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Klima
im Wandel - Climate Change |
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Klimawandel |
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Klimawandel |
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NAS-Report:
Surface Temperature Reconstructions for the Last 2,000 Years
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'High
Confidence' That Planet Is Warmest in 400 Years
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| There is sufficient evidence from tree rings, boreholes, retreating glaciers,
and other "proxies" of past surface temperatures to say with a high level
of confidence that the last few decades of the 20th century were warmer
than any comparable period in the last 400 years, according to a new report
from the National Research Council. |
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Less
confidence can be placed in proxy-based reconstructions of surface temperatures
for A.D. 900 to 1600, said the committee that wrote the report, although
the available proxy evidence does indicate that many locations were warmer
during the past 25 years than during any other 25-year period since 900.
Very little confidence can be placed in statements about average global
surface temperatures prior to A.D. 900 because the proxy data for that
time frame are sparse, the committee added.
Scientists
rely on proxies to reconstruct paleoclimatic surface temperatures because
geographically widespread records of temperatures measured with instruments
date back only about 150 years. Other proxies include corals, ocean
and lake sediments, ice cores, cave deposits, and documentary sources,
such as historic drawings of glaciers. The globally averaged warming
of about 1 degree Fahrenheit (0.6 degrees Celsius) that instruments have
recorded during the last century is also reflected in proxy data for that
time period, the committee noted. |  |
The
report was requested by Congress after a controversy arose last year over
surface temperature reconstructions published by climatologist Michael
Mann and his colleagues in the late 1990s. The researchers concluded
that the warming of the Northern Hemisphere in the last decades of the
20th century was unprecedented in the past thousand years. In particular,
they concluded that the 1990s were the warmest decade, and 1998 the warmest
year. Their graph depicting a rise in temperatures at the end of
a long era became known as the "hockey stick."
The
Research Council committee found the Mann team's conclusion that warming
in the last few decades of the 20th century was unprecedented over the
last thousand years to be plausible, but it had less confidence that the
warming was unprecedented prior to 1600; fewer proxies - in fewer locations
- provide temperatures for periods before then. Because of larger
uncertainties in temperature reconstructions for decades and individual
years, and because not all proxies record temperatures for such short timescales,
even less confidence can be placed in the Mann team's conclusions about
the 1990s, and 1998 in particular.
The
committee noted that scientists' reconstructions of Northern Hemisphere
surface temperatures for the past thousand years are generally consistent.
The reconstructions show relatively warm conditions centered around the
year 1000, and a relatively cold period, or "Little Ice Age," from roughly
1500 to 1850. The exact timing of warm episodes in the medieval period
may have varied by region, and the magnitude and geographical extent of
the warmth is uncertain, the committee said. None of the reconstructions
indicates that temperatures were warmer during medieval times than during
the past few decades, the committee added.
The
scarcity of precisely dated proxy evidence for temperatures before 1600,
especially in the Southern Hemisphere, is the main reason there is less
confidence in global reconstructions dating back further than that.
Other factors that limit confidence include the short length of the instrumental
record, which is used to calibrate and validate reconstructions, and the
possibility that the relationship between proxy data and local surface
temperatures may have varied over time. It also is difficult to estimate
a mean global temperature using data from a limited number of sites.
On the other hand, confidence in large-scale reconstructions is boosted
by the fact that the proxies on which they are based generally exhibit
strong correlations with local environmental conditions. Confidence
increases further when multiple independent lines of evidence point to
the same general phenomenon, such as the Little Ice Age.
Collecting
additional proxy data, especially for years before 1600 and for areas where
the current data are relatively sparse, would increase our understanding
of temperature variations over the last 2,000 years, the report says.
In addition, improving access to data on which published temperature reconstructions
are based would boost confidence in the results. The report also
notes that new analytical methods, or more careful use of existing methods,
might help circumvent some of the current limitations associated with large-scale
reconstructions.
The
committee pointed out that surface temperature reconstructions for periods
before the Industrial Revolution - when levels of atmospheric greenhouse
gases were much lower - are only one of multiple lines of evidence supporting
the conclusion that current warming is occurring in response to human activities,
and they are not the primary evidence.
The
National Research Council is the principal operating arm of the National
Academy of Sciences and the National Academy of Engineering. It is
a private, nonprofit institution that provides science and technology advice
under a congressional charter.
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| Source: NATIONAL
RESEARCH COUNCIL, Division on Earth and Life Studies,Board on Atmospheric
Sciences and Climate, 2006 |
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