
China's Loess Plateau. Credit: Gaojun Li
Temperatures in central China are 10 to 14 degrees Fahrenheit hotter
today than they were 20,000 years ago, during the last ice age, UCLA
researchers report — an increase two to four times greater than many
scientists previously thought.
The findings, published today in the early online edition of the journal Proceedings of the National Academy of Sciences, could help researchers develop more accurate models of past climate change and better predict such changes in the future.
"Previously, we could only infer temperature on land through changes
in climate archives like tree rings or pollen over time," said lead
author Robert Eagle, a UCLA researcher in the department of Earth and
space sciences. "This is the first time that temperature has been
determined accurately on land at the time of the last ice age."
To make their temperature measurements, the scientists used a
technique known as clumped isotope thermometry, which detects subtle
atomic differences in calcium carbonate, a compound commonly found in
rocks, snail shells and wind-blown dust deposits known as loess. The
method is the most accurate land-based temperature-determination tool
available today.
"We can now tell what temperatures were on land 20,000 years ago
with more accuracy than was ever previously possible," said senior
author Aradhna Tripati, a UCLA assistant professor in the department of
Earth and space sciences and the department of atmospheric and oceanic
sciences.
Tripati and Eagle chose to study the Loess Plateau in central China,
a 250,000-square-mile agricultural region some 500 miles southwest of
Beijing, because of its wide expanses of loess, the silty sediments that
give the area its name and which contain deposits from the last ice
age.
"We can calculate temperatures and reconstruct the chemistry of
rainwater from the past ice age, then compare this to the present day
climate in specific regions," Eagle said. "We can then use this
information to validate current climate models and study atmospheric
processes."
The researchers collected two unique ice age sample types from the
Loess Plateau region: fossilized land-snail shells and soil deposits.
While snails calcify quickly over just a few years, soil carbonates grow
over longer time periods, ranging from a few hundred to thousands of
years. Eagle and Tripati used clumped isotope thermometry to determine
the temperature at which these samples formed roughly 20,000 years ago.
"One of the most important aspects of the study was showing that we
could get the same result from such different types of carbonates," said
Tripati, who is also a member of UCLA's Institute of the Environment
and Sustainability. "Even though these materials integrate over very
different time frames, they gave us the same result."
Comparing the findings with climate models
When Eagle and Tripati matched their findings against climate models
predicting the change in temperature in central China from the previous
ice age to the present, they found that those models that took into
account atmospheric processes tended to be more accurate.
"The climate models that did the best job of resolving temperature
changes in this region were the ones that accurately depicted very
large-scale atmospheric processes, such as patterns of winds in the
atmosphere, the position of the jet stream and various atmospheric
fronts," Tripati said. "The models that didn't resolve these atmospheric
phenomena tended to do a poorer job of predicting temperature.
"It's so important to have models that accurately depict regional
climates on land for the study of past and future climate change. We
were surprised at how poorly most climate models predicted temperature
change in central China and also surprised at how sensitive this region
has been to changes in climate forcing."
Since the last ice age, numerous factors have influenced changes in
global wind and precipitation patterns in Earth's atmosphere.
Atmospheric processes move in relation to a standing, stationary wave,
which is an oscillating reference point that wraps around our planet
like an invisible piece of string. The position of that wave around our
planet has changed over time. Contributing factors have been a rise in
carbon dioxide and other greenhouse gases, changes in incoming solar
radiation and changes in the amount of ice covering the Earth's surface.
For example, ice sheets can deflect the stationary waves so that
winds and precipitation patterns fall more frequently in certain
locations on the planet. But as ice has melted over the last 20,000
years, the stationary waves have shifted, influencing the circulation of
the atmosphere.
"Clumped isotope thermometry has allowed us to say with more
confidence how temperatures have warmed in central China, and how the
chemistry of rainfall has changed. The climate models that did the best
job of simulating temperature changes seemed to also be the ones to give
the best depiction of changes in water cycling in this region," Tripati
said. "Our results suggest that in this region, temperature, water
cycling and winds are very sensitive to changing climate forcing. Rises
in greenhouse gas levels, melting ice sheets and changes in solar
radiation can all affect not only temperature but precipitation and
winds as well."
"We have not dissected out the specific role of greenhouse gases,
such as carbon dioxide, in this study, but they are certainly a
contributing factor to temperature change and ice-sheet extent," Eagle
said.
The climate model developed by researchers at France's Institut
Pierre Simon Laplace des sciences de l'Environnement Global (the IPSL
model) closely matched the data for this region in this study, but it
has traditionally been one of the less frequently used climate models
for predicting future climate change.
"That is quite extraordinary," said Eagle, "because while more
commonly used models have simulated a very small amount of temperature
change in the region, that prediction was not validated by our data."
Types of sediment similar to that found in central China exist in
the Midwestern U.S., ranging from Mississippi to Nebraska, and they are
currently being studied by scientists at UCLA.
"One of the things we're doing is measuring samples from the loess
deposits in the Midwestern U.S. to see how climate has changed in these
regions," Tripati said. "These deposits were also formed at the time of
the last ice age and contain similar types of snail and soil carbonates
that we analyzed in central China. It will be interesting to repeat a
similar investigation in this region."
This research was funded by the National Science Foundation. Significant contributions to the research were made by Eagle and
Tripati at UCLA; Gaojun Li, a faculty member at Nanjing University in
China; UCLA collaborators Jonathan L. Mitchell, Ulrike Seibt and David
Neelin; and Camille Risi, a research scientist at the Laboratory of
Dynamic Meterology (LMD/IPSL) at the Center for National Scientific
Research (CNRS) in Paris, France.
Contact: Stuart Wolpert
swolpert@support.ucla.edu
310-206-0511
University of California - Los Angeles
UCLA is California's
largest university, with an enrollment of more than 40,000 undergraduate
and graduate students. The UCLA College of Letters and Science and the
university's 11 professional schools feature renowned faculty and offer
337 degree programs and majors. UCLA is a national and international
leader in the breadth and quality of its academic, research, health
care, cultural, continuing education and athletic programs. Six alumni
and six faculty have been awarded the Nobel Prize.