Past Climate Variability and Change in the Arctic and at High Latitudes

Case Study Summary
Past Climate Variability and Change in the Arctic and at High Latitudes
Based on these case studies, Arctic amplification appears to have operated at times when the Northern Hemisphere
was both colder and warmer than recent levels, and in response to different forcings. This result is fully consistent with
expectations based on physical understanding and models (e.g., Serreze and Francis, 2006). The magnitude of Arctic
amplification probably depends on the extent to which slow versus fast feedbacks are engaged, and whether hemispherically
uniform feedbacks (water vapor, greenhouse gas) are triggered.
3.6 SUMMARY
3.6.1 Major Features of Arctic Climate
in the Past 65 m.y.
Section 3.4 summarized some of the extensive
evidence for changes in Arctic temperatures,
and to a lesser extent in Arctic precipitation,
during the last 65 m.y. To some degree it
also discussed “attribution”—the best scientific
understanding of the causes of the climate
changes. In this subsection, a brief synopsis is
provided; for citations, the reader is referred to
the extensive discussion just above.
At the start of the Cenozoic, 65 Ma, the Arctic
was much warmer year around than it was
recently; forests grew on all land regions and
no perennial sea ice or GreenlAnd ice Sheet
existed. Gradual but bumpy cooling has dominated
most of the last 65 million years, and falling
atmospheric CO 2 concentration apparently
is the most important contributor to the cooling—although
possible changing continental
positions and their effect on atmospheric or
oceanic circulation may also contribute. One
especially prominent “bump,” the Paleocene-
Eocene Thermal Maximum about 55 Ma,
warmed the Arctic oceAn more than 5°C and
the Arctic landmass about 8°C, probably in a
few centuries to a millennium or so, followed
by cooling for about 100 k.y. Warming from
release of much CO 2 (possibly initially as seafloor
methane that was then oxidized to CO 2)
is the most likely explanation. In the middle
Pliocene (about 3 Ma) a modest warming was
sufficient to allow deciduous trees on Arctic
land that at present supports only High Arctic
polar-desert vegetation; whether this warming
originated from changes to circulation, CO 2, or
some other cause remains unclear.
About 2.7 Ma, the cooling reached the threshold
beyond which extensive continental ice sheets
developed in the North American and eurASiAn
Arctic, and it marked the onset of the Quaternary
Ice Age. Initially, the growth and shrinkage
of the ice ages were directly controlled by
changes in northern sunshine caused by features
of Earth’s orbit (the 41-k.y. cycle of sunshine that
is tied to the obliquity (tilt) of Earth’s axis is
especially prominent). More recently, a 100-k.y.
cycle has become more prominent, perhaps because
the ice sheets became large enough that
their behavior became important. Short, warm
interglacials (usually lasting about 10,000 years,
although the one about 440,000 years ago lasted
longer) have alternated with longer glacial intervals.
Recent work suggests that, in the absence
of human influence, the current interglacial
would continue for a few tens of thousands of
years before the start of a new ice age (Berger
and Loutre, 2002). Although driven by the orbital
cycles, the large temperature differences
between glacials and interglacials, and the globally
synchronous response, reflect the effects of
strong positive feedbacks, such as changes in
atmospheric CO 2 and other greenhouse gases
and in the areal extent of reflective snow and ice.
Interactions among the various orbital cycles
have caused small differences between successive
interglacials. More summer sunshine was
received in the Arctic during the interglacial of
about 130–120 ka than has been received in the
current interglacial. Thus, summer temperatures
in many places were about 4°–6°C warmer
than recently, and these higher temperatures
reduced ice on GreenlAnd (Chapter 5, History
of the Greenland Ice Sheet), raised sea level, and
melted widespread small glaciers and ice caps.
The cooling into and warming out of the most
recent glacial were punctuated by numerous
abrupt climate changes, and conditions persisted
for millennia between jumps that were completed
in years to decades. These events were
very pronounced around the north AtlAntic, but
they had a much smaller effect on temperature
elsewhere in the Arctic. Temperature changes
Arctic amplification
appears to have operated
at times when the
Northern Hemisphere
was both colder and
warmer than recent
levels, and in response to
different forcings.
Recent work suggests
that, in the absence of
human influence, the
current interglacial would
continue for a few tens
of thousands of years
before the start of a
new ice age.
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