Ice sheet retreat and forest expansion turned ancient subtropical
drylands into oases
Studying the climate of the distant past to get a sense of what the
future holds

Date:
March 14, 2022
Source:
University of Connecticut
Summary:
Researchers focused on the climate of the Pliocene, over 3 million
years ago, the last time Earth has seen concentrations of over 400
PPM CO2 in the atmosphere, similar to today's concentrations. The
Pliocene prompts a long-standing question: despite the similarity
to the present-day, why were dry areas like the Sahel in Africa
and Northern China much wetter and greener in the Pliocene than
they are today?


FULL STORY ==========================================================================
As human-caused greenhouse gas emissions continue to rise beyond limits
for what our species has experienced, researchers are looking to a
mystery in the past to answer questions about what may lay ahead.


==========================================================================
This work, published today in Nature Communications by an international
team of scientists, is part of a project called the 2nd Pliocene Model Intercomparison Project, or PlioMIP2.

The team focused on the climate of the Pliocene, over 3 million years
ago, the last time Earth has seen concentrations of over 400 PPM CO2in
the atmosphere, similar to today's concentrations. The Pliocene prompts
a long-standing question, says UConn Department of Geosciences researcher
and lead author Ran Feng: despite the similarity to the present-day, why
were dry areas like the Sahel in Africa and Northern China much wetter
and greener in the Pliocene than they are today? The Pliocene was warmer
than present-day conditions by 2 to 3DEGC, and everything we know about
the physics of the climate system suggests the Pliocene should have been
drier in the subtropics, says co-author Tripti Bhattacharya, Thonis Family Professor of Earth and Environmental Sciences at Syracuse University.

"Our paper was motivated by a desire to understand this apparent
discrepancy and see whether there are processes that can account for
wetter Pliocene subtropics," Bhattacharya says.

The answer, the researchers found, is more complex than simply looking
at CO2.



========================================================================== Evidence from the geologic record -- which includes a wide variety of sedimentary and paleobotanical indicators of past climate -- show that
the Sahel and subtropical Eurasian regions were once home to lusher
landscapes with drastically different hydroclimates. Along with proxy
data, the team utilized a suite of the latest state-of-the-art model simulations to identify the factors responsible for subtropical rainfall changes in the Pliocene.

Previous studies suggest the only explanation for the Pliocene discrepancy
was that there must be some mechanism unaccounted for in models to
explain the Pliocene. However, to their surprise, the researchers found
that current generation models perform well at simulating wet conditions
on Pliocene subtropical continents.

"We discovered the hydroclimate in the dry areas like the Sahel and
subtropical East Asia get much wetter when we prescribed vegetation and
ice sheet changes in the Pliocene simulations," says Feng.

Feng explains this work is providing a new perspective when studying hydrological cycle responses to CO2 changes: long-term changes in
terrestrial conditions like the shifting range of the biomes and the
ice sheets are important.

"Continental greening and ice sheet retreat have profound impacts on
the surface temperature through lowering the surface albedo -- the
ability of the Earth's surface to reflect sunlight back to space -- and
a profound effect on the hydrological cycle through allowing for greater evaporation and altering paths of moisture transport. In the long run,
there's much bigger change in hydrological cycle, compared to what we
are anticipating today," says Feng.

"Currently, few of these changes is considered when predicting climate conditions for the next 10 years, or next 50 years." This is cause for concern, says Feng, because changes in the Earth system's hydrological
cycle will mean places already receiving excessive amounts of summer
rainfall such as Southeastern Asia, Northern India, and West Africa,
are going to see even more summer rainfall as continental greening
increases and the ice sheets continue to recede.

Additionally, this work redefines the way we see the Pliocene climate,
says Bhattacharya. "The other nice takeaway is that the Pliocene does
not really challenge our fundamental understanding of the physics
of climate. Our study suggests that we do not need exotic physical
mechanisms to explain the Pliocene. Rather, we can explain regional
patterns of change in aridity by including earth system feedbacks in
models and considering the relationship between earth system sensitivity
and rainfall changes. This ultimately increases our confidence that
models do a good job at simulating the past and can be trusted to
provide reliable projections of future climate." Feng says that when
thinking about the long-term health of our planet, we must regard the
whole planet as a system, and look at these long-term responses and
their wide-ranging impacts.

"For us as a species, we need to have long-term plans, beyond the next
several decades. By looking back to past climates and learning what the
world was like, we can better prepare for the future of our society."

========================================================================== Story Source: Materials provided by University_of_Connecticut. Original
written by Elaina Hancock. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Ran Feng, Tripti Bhattacharya, Bette L. Otto-Bliesner, Esther
C. Brady,
Alan M. Haywood, Julia C. Tindall, Stephen J. Hunter, Ayako
Abe-Ouchi, Wing-Le Chan, Masa Kageyama, Camille Contoux, Chuncheng
Guo, Xiangyu Li, Gerrit Lohmann, Christian Stepanek, Ning Tan,
Qiong Zhang, Zhongshi Zhang, Zixuan Han, Charles J. R. Williams,
Daniel J. Lunt, Harry J.

Dowsett, Deepak Chandan, W. Richard Peltier. Past
terrestrial hydroclimate sensitivity controlled by Earth
system feedbacks. Nature Communications, 2022; 13 (1) DOI:
10.1038/s41467-022-28814-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220314105614.htm

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