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Unformatted text preview: locene range. Shifting the
scale to align the 1870 SST with the lowest Holocene value raises
the paleo curve by 0.5°C. Even in that case, the 2001–2005 WEP Fig. 5. Modern sea surface temperatures (5, 6) in the WEP compared with paleoclimate proxy data (28). Modern data are the 5-year running mean, while the
paleoclimate data has a resolution of the order of 1,000 years. Hansen et al. PNAS September 26, 2006 vol. 103 no. 39 14291 ENVIRONMENTAL
SCIENCES Fig. 4. Comparison of modern surface temperature measurements with
paleoclimate proxy data in the WEP (28) (A), EEP (3, 30, 31) (B), Indian Ocean
(40) (C), and Vostok Antarctica (41) (D). SST is at least as great as any Holocene proxy temperature at that
location. Coarse temporal resolution of the Holocene data, 1,000
years, may mask brief warmer excursions, but cores with higher
resolution (29) suggest that peak Holocene WEP SSTs were not
more than 1°C warmer than in the late Holocene, before modern
warming. It seems safe to assume that the SST will not decline this
century, given continued increases of GHGs, so in a practical sense
the WEP temperature is at or near its highest level in the Holocene.
Fig. 5, including WEP data for the past 1.35 million years, shows
that the current WEP SST is within 1°C of the warmest interglacials in that period.
The Tropical Pacific is a primary driver of the global atmosphere
and ocean. The tropical Pacific atmosphere–ocean system is the
main source of heat transported by both the Pacific and Atlantic
Oceans (2). Heat and water vapor fluxes to the atmosphere in the
Pacific also have a profound effect on the global atmosphere, as
demonstrated by El Niño Southern Oscillation climate variations.
As a result, warming of the Pacific has worldwide repercussions.
Even distant local effects, such as thinning of ice shelves, are
affected on decade-to-century time scales by subtropical Pacific
waters that are subducted and mixed with Antarctic Intermediate
Water and thus with the Antarctic Circumpolar Current.
The WEP exhibits little seasonal or interannual variability of SST,
typically 1°C, so its temperature changes are likely to reflect large
scale processes, such as GHG warming, as opposed to small scale
processes, such as local upwelling. Thus, record Holocene WEP
temperature suggests that global temperature may also be at its
highest level. Correlation of local and global temperature change
for 1880–2005 (Fig. 9, which is published as supporting information
on the PNAS web site) confirms strong positive correlation of
global and WEP temperatures, and an even stronger correlation of
global and Indian Ocean temperatures.
The Indian Ocean, due to rapid warming in the past 3–4 decades,
is now warmer than at any time in the Holocene, independent of any
plausible shift of the modern temperature scale relative to the
paleoclimate data (Fig. 4C). In contrast, the EEP (Fig. 4B) and
perhaps Central Antarctica (Vostok, Fig. 4D) warmed less in the
past century and are probably cooler than their H...
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