From: gpetty@rain.atms.purdue.edu (Grant W. Petty)
Newsgroups: sci.environment,sci.geo.meteorology,talk.environment,
	alt.save.the.earth,alt.philosophy.objectivism,alt.politics.media,
	alt.journalism
Subject: Re: Questions on Global Warming?
Date: 22 Dec 1997 23:46:59 GMT

In article <67ceg6$kio$3@snipp.uninett.no>, Onar Aam <onar@hsr.no> wrote:

>>As a remote sensing specialist who is aware of what has been done with
>>the MSU data, I have high confidence in Spencer and Christy's analysis
>>of tropospheric temperature.  But I *also* believe that the satellite
>>record is simply too short to draw any conclusions from concerning
>>global warming trends. If it were 50 years long, it might be a
>>different story.
>
>What then is your sentiment on the divergence between GISS and MSU/
>radiosondes since about 1980? GISS differs from MSU with about
>0.1 C/decade.

There are three possible explanations that I have heard or can think of
for the divergence:

1) For reasons no one understands, lower tropospheric temperatures
(measured by MSU/raobs) and surface temperatures (measured by GISS)
are somehow physically decoupled to the degree that they can actually
exhibit different trends.

2) The GISS data have time-dependent biases, e.g., due to urban heat
island effects, etc.

3) Both the MSU and raob data have time-dependent biases which happen
to be nearly identical, despite the independence of the two data sets.


<Warning: I am now leaving my zone of expertise and entering the zone
of semi-informed speculation.>

Personally, I find (3) a bit implausible, unless I have been
misinformed concerning the true degree of independence between the two
data sets.  Option (1) likewise strikes me as physically implausible,
unless there has been a systematic change in the mean lapse rate in
the lower troposphere (which the raobs ought to be able to observe).
This leaves (2).  Despite my general feeling that greenhouse warming
represents a genuine threat, I have no difficulty believing that some
of the trends reported in the surface data may be spurious.  For
example, if I understand correctly, urban heat island effects were
supposedly eliminated from the GISS data set by including only
stations in towns with less than 100,000 population. Yet my personal
subjective experience and intuition tell me that even much smaller
communities experience measurable heat island effects.  Given a choice
between the two, I would much rather trust the satellite record.  I
only wish the satellite record were about 50 years longer, since I
can't believe it should be possible to discern a long-term climate
trend in only 18 years of data.


	Grant

--
Grant W. Petty                        |Assoc. Prof.,  Atmospheric Science
Dept. of Earth & Atmospheric Sciences |Voice:  (765)-494-2544
Purdue University, 1397 CIVL Bldg.    |Fax:    (765)-496-1210
West Lafayette, IN 47907-1397, USA    |Email: gpetty@purdue.edu