Previous research had extended back roughly 1,500 years, and suggested
that the rapid temperature spike of the past century, believed to be a
consequence of human activity, exceeded any warming episode during those
years. The new work confirms that result while suggesting the modern
warming is unique over a longer period.
Even if the temperature increase from human activity that is projected
for later this century comes out on the low end of estimates, scientists
said, the planet will be at least as warm as it was during the warmest
periods of the modern geological era, known as the Holocene, and
probably warmer than that.
That epoch began about 12,000 years ago, after changes in incoming
sunshine caused vast ice sheets to melt across the Northern Hemisphere.
Scientists believe the moderate climate of the Holocene set the stage
for the rise of human civilization roughly 8,000 years ago and continues
to sustain it by, for example, permitting a high level of food
production.
In the new research, scheduled for publication on Friday in the journal Science, Shaun Marcott, an earth scientist at Oregon State University,
and his colleagues compiled the most meticulous reconstruction yet of
global temperatures over the past 11,300 years, virtually the entire
Holocene. They used indicators like the distribution of microscopic,
temperature-sensitive ocean creatures to determine past climate.
Like previous such efforts, the method gives only an approximation.
Michael E. Mann, a researcher at Pennsylvania State University who is an
expert in the relevant techniques but was not involved in the new
research, said the authors had made conservative data choices in their
analysis.
“It’s another important achievement and significant result as we continue to refine our knowledge and understanding of climate change,” Dr. Mann said.
Though the paper is the most complete reconstruction of global
temperature, it is roughly consistent with previous work on a regional
scale. It suggests that changes in the amount and distribution of
incoming sunlight, caused by wobbles in the earth’s orbit, contributed
to a sharp temperature rise in the early Holocene.
The climate then stabilized at relatively warm temperatures about 10,000
years ago, hitting a plateau that lasted for roughly 5,000 years, the
paper shows. After that, shifts of incoming sunshine prompted a long,
slow cooling trend.
The cooling was interrupted, at least in the Northern Hemisphere, by a
fairly brief spike during the Middle Ages, known as the Medieval Warm
Period. (It was then that the Vikings settled Greenland, dying out there
when the climate cooled again.)
Scientists say that if natural factors were still governing the climate,
the Northern Hemisphere would probably be destined to freeze over again
in several thousand years. “We were on this downward slope, presumably
going back toward another ice age,” Dr. Marcott said.
Instead, scientists believe the enormous increase in greenhouse gases
caused by industrialization will almost certainly prevent that.
During the long climatic plateau of the early Holocene, global
temperatures were roughly the same as those of today, at least within
the uncertainty of the estimates, the new paper shows. This is
consistent with a large body of past research focused on the Northern
Hemisphere, which showed a distribution of ice and vegetation suggestive
of a relatively warm climate.
The modern rise that has recreated the temperatures of 5,000 years ago
is occurring at an exceedingly rapid clip on a geological time scale,
appearing in graphs in the new paper as a sharp vertical spike. If the
rise continues apace, early Holocene temperatures are likely to be
surpassed within this century, Dr. Marcott said.
Dr. Mann pointed out that the early Holocene temperature increase was
almost certainly slow, giving plants and creatures time to adjust. But
he said the modern spike would probably threaten the survival of many
species, in addition to putting severe stresses on human civilization.
“We and other living things can adapt to slower changes,” Dr. Mann said.
“It’s the unprecedented speed with which we’re changing the climate
that is so worrisome.”
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