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contributions was an assessment as to whether the current multi-year drought being
experienced in the western USA has any precedent. The research team was led by
Edward Cook of the National Oceanic and Atmospheric Administration (NOAA)
and their work was published in Science in 2004. The drought experienced by the
western USA at the end of the 20th century and now is both severe and unpreced-
ented over the period of detailed hydroclimatic measurements, which mainly covers
the 20th century. If the early 21st century has its palaeoclimatic analogue back in
the medieval warm period (MWP) or medieval climatic anomaly (MCA), to see
whether the current drought is a likely response to global warming what is needed is
to ascertain conditions during and prior to the MWP, so as to capture all of the MWP
episode. Edward Cook and colleagues looked at palaeoclimatic dendrochronological
indicators in a geographical grid across the USA and southern Canada. They found
that the current drought does appear to be analogous to one that took place in the
western half of the USA and western half of southern Canada during the MWP.
However, the recent drought up to July 2004 (when the paper was submitted) was
not as severe as the one during the MWP between
900 and 1300. They con-
cluded that the western US drought is likely to get worse with continued global
warming.
Further corroboration of analogous droughts in the region, but much earlier still,
comes from Peter Fawcett, Josef Werne, Scott Anderson et al. in 2011 and reviewed
by John Williams in the same issue of Nature . They noted the above concerns of past
mega-droughts and particularly in the southwestern USA. Multi-year droughts during
the instrumental period and decadal-length droughts of the past two millennia were
shorter and climatically different from the future permanent, 'dust-bowl-like' mega-
drought conditions, lasting decades to a century, that some predicted as a consequence
of warming. They used molecular palaeotemperature proxies to reconstruct the mean
annual temperature in earlier Pleistocene (the rest of the Quaternary prior to our
interglacial) sediments from the Valles Caldera, New Mexico, that were laid during
previous interglacials. They found that the driest conditions occurred during the
warmest phases of interglacials, when the mean annual temperature was comparable
to or higher than today's, and that these droughts lasted over a thousand years. It
seems that with south-western North America, water variability seems to be the rule
rather than the exception during interglacial periods and that during the warm parts
of previous interglacials there were extended mega-droughts that lasted far longer
than those occurring during human history.
In 2010 the National Research Council - whose members are drawn from the
councils of the National Academy of Sciences, the National Academy of Engineer-
ing and the Institute of Medicine in the USA - provided their perspective on US
climate change and impacts via their Committee on Stabilization Targets for Atmo-
spheric Greenhouse Gas Concentrations in the report Climate Stabilization Targets:
Emissions, Concentrations, and Impacts over Decades to Millennia . The ground it
covered overlapped much of that the USGCRP and IPCC Working Group I assess-
ments already covered (and so will not be reiterated here).
In particular, the report demonstrated that stabilizing atmospheric carbon dioxide
concentrations will require deep reductions in the amount of carbon dioxide emit-
ted. Because human carbon dioxide emissions exceed removal rates through natural
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