13. Placing 20th century Arctic warming in a millennial
perspective
Much scientific effort has been devoted to learning how 20th
and 21st century warmth compares with warmth during earlier
times (e.g., National Research Council, 2006; Jansen et al., 2007).
Owing to the orbital changes affecting midsummer insolation
(a drop in June insolation of about 1Wm2 at 75 N. and 2Wm2 at
90 N during the last millennium; Berger and Loutre, 1991), additional
forcing was needed in the 20th century to give the same
summertime temperatures as achieved in the Medieval Warm
Period.
Thin, cold ice caps in the eastern Canadian Arctic preserve intact
vegetation beneath them that was killed by the expanding ice. As
these ice caps melt, they expose this dead vegetation, which can be
dated by radiocarbon with a precision of a few decades. A recent
compilation of more than 50 radiocarbon dates on dead vegetation
emerging from beneath thin ice caps on northern Baffin Island
shows that some ice caps formed more than 1600 years ago and
persisted through Medieval times before melting early in the 21st
century (Anderson et al., 2008).
Records of the melting from ice caps offer another view by
which 20th century warmth can be placed in a millennial
perspective. The most detailed record comes from the Agassiz Ice
Cap in the Canadian High Arctic, for which the percentage of
summer melting of each season’s snowfall is reconstructed for the
past 10 ka (Fisher and Koerner, 2003). The percent of melt follows
the general trend of decreasing summer insolation from orbital
changes, but some brief departures are substantial. Of particular
note is the significant increase in melt percentage during the past
century; current percentages are greater than any other melt
intensity since at least 1700 years ago, and melting is greater than
in any sustained interval since 5e4 ka ago.
As reviewed by Smol and Douglas (2007b), changes in lake
sediments and their biota record climatic and other changes in the
lakes. Extensive changes especially in the post-1850 interval are
most easily interpreted in terms of warming above the Medieval
warmth on Ellesmere Island and probably in other regions,
although explanations have been debated and explored (also see
Douglas et al., 1994; Douglas, 2007; Smol et al., 2005; Smol and
Douglas, 2007a,b). D’Arrigo et al. (2006) show tree-ring evidence
from a few North American and Eurasian sites that imply that
summers were cooler in the MedievalWarm Period than in the late
20th century, although the statistical confidence is weak. Tree-ring
and treeline studies in western Siberia (Esper and Schweingruber,
2004) and Alaska (Jacoby and RD’Arrigo, 1995) suggest that
warming since 1970 has been optimal for tree growth and follows
a circumpolar trend. Hantemirov and Shiyatov (2002) have records
from the Yamal Peninsula, Russia, well north of the Arctic Circle,
that show how summer temperatures of recent decades are the
most favorable for tree growth within the past 4 ka.
The National Research Council (2006; p. 3) evaluated the
available published data on globally and hemispherically averaged
temperatures for the last millennium, and found that “Presently
available proxy evidence indicates that temperatures at many, but
not all, individual locations were higher during the past 25 years
than during any period of comparable length since A.D. 900”.
Greater uncertainties for hemispheric or global reconstructions
were identified in assessing older comparisons.
Although high-resolution records of the past millennium in the
Arctic are rare, 14 new Arctic lake-sediment records were recently
published in a dedicated issue of the Journal of Paleolimnology
(2009, v.41-1). Six of these records extend back at least 1 ka at
decadal resolution. These six records were combined with an
additional 17 published records from lakes, ice, and trees to
reconstruct summer temperature for the Arctic for the past 2 ka
that could be compared with climate-model simulations (Kaufman
et al., 2009). The combination of data and models indicate that the
first-order cooling trend dominating most of the past 2 ka is linked
to decreasing summer insolation from orbital factors, and that this
trend was reversed during the 20th Century, despite continued
reduction of summer insolation across the Arctic. The warmest 50-
year interval in the 2 ka composite record occurred between 1950
and 2000 AD.
And from the conclusion:
Within the Holocene the general decreases in summer temperatures have
been driven by precession, with modulations to the cooling trend
resulting from changes in the frequency of sulfur-rich explosive
volcanic eruptions, and to small changes in solar luminosity. The
strong warming trend of the past century across the Arctic, and of
the past 50 years in particular, stands in stark contrast to the firstorder
Holocene cooling trend, and is very likely a result of increased
greenhouse gases that are a direct consequence of anthropogenic
activities.
It's a '
denier website' that publicised the graph (plotted by a now-deceased denier, David Lappi), and deniers, like the site you took it from, who ignore that it only goes up to 1905.
We have seen it here before:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x233629
= new reply since forum marked as read
