Temperature trends in Garnaut 2011

I remain uncon­vinced by Dr Gar­naut’s argu­ments that glob­al aver­age tem­per­a­ture trends in the past decade should, if any­thing, add to a sense of alarm about man-made “green­house-gas” emissions.

There is much in the updat­ed review that I have not had an oppor­tu­ni­ty yet to con­sid­er. (Update: There’s a more com­pre­hen­sive cri­tique of the updat­ed review here.) But on one top­ic where I believed Dr Gar­naut must have been kid­ding us in his 2008 report—the expert sta­tis­ti­cal review of the (now known to have been ‘doc­tored’) tem­per­a­ture trend since 1860—he con­tin­ues, in my view, to over­state the con­clu­sions of the sta­tis­ti­cal advice he has been given.

Dr Gar­naut wants us to be even more alarmed than we were in 2008. He says that the updat­ed sta­tis­ti­cians’ report he obtained for his lat­est review demon­strates that there remains a ris­ing trend in tem­per­a­tures over “recent years”: 

The sta­tis­ti­cal­ly sig­nif­i­cant warm­ing trend has been con­firmed by obser­va­tions over recent years: glob­al tem­per­a­tures con­tin­ue to rise around the mid­points of the range of the pro­jec­tions of the Inter­gov­ern­men­tal Pan­el on Cli­mate Change (IPCC) and the pres­ence of a warm­ing trend has been confirmed;

But in my read­ing of the paper from the two aca­d­e­m­ic sta­tis­ti­cians I can­not find any direct sup­port for that assess­ment. What Breusch and Vahid say in their updat­ed report—as in their orig­i­nal report—is more cau­tious and leads to a weak­er con­clu­sion.

The sta­tis­ti­cians say three things; first, some­thing no-one, whether warmist or scep­tic, disputes: 

We con­clude that there is suf­f­cient sta­tis­ti­cal evi­dence in the tem­per­a­ture data of the past 130–160 years to con­clude that glob­al aver­age tem­per­a­tures have been on a warm­ing trend”
They say, sec­ond, that the sta­tis­tics them­selves have no mean­ing with­out some hypoth­e­sis aris­ing from an under­stand­ing of the under­ly­ing phys­i­cal real­i­ty, and on this point they have no con­tri­bu­tion to make:
…[W]e do not claim that we have uncov­ered the nature of the trend in the tem­per­a­ture data. There are many mech­a­nisms that can gen­er­ate trends and lin­ear trends are only a first order approx­i­ma­tion (see Granger 1988). It is impos­si­ble to uncov­er detailed trend pat­terns from such tem­per­a­ture records with­out cor­rob­o­rat­ing data from oth­er sources and close knowl­edge of the under­ly­ing cli­mate system
Third, the sta­tis­ti­cians agree that the over­all 1860–2010 warm­ing trend seems to con­tin­ue up to 2010 if your per­spec­tive is that of a per­son in 1980 who looks for­ward over 30 years. That per­son would be uncer­tain about future trends but the cur­rent tem­per­a­ture would fall more than the 1‑s­tan­dard-devi­a­tion above the expect­ed trend as seen from the 1980s. In sta­tis­ti­cal terms, “there is suf­fi­cient evi­dence in all three tem­per­a­ture series to reject the hypoth­e­sis of no drift in favour of a warm­ing trend in glob­al temperatures.”

But this last point, that Dr Gar­naut makes much of, says noth­ing at all about the tem­per­a­ture stat­sis over the last decade. Almost all of the warm­ing trend since 1980s took place pri­or to 2001. You don’t need a sta­tis­ti­cal mod­el to see this. You only have to look at the tem­per­a­ture record.

ChristyLowerTropTempGraph tmb Here, for exam­ple, is a graph tak­en from this week’s U.S. Con­gres­sion­al tes­ti­mo­ny of John Christy, Dis­tin­guished Pro­fes­sor of Atmos­pher­ic Sci­ence, Alabama’s State Cli­ma­tol­o­gist and Direc­tor of the Earth Sys­tem Sci­ence Cen­ter at The Uni­ver­si­ty of Alaba­ma in Huntsville. Dr Christy has been a Lead Author and Con­tribut­ing Author to the UN IPCC Assessments. 

The graph shows, from top-to-bot­tom, the “anom­alous” excess of low­er-tro­pos­phere (air) tem­per­a­ture over the 1980–1990 lev­el (TLT); the sea-sur­face tem­per­a­ture anom­alies (SLT); the air-tem­per­a­ture minus the influ­ence of the sea-sur­face tem­per­a­ture anom­alies; the influ­ence of vol­canic ash on the cool­ing of air tem­per­a­tures (VOL); and, final­ly, at the bot­tom, the air tem­per­a­ture anom­alies minus both sea-sur­face and vol­canic influ­ences. The hor­i­zon­tal ‘slices’ in the graph are each 1°C in height.

What you see in both the top and bot­tom lines, with­out the help of any sta­tis­ti­cal mod­el, is that there has been no net warm­ing since 1999 (although the ampli­tude of the vari­a­tions is much small­er after con­found­ing fac­tors are removed and the 1998 El-Nino spike in TLT is small­er when the SST is sub­tract­ed ). This is hard­ly grounds for the increased alarm that Dr Gar­naut urges. 

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