[asa] Rush Limbaugh Science Lesson

On Thu, May 29, 2008 at 12:12 PM, Murray Hogg <muzhogg@netspace.net.au>
wrote:

> Hi Rich,
>
> This historical perspective on the global warming issue is fascinating
> stuff - particularly as the impression often given in the media is that
> global warming was first discovered about six minutes after tea-time last
> Tuesday. It appears that the full story isn't getting the exposure it
> deserves.
>
> A few questions on the historical data, though:
>
> (1) how are we able to determine historical CO2 levels? [my guess:
> ice-coring?]

Yes.

>
>
> (2) how far back are we able to make reasonable assessments of CO2 levels?

Direct measurement happened starting in the late 50s. We can directly sample
CO2 levels from trapped gas in ice cores. Temperature can be measured in the
same by comparing the O16 and O18 levels. These have gone back 800000 years.
Temperature proxies include benthic foraminphora and these go back millions
of years. It was warmer than now IIRC 3 million years ago before our current
ice age cycling. One troubling thing there is given the estimates of CO2 we
should be warmer based on this data. Climate sensitivity is an equilibrium
phenomenon so we could be much worse off in the longer term if the climate
sensitivity of 3 mya is the real one.

>
> (3) what explanation is given to account for the previous historical high
> 80k years ago?

800k ago was as far back as the ice core records went. CO2 levels tracked
the temperatures for the last 6 ice age cycles. Current CO2 is higher than
all of the inter-glacial maxima. One of things that is controversial (at
least with the skeptics) with respect to the record is that the CO2 levels
lagged the temperature increases.

GHG can either act as a feedback mechanism or a direct forcing. Currently,
CO2 acts as a direct forcing and H20 is a feedback mechanism. The best
guess is that CO2 increased as the result of warming due to orbital
precession which in turn warmed things further as a feedback mechanism. CO2
increased in the middle of the change suggesting some validity to this
hypothesis.

One thing that is different this time around is the rate of change which is
currently orders of magnitude faster than any of the changes in the past.
When skeptics say whether a warmer or cooler Earth is better they are not
taking into account that rapid change in either direction (in a geological
sense) is often bad because we don't have time to adapt.

>
>
> and
>
> (4) does the data give any support to the argument that the current
> increase is just part of a cycle? [my guess: it's the level of current CO2
> emissions which is the critical consideration as it swamps any historical
> trend from the pre-industrial period]

Not at all. CO2 and temperature has been cycling following the ice age
cycles but now has been just increasing in the 20th Century and the CO2
literally went off the charts. Since the 1950s measured CO2 has always
increased year over year. Natural climate variability is more cyclical. For
example, the solar output follows the 11-year sunspot cycle. El Nino is also
a cyclical phenomenon. Orbital mechanics have cycle times in the 10s and
100s of thousands of years. In the 20th Century we have gone through
multiple cycles of all of these effects except for the orbital changes which
are too slow, but we have had increasing temperature consistently since
1980, slightly modulated by these cyclical effects.

One way that climate models have been helpful in understanding this is you
can factor in all the natural variability and the climate models are all
wrong starting in the second half of the 20th Century. (The first half
matched well.) When you add in the anthropogenic factors the models followed
the record except for the two years following any major volcanic eruption
and then the warming continues afresh. The folks at the Real Climate blog
are making a bet that it will get warmer rather than cooler over the next
five years, but they want an escape clause for a volcanic eruption.

Rich

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Received on Thu May 29 15:56:10 2008