Geoscience Reference
In-Depth Information
Kaufman, David Schneider and Nicholas McKay led a North American and European
team to bring together a synthesis of decadally resolved proxy temperature records,
published in Science in 2009. Unlike the Mann team's meta-analysis of global climate
proxies this study looked purely at northern hemisphere proxies from poleward of
60 N and covering the past 2000 years. Although not global, like Greenland and
Antarctica ice-core derived temperature analyses, this analysis is of relevance for
much of the northern hemisphere even if caution is needed interpreting it at the whole-
hemisphere, or even global, scale. The conclusion they drew from their analysis was
that the long-term trend (i.e. excluding things like the MCA and Little Ice Age) of
northern hemisphere temperatures was one of gradual cooling through to the 20th
century. Indeed, the northern hemisphere cooling seems to have ended at the end
of the Little Ice Age. What then happens in the 20th century is warming that takes
temperatures above anything this proxy analysis reveals in the previous nineteen
centuries.
In terms of present-day questions of global warming, this means that this Little Ice
Age recovery period is critical. It was during this time (after inception of the Industrial
Revolution) that significant amounts of greenhouse gases (though small at first) were
added to the atmosphere from human activity. Whereas before the 20th century
the global climate was largely (but not solely) determined by non-human factors,
it was during the 20th century that anthropogenic warming increasingly became
manifest.
This point is an important one because it is possible to mistake (and indeed some
vocal people have mistaken) anthropogenic global warming for some other factor.
That the early 20th century saw a warming out of the Little Ice Age does not mean that
there was just one factor at work. Some (such as the popular thriller author Michael
Crichton) put this climate change down to 'natural' climatic vagaries, whereas others
(such as some astronomers) put it down to changes in solar output. However, the IPCC,
the considered scientific authority on climate change, is clear that many factors affect
climate, including changes in the Sun's output, but that this does not prevent human-
generated greenhouse gases becoming an ever increasingly powerful factor, from
small beginnings with the Industrial Revolution to a dominant (if not the dominant)
climate change factor by the end of the 20th century.
Before leaving our climate review of the pre-industrial Holocene, it is worth noting
that there has been some discussion with regards to human influences on the global
climate system over this time (see Anthropocene, section 4.4). Analysis of methane
concentrations from Antarctic ice cores covering the past 2000 years reveal an inter-
esting 13 C isotope pattern embedded within the record of change in overall methane
concentrations, containing both 13 C and 12 C.
As noted previously, plant photosynthesis sequesters more 12 C from the atmosphere
than 13 C but this preference not only differs between C 3 and C 4 plants. The propor-
tions of 13 C in methane given off by microbial decomposition of organic material in
wetlands and of 13 C in fossil fuels are also different (this last itself has only been a
significant factor for the past 200 years and is reasonably well documented). There-
fore, in theory it should be possible to begin to disentangle the way various methane
sources contributed to atmospheric concentrations. Analysis of methane trapped in
ice-core bubbles has showed that changes in
13 CH 4 (represented as
13 CH 4 )were
δ
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