Re: [asa] AGW discussion

From: Randy Isaac <>
Date: Fri Dec 04 2009 - 13:28:33 EST

There's a fascinating report in the latest issue of Science that extends
what I was describing in a previous post where I commented that ice core
data over the last 420ky showed a very close correlation between atmospheric
CO2 and climate. This report is "Coupling of CO2 and Ice Sheet Stability
Over Major Climate Transitions of the Last 20 Million Years," by Aradhna K.
Tripati, Christopher D. Roberts, and Robert A. Eagle. If you can access it,
the link is It is
in the Dec 4 2009 issue of Science, p. 1394. They use studies on
foraminifera to study such correlation between CO2 and climate over the last
20my. Their summary is below. This is a significant concern. It is an
independent line of investigation that corroborates that we already have
increased atmospheric CO2 levels to a level that historically had very
little ice caps.

These results show that changes in pCO2 and
climate have been coupled during major glacial
transitions of the past 20 My, just as they have
been over the last 0.8 My, supporting the hypothesis
that greenhouse gas forcing was an
important modulator of climate over this interval
via direct and indirect effects. Variations in pCO2
affect the radiative budget and energy balance of
the planet. Such changes will inevitably have consequences
for temperature, the hydrologic cycle,
heat transport, and the accumulation and ablation
of sea ice and glacial ice. The data presented here
do not preclude alternative mechanisms for driving
climate change over the past 20 Ma. However,
they do indicate that changes in pCO2 were
closely tied to the evolution of climate during the
Middle and Late Miocene and the Late Pliocene
glacial intensification, and therefore, it is logical to
deduce that pCO2 played an important role in
driving these transitions. High-resolution records
of pCO2 and other climate parameters should help
to resolve whether pCO2 was a trigger and/or
feedback (or both).
These results provide some constraints on
pCO2 thresholds for the advance and retreat of
continental ice sheets in the past, which is also
relevant in the context of anthropogenic climate
change because it is uncertain how continental
ice sheets will respond over the coming centuries
to increased levels of pCO2 (1). By comparing
our reconstruction to the published data
sets described above, we are able to estimate
past thresholds for the buildup of ice in different
regions. When pCO2 levels were last similar
to modern values (that is, greater than 350 to
400 ppmv), there was little glacial ice on land or
sea ice in the Arctic, and a marine-based ice mass
on Antarctica was not viable. A sea ice cap on the
Arctic Ocean and a large permanent ice sheet
were maintained on East Antarctica when pCO2
values fell below this threshold. Lower levels
were necessary for the growth of large ice masses
on West Antarctica (~250 to 300 ppmv) and
Greenland (~220 to 260 ppmv). These values are
lower than those indicated by a recent modeling
study, which suggested that the threshold on East
Antarctica may have been three times greater
than in the Northern Hemisphere (35).
This work may support a relatively high climate
sensitivity to pCO2. pCO2 values associated with
major climate transitions of the past 20 Ma are
similar to modern levels. During theMid-Miocene,
when pCO2 was apparently grossly similar to
modern levels, global surface temperatures were,
on average, 3 to 6C warmer than in the present
(2, 25).We suggest that the Mid-Miocenemay be
a useful interval to study to understand what effect
sustained high pCO2 levels (i.e., a climate in
equilibrium with near-modern pCO2 values) may
have on climate.

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Received on Fri Dec 4 13:28:40 2009

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