[The annual summer monsoon in South Asia begins this month. A new study points to more destructive storms.]
Scientists have known for years
that climate
change is disrupting monsoon season. Past
research based on computer models has suggested that the global
heating caused by greenhouse gases, and the increased moisture in the warmed
atmosphere, will result in rainier summer monsoon seasons and unpredictable,
extreme rainfall events.
The new paper, published Friday in the journal Science
Advances, adds evidence for the theory by looking back over the past million
years to give a sense of monsoons to come.
The monsoon season, which generally
runs from June to September, brings enormous amounts of rain to South Asia that
are crucial to the region’s agrarian economy. Those rains affect the lives of a
fifth of the world’s population, nourishing or destroying crops, causing devastating
flooding, taking
lives and spreading pollution. The changes wrought by climate change
could reshape the region, and history, the new research suggests, is a guide to
those changes.
The researchers had no time
machine, so they used the next best thing: mud. They drilled core samples in
the Bay of Bengal, in the northern Indian Ocean, where the runoff from monsoon
seasons drains away from the subcontinent.
The core samples were 200 meters
long, and provided a rich record of monsoon rainfall. Wetter seasons put more
fresh water into the bay, reducing the salinity at the surface. The plankton
that live at the surface die and sink to the sediment below, layer after layer.
Working through the core samples, the scientists analyzed the fossil shells of
the plankton, measuring oxygen isotopes to determine the salinity of the water
they lived in. The high rainfall and low salinity times came after periods of
higher concentrations of atmospheric carbon dioxide, lower levels of global ice
volume and subsequent increases in regional moisture-bearing winds.
Now that human activity is boosting
levels of atmospheric greenhouse gases, the research suggests, we can expect to
see the same monsoon patterns emerge.
Steven Clemens, a professor of
earth, environmental and planetary sciences at Brown University and lead author
of the study, said “we can verify over the past million years increases in
carbon dioxide in the atmosphere have been followed by substantial increases in
rainfall in the South Asian monsoon system.” The predictions of the climate
models are “wonderfully consistent with what we see in the past million years,”
he said.
Anders Levermann, a professor of
the dynamics of the climate system at the Potsdam Institute in Germany who was
not involved in the new paper but has produced research on climate model
monsoon projections, said that he was pleased to see research that supported
the findings of forward-looking climate models. “It’s a tremendous body of information,”
he said, “and it’s really nice to see in actual data that reflects more than a
million years of our planet’s history, to see the physical laws that we
experience every day leave their footprints in this extremely rich
paleo-record.”
Dr. Levermann added that the
consequences for the people of the Indian subcontinent are dire; the monsoon
already drops tremendous amounts of rain, and “can always be destructive,” he
said, but the risk of “catastrophically strong” seasons is growing, and the
increasingly erratic nature of the seasons holds its own risks. “And it is
hitting the largest democracy on the planet; in many ways, the most challenged
democracy on the planet,” he said.
Dr. Clemens and other researchers
took their samples during a two-month research voyage on a converted oil
drilling ship, the JOIDES Resolution. It carried a crew of 100 and 30
scientists on a trip that began in November 2014. “We were out over Christmas,”
he recalled, and while “it’s difficult to be away from family that long,” the
payoff has finally arrived. “We’ve been at this years,” he said, “creating
these data sets. It’s satisfying to have this finally come out.”
John Schwartz is a reporter on the climate desk. In nearly two decades at The Times, he has also covered science, law and technology. @jswatz • Facebook