As soon as
I read the comments by Jan Esper last week on his team's Lapland tree study I knew that climate change "skeptics" would exaggerate its significance. The study indicates a long term cooling trend greater than previously expected, and that the previous warm periods of the last couple of thousand years were a bit warmer than earlier estimates.
That's quite a lot to get from one set of trees in one tiny part of the Northern Hemisphere, I thought.
True to form, climate change skeptics who only get their information from
Watts Up With That were thrilled with the paper. Strangely, it has only turned up on
Tim Blair's site in Australia, not Andrew Bolt's yet, but give it another day.
You can bet your last dollar that no more than a few percent of those who note this would read the commentary on the paper at
Real Climate, which deals with it as scientists in the field would - pointing out some of its strengths, but also its weakenesses and the reasons to be somewhat cautious about its authors' broader suggestions about the significance of their study. Here's the important section:
Orbital forcing is indeed substantial on the millennial timescale for
high-latitudes during the summer season, and the theoretically expected
cooling trend is seen in proxy reconstructions of Arctic summer
temperature trends (Kaufman et al, 2009). But insolation forcing is near
zero at tropical latitudes, and long-term cooling trends are not seen
in non-tree ring, tropical terrestrial proxy records such as the Lake
Tanganyika (tropical East Africa) (Tierney et al, 2010) (see below).
Long-term orbital forcing over the past 1-2 millennia is also minimal
for annual, global or hemispheric insolation changes, and other natural
forcings such as volcanic and solar radiative forcing have been shown
to be adequate in explaining past long-term pre-industrial temperature
trends in this case (e.g. Hegerl et al, 2007). Esper et al’s speculation
that the potential bias they identify with high-latitude,
summer-temperature TRW tree-ring data carry over to a bias in
hemispheric temperature reconstructions based on multiple types of proxy
records spanning tropics and extratropics, ocean and land, and which
reflect a range of seasons, not just summer (e.g. Hegerl et al, 2006;
Mann et al, 1999;2008) is therefore a stretch.
Indeed, there are a number of lines of evidence that contradict that
more speculative claim. For example, if one eliminates tree-ring data
entirely from the
Mann et al (2008)
“EIV” temperature reconstruction (see below; blue curve corresponds to
the case where all tree-ring data have been withheld from the multiproxy
network), one finds not only that the resulting reconstruction is
broadly similar to that obtained with tree-ring data, but in fact the
pre-industrial long-term cooling trend in hemispheric mean temperature
is actually
lessened when the tree-ring data are eliminated—precisely the opposite of what is predicted by the Esper
et al hypothesis.
Journalists should only be partially blamed for the bad coverage of
the latest Jan Esper paper. Some of them wrote stories without
interviewing the authors, which is wrong, but the press release issued
by JG University in Mainz helps the denialist fringe by including a
couple of odd quotes from Esper himself. Take a look at what he says:
http://www.uni-mainz.de/eng/15491.php
“We found that previous estimates of historical temperatures during
the Roman era and the Middle Ages were too low,” says Esper. “Such
findings are also significant with regard to climate policy, as they
will influence the way today’s climate changes are seen in context of
historical warm periods.”
And some of the Real Climate team do get stuck into that:
I wonder if you guys could please comment on this press release,
because it’s very hard for journalists to deal with such vague
statements. Do you really think Esper is advocating lowering the tone of
the IPCC reports?
[
Response: I have no idea. I'd say
it was more related to emphasizing the potential implications of one's
own work over anyone else's - a frequent occurrence in press releases. I
generally find it prudent to wait for the work on the implications to
be done (
for instance). - gavin]
[Response: Gavin is again quite
generous. It would appear that Esper's misleading statements and
overstatement of larger implications directly fed the sort of denialist
frame represented in the Daily Mail article. It is of course impossible,
and unwise, to guess at whether or not that was his intent. -mike]
Interestingly, John Nielsen-Gammon a couple of weeks ago
had a long and useful post in which he looked at how a paper on one particular bit of one Antarctic ice shelve had its significance over-inflated by the skeptic press too. He also noted at one point that the press release for that study did what seemed to be some exaggeration of the significance of the findings.
If climate scientists don't want the public to be so easily confused (and for their results to not be so readily twisted by "skeptics" who are motivated to twist it), they really need to be careful with how their work is explained in their own press releases.
