Wednesday, May 23, 2012

4629.txt

date: Tue Jun 3 16:56:57 2008
from: Phil Jones <p.jonesatXYZxyz.ac.uk>
subject: Re:
to: John Finn <john.finnatXYZxyzmail.co.uk>

John,
Any tropospheric aerosol forcing (mainly sulphates) may affect the climate more
locally than further afield. With climate models run with just sulphate aerosol
forcing (in eastern Nortn America, Europe and China) there are effects locally,
but also effects downstream a long way from the source. The forcing is regionally
specific, but the effects can be more widespread - at least within the NH. So your
logic about where the greatest cooling might be may not be correct, but then no-one
will know the true answer.
Warming/cooling trends may be great in polar regions, but variability is greater there.
This means that the trend may not be as significant as an area more
in the mid or low latitudes, so it is important to consider the significance of the
trends,
rather than the magnitude. The area of the polar region is also much smaller than regions
of the same latitude belts nearer the equator.

Have you looked at the IPCC AR4 report - Ch 2 on forcing and the modelling chapters?
The chapters are on-line at the WG1 web site. See Figure 2.12.
Aerosols have a direct effect on climate which tends to be more local, but there is
an indirect effect (through clouds) that causes effects further afield. A number of
models
produce a large indirect effect over the Southern Oceans, but it is difficult to assess
this as there are few observations there.
Cheers
Phil
At 12:35 03/06/2008, you wrote:

Dear Professor Jones<?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />

Commenting on the recently published paper which claims to explain the ~1945 sea
temperature anomaly, you are reported as saying

the study lends support to the idea that a period of global cooling occurred later
during the mid-twentieth century as a result of sulphate aerosols being released during
the 1950s with the rise of industrial output. These sulphates tended to cut sunlight,
counteracting global warming caused by rising carbon dioxide

Theres something bothering me about this statement. To illustrate Id like to draw your
attention to another paper (Climate Change over past Millenia) by Mann and Jones (i.e.
you). In Section 5.1.4 you write

Compared to greenhouse gas forcing, sulphate aerosol forcing is far more uncertain,
principally because of limited understanding of the radiative properties of the aerosols
and their effects on clouds. This forcing is also REGIONALLY SPECIFIC and must be
estimated from past fossil fuel use (see, e.g., Crowley [2000, and references therein]
for further discussion).

Note my emphasis on regionally specific. From which I take it that, due to the short
residence time of aerosols in the atmosphere, the greatest climatic effect (i.e.
cooling) due to aerosols will be close to the source of the emissions. A fact that is
supported by several other papers. For example from Levitus et al (2005), we have

The second is that the natural and anthropogenic aerosols are not well-mixed
geographically and can have a substantial effect on regional warming rates

Here Levitus is giving the second of 3 reasons for non-uniform heating of the oceans.

My understanding is, therefore, that if sulphate aerosols cause cooling then the
greatest cooling will occur in the industrialised regions which produce the aerosols. I
hope my logic is correct here. So now consider the following data which is from the GISS
(land-only) surface temperature record.

64N-90N -0.93
44N-64N -0.17
24N-44N -0.21
24N-EQU -0.09

The figures show the total cooling for each latitude band for the 32 year period between
1944 and 1975. Note that the cooling is greatest (by far) in the latitudes north of 64
deg and that cooling tends to be less as we get closer to the equator. This is almost a
mirror image of the warming since 1975, i.e. greatest warming at northerly latitudes and
less at the equator. But the important point is that the mid latitudes which include the
big post-war industrialised regions (US and Europe) experienced much less cooling than
the non-industrialised Arctic regions.

This, to me, doesnt seem to square with the aerosol cooling theory.

Thanks for any help you can give.

John Finn


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University of East Anglia
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