Re: CO2 tightly linked with climate for 420 million years

In article <4ecada1982rblackall@xxxxxxxxxxxxxxxxxxxx>,
Rodney Blackall <rblackall@xxxxxxxxxxxxxxxxxxxx> wrote:
In article <1175106286.025077.151070@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Roger Coppock <rcoppock@xxxxxxxx> wrote:

" . . . climate sensitivity [to 2X CO2] greater than 1.5 °C
has probably the Earth's climate system over the past
420 million years, regardless of temporal scaling."

The trouble with going back so far into the past is that the geography of
Earth's surface has changed an awful lot in that time. If you disregard the
colossal oceanographic changes caused by the linking of the Pacific and
Atlantic; Arctic and Pacific and Arctic and Atlantic, let alone the building
of the great mountain ranges you could get some very unreal answers.

As always, it depends on what question you're trying to answer.
They're not answering the question of 'what is the precise sensitivity
of the current climate system to 2xCO2?' -- agreed. The excerpt doesn't
suggest to me that they thought they were either.

What they do present, assuming everything holds up, is that
over the last 420 My, over all the different continental arrangements,
all the different ocean circulations, all the mountain arrangements,
the assortments of glacial arrangements, varying CO2 levels (part of
that period includes lower than present CO2 levels, most is higher
to much higher than present), ... that it was seldom the case that
the sensitivity to 2xCO2 was lower than 1.5 C.

To the extent that some models for the present call for as little as
1.5 C for 2xCO2, this work suggests that we should spend some time
understanding what exactly it is that is special about the present
(vs. the 420 My) to make for such a low sensitivity. If it's, say,
the presence of an ice cap on Greenland, we should be much more concerned
than if it is, say, the presence of western boundary currents (Gulf
Stream, Kuroshio, ...).


--
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Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences
.