Over the past century, global average temperatures appear to have
risen faster than at any time since the end of the last ice age
11,300 years ago, and perhaps longer. Meanwhile, the magnitude of
the increase has been unmatched in at least the past 4,000 years.
Researchers say those are the implications of a new study that
uses natural stand-ins for thermometers to trace temperature trends
back to the beginning of the current warm, interglacial period.
Significantly, the studys findings suggest the current warming trend
cannot be explained by naturally occurring temperature variability,
a lingering issue in the debate over the impact of human activity on
The main trigger for the current warming trend, especially since
the middle of the last century, has been rising emissions of heat-
trapping carbon dioxide as people burn fossil fuels and change land-
use patterns, researchers say.
Although other so-called paleoclimate records reach farther back
into geological time, the team focused on the Holocene epoch, in
which human civilizations emerged and evolved.
"To our knowledge, based on this reconstruction, the rate of
change today is unprecedented" in the Holocene, says Shaun Marcott,
an atmospheric scientist at Oregon State University who led a team
formally reporting the results in Friday's issue of the journal
Science. Indeed, it may be unprecedented in the past 22,000 years,
he adds, when previous paleoclimate research he and his colleagues
have conducted is taken into account.
Other researchers have focused on the Holocene as well, notably
Michael Mann, a Penn State University climatologist, and his
colleagues. But their reconstructions have taken the record back
only about 1,500 years.
The new work, using different thermometer stand-ins, or proxies,
not only reaches results similar to these previous efforts covering
the recent past. It also accounts for natural variations in climate
over longer time scales in ways that suggest rising temperatures
will exceed the range of natural fluctuations. The long-term
variations would include changes in Earths orbit, for instance.
Based on the reconstructed temperatures records, natural
variability over the study's time span accounts for roughly 1 degree
C from coldest to warmest compared with the current climate,
observes David Anderson, branch chief for the Paleoclimate Program
in at the National Oceanic and Atmospheric Administration's National
Climatic Data Center office in Boulder, Colo.
"If you go grab the mount of warming expected just within the
next 80 years, that's more like 3 degrees," says Dr. Anderson, who
was not a member of the study team three times the change one would
expect from natural variability alone, and all in the warm
According to the reconstruction, global average temperatures
increased by about 0.6 degrees Celsius (1 degree Fahrenheit) from
11,300 to 9,500 years ago. Temperatures remained relatively constant
for about 4,000 years. From about 4,500 years ago to roughly 100
years ago, global average temperatures cooled by 0.7 degrees C.
But over just the past century, the climate recouped the lost
warmth driven to an increasing degree by rising carbon-dioxide
emissions from burning fossil fuel and from land-use changes.
The team estimates that now, even if CO2 emissions follow the
most optimistic path envisioned by the UN's Intergovernmental Panel
on Climate Change, by 2100 global average temperatures will set a
Holocene record. …