On Tue, 16 Sep 2008 08:27:44 +0100 someone who may be Terry Fields
wrote this:-
Since Al Gore's famous graph of CO2/temperature over the millennia was
shown to be flawed - in that the CO2 levels *lagged* climate change -
"Minor" detail - it has not been shown to be flawed.
http://www.realclimate.org/index.php/archives/2007/04/the-lag-between-temp-and-co2/
is a good introduction to this subject from 2007.
"The lag between temperature and CO2. (Gore’s got it right.)
"When I give talks about climate change, the question that comes up
most frequently is this: 'Doesn’t the relationship between CO2 and
temperature in the ice core record show that temperature drives CO2,
not the other way round?'
"On the face of it, it sounds like a reasonable question. It is no
surprise that it comes up because it is one of the most popular
claims made by the global warming deniers. It got a particularly
high profile airing a couple of weeks ago, when congressman Joe
Barton brought it up to try to discredit Al Gore’s congressional
testimony. Barton said:
"'In your movie, you display a timeline of temperature and compared
to CO2 levels over a 600,000-year period as reconstructed from ice
core samples. You indicate that this is conclusive proof of the link
of increased CO2 emissions and global warming. A closer examination
of these facts reveals something entirely different. I have an
article from Science magazine which I will put into the record at
the appropriate time that explains that historically, a rise in CO2
concentrations did not precede a rise in temperatures, but actually
lagged temperature by 200 to 1,000 years. CO2 levels went up after
the temperature rose. The temperature appears to drive CO2, not vice
versa. On this point, Mr. Vice President, you’re not just off a
little. You’re totally wrong.'
"Of course, those who've been paying attention will recognize that
Gore is not wrong at all. This subject has been very well addressed
in numerous places. Indeed, guest contributor Jeff Severinghaus
addressed this in one of our very first RealClimate posts, way back
in 2004. Still, the question does keep coming up, and Jeff recently
received a letter asking about this. His exchange with the letter
writer is reproduced in full at the end of this post. Below is my
own take on the subject.
"First of all, saying 'historically' is misleading, because Barton
is actually talking about CO2 changes on very long
(glacial-interglacial) timescales. On historical timescales, CO2 has
definitely led, not lagged, temperature. But in any case, it doesn't
really matter for the problem at hand (global warming). We know why
CO2 is increasing now, and the direct radiative effects of CO2 on
climate have been known for more than 100 years. In the absence of
human intervention CO2 does rise and fall over time, due to
exchanges of carbon among the biosphere, atmosphere, and ocean and,
on the very longest timescales, the lithosphere (i.e. rocks, oil
reservoirs, coal, carbonate rocks). The rates of those exchanges are
now being completely overwhelmed by the rate at which we are
extracting carbon from the latter set of reservoirs and converting
it to atmospheric CO2. No discovery made with ice cores is going to
change those basic facts.
"Second, the idea that there might be a lag of CO2 concentrations
behind temperature change (during glacial-interglacial climate
changes) is hardly new to the climate science community. Indeed,
Claude Lorius, Jim Hansen and others essentially predicted this
finding fully 17 years ago, in a landmark paper that addressed the
cause of temperature change observed in Antarctic ice core records,
well before the data showed that CO2 might lag temperature. In that
paper (Lorius et al., 1990), they say that:
"'changes in the CO2 and CH4 content have played a significant part
in the glacial-interglacial climate changes by amplifying, together
with the growth and decay of the Northern Hemisphere ice sheets, the
relatively weak orbital forcing'
"What is being talked about here is influence of the seasonal
radiative forcing change from the earth's wobble around the sun (the
well established Milankovitch theory of ice ages), combined with the
positive feedback of ice sheet albedo (less ice = less reflection of
sunlight = warmer temperatures) and greenhouse gas concentrations
(higher temperatures lead to more CO2 leads to warmer temperatures).
Thus, both CO2 and ice volume should lag temperature somewhat,
depending on the characteristic response times of these different
components of the climate system. Ice volume should lag temperature
by about 10,000 years, due to the relatively long time period
required to grow or shrink ice sheets. CO2 might well be expected to
lag temperature by about 1000 years, which is the timescale we
expect from changes in ocean circulation and the strength of the
"carbon pump" (i.e. marine biological photosynthesis) that transfers
carbon from the atmosphere to the deep ocean.
"Several recent papers have indeed established that there is lag of
CO2 behind temperature. We don't really know the magnitude of that
lag as well as Barton implies we do, because it is very challenging
to put CO2 records from ice cores on the same timescale as
temperature records from those same ice cores, due to the time delay
in trapping the atmosphere as the snow is compressed into ice (the
ice at any time will always be younger older than the gas bubbles it
encloses, and the age difference is inherently uncertain). Still,
the best published calculations do show values similar to those
quoted by Barton (presumably, taken from this paper by Monnin et al.
(2001), or this one by Caillon et al. (2003)). But the calculations
can only be done well when the temperature change is large, notably
at glacial terminations (the gradual change from cold glacial
climate to warm interglacial climate). Importantly, it takes more
than 5000 years for this change to occur, of which the lag is only a
small fraction (indeed, one recently submitted paper I'm aware of
suggests that the lag is even less than 200 years). So it is not as
if the temperature increase has already ended when CO2 starts to
rise. Rather, they go very much hand in hand, with the temperature
continuing to rise as the the CO2 goes up. In other words, CO2 acts
as an amplifier, just as Lorius, Hansen and colleagues suggested.
"Now, it there is a minor criticism one might level at Gore for his
treatment of this subject in the film (as we previously pointed out
in our review). As it turns out though, correcting this would
actually further strengthen Gore's case, rather than weakening it."
Feel free to read the rest and follow the references.
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54