Geoscience Reference
In-Depth Information
variability in the Northern Hemisphere from 1610 to 1800 to irradiance changes.
Suspicion that volcanism played a role (e.g., Crowley,
2000
) has recently gained
additional support. Precisely dated records from Arctic Canada and Iceland show
that ice growth began abruptly between the years 1275 and 1300, and intensified
between the years 1430 and 1455, coinciding with two periods of intense explo-
sive volcanism. Results from a climate model simulation suggest that while the
volcanism itself caused summer cooling (through aerosol loading), cold summers
can be maintained long after volcanic aerosols are removed through sea ice/ocean
feedbacks (Miller et al.,
2012
).
10.7.3
The Twentieth Century and Beyond
The pattern of surface air temperature change from the twentieth century onward
is widely viewed in terms of an overall upward trend linked to increasing concen-
trations of atmospheric greenhouse gases with superposed imprints of natural var-
iability operating on a variety of timescales. Considerable attention has been paid
to a period of strong Arctic warming that occurred from the 1920s to the 1940s
(e.g., Overland et al,
2004
; Polyakov et al.,
2002
; Serreze and Francis,
2006
), fol-
lowed by cooling through about 1970 and then renewed warming. Until recently,
the maximum warmth of the 1920s-1040s event was comparable to temperatures
observed today. As has been frequently pointed out (e.g., Serreze et al.,
2000
;
Polyakov and Johnson,
2000
; Polyakov et al.,
2002
; Semenov and Bengtsson,
2003
, Chylek et al.,
2009
), such low frequency variability in the Arctic can greatly
complicate separate natural fluctuations climate from those attributed to anthro-
pogenic influences.
In being largely isolated to the northern high latitudes (although having a clear
expression in the global mean), the earlier twentieth-century event is quite differ-
ent from more recent warming, which essentially covers all latitudes (Serreze and
Francis,
2006
). Impacts of changing Atlantic heat transport to the Arctic reducing
sea ice extent likely played an important role; this is supported by evidence that
the warmth was strongest in the Atlantic sector and that that it was most conspic-
uous during autumn and winter, at which time the effects of reduced ice cover on
air temperature would be expected to be most pronounced. This in turn invokes
links with altered patterns of atmospheric circulation (Bengtsson, Semenov, and
Johannessen,
2004
). However, the warming period did not coincide with a strongly
positive phase of the NAO (Polyakov et al.,
2009
), as would be expected based on
the conventional paradigm that a positive NAO fosters a stronger ocean heat trans-
port into the Arctic. T.L. Delworth and T.R. Knutson (
2000
) attempted to explain
the phenomenon using a coupled ocean-atmosphere climate model. They included
observed, time-varying concentrations of greenhouse gases and sulfate aerosols
in six realizations of 135-year duration. These runs were compared to those from
a 1,000-year control simulation with a constant atmospheric composition. The
warming event was reproduced by only one of the six realizations, suggesting a
strong role of natural climate variability. But as warming events of this magnitude
