"There is a preference in the atmospheric observations chapter of IPCC AR4 to stay with the 1961-1990 normals. This is partly because a change of normals confuses users, e.g. anomalies will seem less positive than before if we change to newer normals, so the impression of global warming will be muted. Also we may wish to wait till there are 30 years of satellite data, i.e until we can compute 1981-2010 normals, which will then be globally complete for some parameters like sea surface temperature."
David Parker, Met Office, to Neil Plummer, National Climate Centre - Melbourne
As always with deniers, taken OUT OF CONTEXT.
From: Phil Jones <p.jones@xxxxxxxxx.xxx>
To: "Parker, David (Met Office)" <david.parker@xxxxxxxxx.xxx>, Neil Plummer <n.plummer@xxxxxxxxx.xxx>
Subject: RE: Fwd: Monthly CLIMATbulletins
Date: Thu Jan 6 08:54:58 2005
Cc: "Thomas C Peterson" <Thomas.C.Peterson@xxxxxxxxx.xxx>
Neil,
Just to reiterate David's points, I'm hoping that IPCC will stick with 1961-90.
The issue of confusing users/media with new anomalies from a
different base period is the key one in my mind. Arguments about
the 1990s being better observed than the 1960s don't hold too much
water with me.
There is some discussion of going to 1981-2000 to help the modelling
chapters. If we do this it will be a bit of a bodge as it will be hard to do
things properly for the surface temp and precip as we'd lose loads of
stations with long records that would then have incomplete normals.
If we do we will likely achieve it by rezeroing series and maps in
an ad hoc way.
There won't be any move by IPCC to go for 1971-2000, as it won't
help with satellite series or the models. 1981-2000 helps with MSU
series and the much better Reanalyses and also globally-complete
SST.
20 years (1981-2000) isn't 30 years, but the rationale for 30 years
isn't that compelling. The original argument was for 35 years around
1900 because Bruckner found 35 cycles in some west Russian
lakes (hence periods like 1881-1915). This went to 30 as it
easier to compute.
Personally I don't want to change the base period till after I retire !
Cheers
Phil
At 09:22 05/01/2005, Parker, David (Met Office) wrote:
Neil
There is a preference in the atmospheric observations chapter of IPCC
AR4 to stay with the 1961-1990 normals. This is partly because a change
of normals confuses users, e.g. anomalies will seem less positive than
before if we change to newer normals, so the impression of global
warming will be muted. Also we may wish to wait till there are 30 years
of satellite data, i.e until we can compute 1981-2010 normals, which
will then be globally complete for some parameters like sea surface
temperature.
Regards
David