Sea Level Influenced Tropical Climate During The Last Ice Age
Washington, May 20
(ANI): A new study looks to the past to learn about the future of
tropical
climate change, and our ability to simulate it with
numerical
models.
Pedro DiNezio of the University
of Hawaii and
Jessica
Tierney of
Woods Hole
Oceanographic Institution investigated preserved geological clues
(called "proxies") of
rainfall
patterns during a time when the planet went into opposite gear
and cooled dramatically in the last ice age.
Land clues included charcoal from
fires, and evidence of more sand dune activity and desiccated lakes, all
indicating drier conditions, and evidence for higher lake levels and
more pollen, indicating
wetter
conditions.
They also looked at records of seafloor sediments containing
preserved shells of dead marine organisms; the shells contain higher or
lower levels of a heavier isotope of oxygen, depending on the relative
salinity of surface waters when the organisms were alive (less salty
waters indicate more rainfall over the ocean).
Together the records show that 26,000 to 19,000 years ago during the
ice age, conditions were drier throughout the center of the Indo-Pacific
warm pool-a vast region of warm ocean waters in the western Pacific
region that is the main source of heat and moisture to Earth's
atmosphere. Wetter conditions prevailed on either side of the warm pool.
They then compared this evidence with results from 12 different
mathematical climate models that simulate Earth's climate, which
incorporate basic laws of physics, chemistry, and fluid dynamics
surrounding air-sea-land-ice interactions. The idea is that the ice age
provides a great test "to evaluate numerical models' ability to simulate
climates radically different from the present one," the scientists said.
Their results surprised them:
Only one model, developed by the Hadley Centre for Climate Prediction
and Research in the England, reproduced the rainfall patterns they found
from the
geological evidence: a pattern of strong, widespread dry
conditions
over Indonesia, Southeast Asia and northern Australia, wetter conditions
in eastern Africa, saltier waters (less rainfall) in the eastern Indian
Ocean and Bay of Bengal and less salty waters (more rainfall) in the
Arabian Sea and the western Pacific.
The scientists say the primary cause for these conditions during
glacial times was lower sea levels, which exposed the now-submerged
Sunda Shelf as dry land and connected what are now Indonesian islands
into one large land mass. However, the finding that only one model is
able to reproduce the patterns of rainfall during the glacial period has
broad implications for simulating tropical climate change.
Climate scientists think that the main weakness of the models is
their limited ability to simulate convection, the vertical air motions
that lift humid air into the atmosphere. Differences in the way each
model simulates convection may explain why model results for the glacial
period are so different and don't match the proxy evidence.
"The good news is, the Hadley model combined with the geological
evidence show a pathway to improve our ability to simulate and predict
tropical rainfall in the future," Tierney said.
"The more we study the mechanisms
that governed
tropical
climate in the past, the better we can predict the climate
changes that will affect the billions of people that live in this vast
region of the world," she added.
The study was published online in the journal Nature Geoscience.
(ANI)
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