DURING the last ice age, the climate may have fluctuated much more in
temperate zones than it did near the poles, say scientists who have studied the
most ancient ice yet discovered outside polar regions. This unexpected finding
turns current thinking on its head: standard computer models predict that
warming and cooling was greater at the poles than in the subtropics.
The ice core drilled to a depth of 300 metres from the Guliya ice cap on
Tibet鈥檚 Qinghai plateau records the climate over the 110 000 years since the end
of the last interglacial period. Cores from Greenland and Antarctica span a
similar interval, but no other nonpolar ice dates back further than 20 000 years
to near the end of the last ice age.
Thick polar ice sheets give the most detailed records of climate change, but
a team from Ohio State University sought subtropical glaciers that might contain
similar data. The team drilled the Tibetan ice cap at an altitude of 6200
metres, in 1992. 鈥淲e went saying we would be thrilled to get back 40 000 years,鈥�
says Ellen Mosley-Thompson. They did much better. Ice at the base of the Chinese
core appears to be more than 500 000 years old, they report in last week鈥檚
Science(vol 276, p 1821).
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The Ohio team were unable to date the ice directly because there was no
annual layering, so they used the indirect method of correlating changes in the
isotope oxygen-18 in both the Guliya core and the well-documented Greenland
core. They assumed that methane levels鈥攎easured only in
Greenland鈥攚ould be the same in both cores. Their correlations do not
extend to the very base of the core, where the ice appears to be much older and
quite compressed. The scarcity of the radioactive isotope chlorine-36 indicates
that this bottom ice is older than 500 000 years鈥攚hich would make it the
oldest ice ever recorded.
Figures for oxygen-18 for both sites showed four warm spells in the last ice
age, but with much greater warming in the subtropics. These 鈥渋nterstadials鈥� each
lasted 5000 to 10 000 years. At the poles, 鈥渢heir magnitude is substantially
less than in the current warm period,鈥� says Mosley-Thompson. However, oxygen-18
measurements show that in China the interstadials 鈥渕uch more closely approximate
[modern] conditions in terms of precipitation and temperature鈥�.
Mosley-Thompson suggests that Tibet warmed more than the poles because the
warming originated in the tropics. Methane could be a crucial link, she told
New 杏吧原创. 鈥淥ur isotopic fluctuations look very similar to methane
fluctuations in the polar cores.鈥�
The Guliya core also shows large climate swings in a cycle averaging 200
years, occurring between 33 000 and 15 000 years ago at the end of the ice age.
The oxygen-18 data imply 鈥渟wings of 20 掳C, which is not realistic,鈥� says
Mosley-Thompson. Something besides temperature seems to be affecting the oxygen
isotopes, but what that might be remains a mystery. So does the origin of the
200-year climate cycle, which is unique to this climate record.
Chronology is the weak point of the new measurements, says Steven Clemens of
Brown University in Rhode Island. 鈥淚t really would be nice to have some
independent assessment of age,鈥� he says. The delay between tropical and polar
changes is crucial to understanding mechanisms, but cannot be measured when the
cores鈥� ages are correlated. Clemens believes that the Ohio team has done the
best they could with the core, however.