Re: [新聞] 研究:全球暖化恐造成南亞降雨大減
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http://0rz.tw/46P75
Global warming could delay, weaken monsoons: study
4 days ago
CHICAGO (AFP) — Global warming could delay the start of the summer monsoon
by five to 15 days within the next century and significantly reduce rainfall
in much of South Asia, a recent study has found.
Rising global temperatures will likely lead to an eastward shift in monsoon
circulation which could result in more rainfall over the Indian Ocean,
Myanmar and Bangladesh but less over Pakistan, India and Nepal, the study
found.
It could also result in longer delays between rainy seasons and intensify the
risk of deadly floods by leading to a significant increase in average
rainfalls in some coastal areas of western India, Sri Lanka and Myanmar.
That could have a major impact on agriculture, human health and the economies
of the region, warned study author Noah Diffenbaugh.
"Almost half of the world's population lives in areas affected by these
monsoons, and even slight deviations from the normal monsoon pattern can have
great impact," said Diffenbaugh, interim director of Purdue University's
Climate Change Research Center.
"Agricultural production, water availability and hydroelectric power
generation could be substantially affected by delayed monsoon onset and
reduced surface runoff."
The atmospheric conditions that lead to reduced rain also can lead to
intensification of extremely hot conditions, said lead author Moetasim
Ashfaq, a graduate student at Purdue.
"In the past when we have seen extremely hot days, we have observed a similar
circulation anomaly," Ashfaq said in a statement.
"These circulation changes decrease moisture flow over the land, and we see
longer periods without rain, along with hot conditions."
Ashfaq used a high-resolution climate model to map how global warming will
affect the complex topography of South Asia by recreating the monsoon season
of past years.
He found that increasing temperatures strengthen some aspects of large-scale
monsoon circulation but weaken the fine-scale interactions of the land with
the moisture in the atmosphere.
"Even with a strong monsoon system, if circulation changes enough to change
where and when rain is delivered, then that could have an impact that has not
been captured in the large-scale evaluations," Ashfaq said.
The study was published in the January issue of the peer-reviewed journal
Geophysical Research Letters.
Study: Climate change to affect monsoon in South Asia
http://0rz.tw/jPcLK
WEST LAFAYETTE, Ind. (AP) — The South Asian summer monsoon — critical to
agriculture in Bangladesh, India, Nepal and Pakistan — could be weakened and
delayed due to rising temperatures in the future, according to a recent
climate modeling study.
A Purdue University research group found that climate change could influence
monsoon dynamics and cause less summer precipitation, a delay in the start of
monsoon season and longer breaks between the rainy periods.
Noah Diffenbaugh, whose research group led the study, said the summer monsoon
affects water resources, agriculture, economics, ecosystems and human health
throughout South Asia.
"Almost half of the world's population lives in areas affected by these
monsoons, and even slight deviations from the normal monsoon pattern can have
great impact," said Diffenbaugh, an associate professor of earth and
atmospheric sciences and interim director of the Purdue Climate Change
Research Center.
"Agricultural production, water availability and hydroelectric power
generation could be substantially affected by delayed monsoon onset and
reduced surface runoff. Alternatively, the model projects increases in
precipitation over some areas, including Bangladesh, which could exacerbate
seasonal flood risks."
FIND MORE STORIES IN: Pakistan | Beijing | Myanmar | Indian Ocean |
Bangladesh | South Asia | Nepal | Intergovernmental Panel | Geophysical
Research Letters | Atmospheric Sciences | Department of Earth | National
Climate Centre
The summer monsoons are responsible for approximately 75% of the total annual
rainfall in major parts of the region and produce almost 90% of India's water
supply, he said.
General circulation models have been used for projections of what may happen
to monsoon patterns for this region, but the models have disagreed as to
whether precipitation will increase or decrease, said Moetasim Ashfaq, lead
author of the study and a graduate student in earth and atmospheric sciences
at Purdue.
"South Asia is a unique region with very complex topography," he said. "It
ranges from 0 meters elevation from sea level in the south to more than 5,500
meters from sea level in the north. So in terms of topography playing a role
in climate and weather, this region of the world is where we expect to see a
large impact. Global models like the ones featured in the Intergovernmental
Panel on Climate Change reports can resolve large-scale interactions but have
difficulty capturing some of the more subtle atmospheric processes."
The research team used a high-resolution climate model believed to have the
greatest detail currently available for this region. A paper detailing the
work was published in the Jan. 3 issue of Geophysical Research Letters.
Co-authors from Purdue include assistant professor Wen-wen Tung and associate
professor Robert J. Trapp, both from the Department of Earth and Atmospheric
Sciences. Additional co-authors include Ying Shi and Xueijie Gao of the
National Climate Centre in Beijing and Jeremy S. Pal of Loyola Marymount
University.
"Our simulations are the most detailed to date for this part of the world,
but it doesn't mean we have the answer," Diffenbaugh said. "It highlights the
importance of spatial complexity in the climate response and suggests that
understanding the potential impacts of future climate change in this region
requires improved understanding of a host of climate processes."
The model projected a delay in the start of monsoon season from five days to
15 days by the end of the 21st century and an overall weakening of the summer
monsoon precipitation over South Asia. Ashfaq said increasing temperatures in
the future strengthen some aspects of large-scale monsoon circulation but
weaken the fine-scale interactions of the land with the moisture in the
atmosphere, which could lead to reduced precipitation over the Indian
subcontinent.
"It is the more subtle, local-scale processes that are key in this case," he
said. "Our model shows a decrease in convective precipitation, which is
critical for summer precipitation in this region. Our findings show it is not
just a question of whether monsoon circulation is stronger or weaker. Even
with a strong monsoon system, if circulation changes enough to change where
and when rain is delivered, then that could have an impact that has not been
captured in the large-scale evaluations."
The atmospheric conditions that lead to reduced precipitation also can lead
to intensification of extremely hot conditions, he said.
"In the past when we have seen extremely hot days, we have observed a similar
circulation anomaly," Ashfaq said. "These circulation changes decrease
moisture flow over the land, and we see longer periods without rain, along
with hot conditions."
The model shows an eastward shift in monsoon circulation, which would mean
more rainfall over the Indian Ocean, Bangladesh and Myanmar, and less over
India, Nepal and Pakistan, Ashfaq said. Less moisture over the land in
combination with the ambient dry summer air would lead to less moisture in
the clouds and reduced rainfall.
Monsoon moisture flow comes from ocean to land. In the summer, the land warms
faster than the ocean. This creates a pressure gradient that draws air masses
from the ocean to the continent, bringing moist air that promotes formation
of a large-scale monsoon system.
Monsoon season, which starts in early June and ends in late September, begins
at the southeast tip of India and moves northwest to the rest of India and
Pakistan.
The climate model used by the research team accurately recreated the monsoon
season of past years, and its future projections are consistent with what has
been seen in recent drought years over this region, Diffenbaugh said.
The team next plans to examine a broader range of global climate models and
to assess the impact of potential future changes on food security and the
economy.
The National Science Foundation partially funded this research.
Dr. Noah Diffenbaugh
http://www.purdue.edu/eas/people/faculty/diffenbaugh.html
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