The Garnaut Climate Review Interim Report—I’m not convinced

The Gar­naut Review’s Inter­im Report accepts the IPC­C’s eval­u­a­tion of the cli­mate risk (and the car­bon-forc­ing the­o­ry) ‘on the bal­ance of prob­a­bil­i­ties’. The Review team’s report says that they have nei­ther the skills nor resources to re-vis­it the cli­mate sci­ence. They acknowl­edge con­tro­ver­sy anduncer­tain­ty in this sci­ence but they decide—not unreasonably—that they have lit­tle option but to work with the pro­jec­tions of the IPCC, which pur­port1 to sum­ma­rize the recent views of a large num­ber of experts.

Most of us will be tempt­ed to do just what the Gar­naut Review team has done and assume that, despite con­tro­ver­sies and uncer­tain­ties, the IPCC sce­nar­ios are a safe bet to be an accu­rate assess­ment ‘on the bal­ance of prob­a­bil­i­ties’. But this isn’t a rea­son to ignore the evi­dence for or against the IPCC sto­ry. For my part, I’m increas­ing­ly uneasy with the IPCC assess­ments. There are too many grounds for rea­son­able doubt about the cen­tral propo­si­tions of a wor­ry­ing tem­per­a­ture rise and the car­bon-forc­ing theory.

I’ll con­fine myself to the first ground for doubt—because it’s the eas­i­est to illustrate—about the basic sto­ry that there is a wor­ry­ing trend in glob­al tem­per­a­tures. Also, I’ll keep this very brief. I have two concerns:

  1. There’s no evi­dence from the recon­struc­tions of the paleo-cli­mate (that is, tem­per­a­ture esti­mates drawn from proxy sources such as ice-cores going back thou­sands of years) that recent glob­al tem­per­a­ture vari­a­tions are out­side the range of his­tor­i­cal, and evi­dent­ly not man-made, cli­mate vari­a­tion. Here’s a pic­ture of a recon­struc­tion that you can read all about on WikiPedia (click). It shows that the prox­ies reveal recent tem­per­a­ture lev­els are close to, or pos­si­bly below, the aver­age over the peri­od since the last ice-age 12,000 years ago
  2. Here’s a sec­ond graph­ic that I con­struct­ed my self (in Excel) from data that’s read­i­ly avail­able on the web. This is a set of esti­mates of glob­al tem­per­a­ture anom­alies : that is, devi­a­tions from aver­age tem­per­a­tures in the peri­od 1961–1990. The esti­mates com­bin­ing actu­al mea­sured land, sea and air tem­per­a­tures are from the UK Mete­o­ro­log­i­cal Research lab­o­ra­to­ry at Hadley. You can get the same data here
    Click for full size
    Click for full size

What I notice in the sec­ond pic­ture is: 

  1. That the range of vari­a­tions mea­sured over this peri­od of time—one and two-third centuries—is about 0.7 degrees cel­sius, maybe a touch more. Not very big at all and not much out of line with the sec­u­lar vari­a­tion in the first picture.
  2. That there has been a jump in the tem­per­a­ture in the late 1990s (1998 was an el Nino year) but that after that the tem­per­a­ture anom­aly did­n’t have any trend at all, up or down, for most of the past 7 years (inset: just an extract from the same data).
  3. There has been a sur­pris­ing drop in Jan­u­ary 2008—the cold­est north­ern win­ter for many years—that is as big as the jump in the late 1990s.

Now none of my obser­va­tions here is new or a sci­en­tif­ic basis for iden­ti­fy­ing a trend in an obvi­ous­ly volatile series such as glob­al tem­per­a­ture anom­alies. But the sto­ry of cli­mate change here does not look so wor­ry­ing to me that I want to rush into any endorse­ment of the­o­ries of big impend­ing threats or calls for dras­tic mit­i­ga­tion strate­gies. It just does­n’t make sense

Although the Gar­naut Review takes the asser­tions of IPCC on face val­ue, some key asser­tions it accepts don’t seem to fit with the facts in the data repro­duced here. Let’s take the state­ment that the inter­im report quotes from a paper dat­ed 2007 from a group of cli­mate sci­en­tists asso­ci­at­ed with the IPCC

Glob­al mean sur­face tem­per­a­ture increase since 1990 has been mea­sured at 0.33ºC, which is in the upper end of the range pre­dict­ed by the IPCC in the Third Assess­ment Report in 2001 (page 21 of the Gar­naut Review Inter­im Report, empha­sis added)

In fact, the HadCRUT3 data in the sec­ond pic­ture above appar­ent­ly con­tra­dict this. If we take the glob­al anom­aly as mea­sured in Jan­u­ary 1990 and Jan­u­ary 2007 we see that they are both 0.22 degrees cel­sius. In oth­er words, pre­cise­ly the same devi­a­tion from the aver­age tem­per­a­ture in the peri­od 1961–1990. There is no appar­ent increase over this peri­od of time.

Of course, vari­a­tion from one fixed point to anoth­er is not sound sta­tis­ti­cal prac­tice, par­tic­u­lar­ly giv­en the range of mea­sure­ment error in the anom­alies (my plot shows only the Hadley Cen­ter’s best esti­mate). But it cer­tain­ly weak­ens the case for extreme action; at least for any­one who remem­bers the awful pan­ic over Y2K.

The role of cit­i­zens who are not cli­mate sci­en­tists is to help the gov­ern­ment decide what lev­els of risk mit­i­ga­tion are pro­por­tion­al to the prob­lem. That is a polit­i­cal ques­tion as much as a sci­en­tif­ic one. Con­sid­er­ing the poten­tial costs of the mit­i­ga­tion strate­gies being considered—the Gar­naut Review Inter­im Report talks of cut­ting Aus­tralian per-cap­tia emis­sions by 70% to 90%—you owe it to your self, and your chil­dren, to ques­tion whether these dra­mat­ic mea­sures are pro­por­tion­ate to the risk as you see it.

1. The rep­re­sen­ta­tive char­ac­ter of the IPCC‑4 report has been chal­lenged by an Aus­tralian researcher who exam­ined the review records

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