date: Sat, 12 Jul 2003 00:07:37 -0400
from: "Michael E. Mann" <mannatXYZxyzginia.edu>
subject: Re: query regarding Soon et. al. rebuttal
to: "Regalado, Antonio" <Antonio.RegaladoatXYZxyz.com>
Thanks for your message. Happy to hear your thinking of doing this article.
Just got back from Japan, so a bit jet lagged, but wanted to at least get an initial
response to you. Please feel free to contact me over the weekend, by email or otherwise, if
I can be of further help.
Some specific comments below.
At 04:17 PM 7/11/2003 -0400, you wrote:
Hi from the Wall Street Journal. I am thinking of citing your rebuttal to
Soon et. al. in a news item I am writing for the newspaper.
You say that it is "only the past few decades druging which n. hemisphere
temperatures have exceeded the bounds of natural variability...". Do you
mean the last few decades are the warmest of the last millennium only, not
all of time, right?
yes, in fact, though, we can now say with a reasonable degree of confidence that Northern
Hemisphere mean temperatures were higher during the past two decades than any other
interval during at least roughly the past *two* millennia (the extension to the past two
millennia is afforded by a paper in press in the journal 'Geophysical Research Letters' by
Phil Jones and myself--a result of that paper was shown in our 'Eos' piece, but we'll issue
a more specific press release on that result when the paper is slated to appear in a few
weeks). Its unclear how Northern Hemisphere average temperature (let alone global average
temperature) varied during prior millennia (see below).
To what degree are n. hemisphere temperatures anomalous
when compared to the entire paleoclimate record?
Its *possible* the conclusion holds for the last 6000 years, or even longer, but that's
speculative. When we go back beyond the past one or two millennia, the issue gets very
tricky--we no longer have annually-detailed proxy records which we can compare directly
against modern thermometer records. It is possible to do so with very long
annually-resolved ice cores, tree-rings, corals, and historical records, which give us a
picture of changes over the past one-to-two millennia, but not with the sorts of evidence
(pollen, ocean sediments, coursely-resolved ice cores, glacial advances and retreats) that
are available to provide longer-term insights. There is a good discussion of these issues
in the 2001 IPCC report, if you would like some additional detailed information:
That having been said, the 'mid-Holocene' interval (about 6000 years ago) when the
astronomical factors influencing the climate favoured greater insolation in the Northern
Hemisphere summer, might have been warmer than the late 20th century. The available
evidence, though limited, suggests this--and a number of older model simulations suggested
that might be the case. Some recent work, using the best available current climate models,
suggests, however, that the temperatures were perhaps comparable to today back then. There
was another period prior to the last Ice Age (more than 120,000 years ago) called the
'Eemian' for which there is tentative evidence that global mean temperatures might have
been even higher than during the mid-Holocene. But 'tentative' is the key phrase--the
evidence is often restricted in where its available, and whether its telling us about
annual conditions (what we would like to know) or only, say, summer growing season
It is almost certain that global mean temperatures were warmer during certain past
geological periods (e.g., the Cretaceous, when we suspect that Co2 levels were higher than
today, and that the globe, w/ Dinosaurs wandering around near the poles, was almost
certainly warmer). These changes occurred over many millions of years, due to the influence
of plate motion on the production of co2 by geological sources (e.g. volcanic outgassing).
Of course, that warming occurred over many millions of years. The present warming is
occurring on a century time scale, so it is the *rate* of recent warming that may be
particularly anomalous in the long-term history of the climate.
Also, when was the last time C02 levels were as high as they are now, do you
There is still some debate about this. We now have excellent CO2 records from ice cores
dating back to more than 400,000 years. The present Co2 concentration appears higher than
at any time during that record. The longer-term evidence is more tenuous (based on trace
gases trapped in ambers, evidence from fossil leaf stomata, etc.), but it is quite likely
that co2 levels were higher as one gets back towards the Cretaceous period (e.g. more than
50 million years ago), precisely how much higher is still a subject of dispute. The present
thinking is that current co2 levels are probably the highest in about 20 million years. See
again e.g. the IPCC 2001 report for details:
happy to be of help
Staff Reporter, The Wall Street Journal
Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
e-mail: mannatXYZxyzginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137