@hotmail...
142.162.13.137
As shown in Figure 1, the actual increase in global surface temperatures has been higher than any of Hansen's projected scenarios

I don't get that from the graph?
2010-09-20 05:22:31whoops
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215
Whoops, I meant lower.
2010-09-20 08:17:29Thumbs up
John Cook

john@skepticalscience...
121.222.93.62
I think Schmidt's graph is unnecessarily complicated for a basic rebuttal - in an ideal world where we all have oodles of time on our hands, I would just plot the scenarios and one temperature index, probably GISS global land/ocean temperature index. That would make the graph a lot simpler, easier to digest. But that's a lot of time and effort so don't consider that a deal breaker from me.
2010-09-20 08:18:35BTW, one liner needs shortening
John Cook

john@skepticalscience...
121.222.93.62
It needs to get down to a maximum of 100 characters to be tweetable. Good luck with that :-)
2010-09-20 08:23:23tweetable?
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215

Are basic rebuttals usually tweeted?  This looks like about the same length as most.

Is this one too complicated too?  It was posted by Peter Hogarth in the comments to the intermediate rebuttal.

I don't have the software to improve the graphic myself.

2010-09-20 08:40:52
Riccardo

riccardoreitano@tiscali...
93.147.82.75
I think the original figure is better because it shows the trend line. Thumbs up for me.
2010-09-20 08:48:38Go with the original graphic
John Cook

john@skepticalscience...
121.222.93.62
If I was plotting the graph from scratch, I'd use just the land and ocean rolling average which IMHO is a better indication of global average than just land temp. You could do it in Excel just using the data which is available online including the 3 scenarios. But meh, go with the original graphic, it's fine. I'm just being unnecessarily obsessive about using simple graphics :-)
2010-09-20 10:27:46will do
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215
Haha okay.  I think the Schmidt figure isn't too complicated.  People should be able to understand it.  I think this version is good to go.
2010-09-20 11:49:04Published
John Cook

john@skepticalscience...
121.222.93.62
Blog posted the advanced version, linking to all 3 at the end of the post
2010-09-20 12:28:21
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.137.228

Dana,

Given that climate sensitivity itself is a concept, not a physical aspect of the atmosphere which is described in the equations of the models, do you know which input parameters have been tweaked since 1988 that would yield a lower climate sensitivity?.

"Hansen's model had a rather high climate sensitivity input parameter" kind of implies that climate sensitivity itself is an input. It's an allegation that I've clashed with a few skeptics over.  

I like how you've turned the tables, using the argument to support current estimates of climate sensitivity. 

 

2010-09-21 02:23:17sensitivity
dana1981
Dana Nuccitelli
dana1981@yahoo...
38.223.231.249

No, I'm not sure exactly what parameters in Hansen's model led to its rather high climate sensitivity.  I know there's a fairly wide range of sensitivities in today's models as well.

Robert made a similar comment, so I used the phrasing from Hansen's study, that he 'employed a climate model which had a climate sensitivity of 4.2°C', or something similar.

2010-09-21 05:46:49
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.245.170
Dana, I know Gavin Schmidt mentions that the 1988 simulation included a very crude ocean model, which did not accurately portray natural variability, I thought you may have found out the details in your research. It's those little details which interest me, that's all. Cheers.  
2010-09-21 17:48:22Simplified graph
John Cook

john@skepticalscience...
121.222.93.62

Turns out I had to plot the Hansen scenarios for myself anyway for some other purpose so while I was at it, I did a simplified coloured version. Dana, are you happy to use this in your basic version?

 

2010-09-22 01:57:48sure
dana1981
Dana Nuccitelli
dana1981@yahoo...
38.223.231.249
Yeah that looks good.  I'll add it to the basic version.
2010-09-24 02:27:43
doug_bostrom

dbostrom@clearwire...
184.77.83.151

This is really nice, Dana. 

As this is the "basic" version, I wonder if there's anything to be done about the terms "linear" and "exponential?" Would it be appropriate to substitute "constant" and "accelerating" for those?

Not a big deal.  

2010-09-26 11:53:22good idea
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215
Sure, that's not a bad idea.  I'll make that change.
2010-09-26 15:34:24Refining rebuttals
John Cook

john@skepticalscience...
124.185.151.34

Like the idea of refining rebuttals - never think they're locked in stone after they're published. I found myself constantly tweaking and refining my rebuttals, improving the wording, adding content, often tweaking the phrases so my earlier, simpler treatments became more nuanced as my understanding improved. So always feel free to improve your work or comment on others' rebuttals if you have a suggestion on how to improve them.

I don't really go back and update the blog posts much though - it's the rebuttals that are the encyclopedic reference whereas I consider the blog posts more a 'snapshot in time'.