The
acid test of a scientific theory is whether it makes predictions that
eventually come true. So consider this old prediction, from a pair of
researchers in Australia and New Zealand. They were summarizing the
results of then-primitive computerized forecasts about global warming:
“The
available evidence suggests that a warmer world is likely to experience
an increase in the frequency of heavy precipitation events, associated
with a more intense hydrological cycle and the increased water-holding
capacity of a warmer atmosphere.”
That was published in 1995, and it was based on research going back to the 1980s. Fast forward to 2014.
In the National Climate Assessment,
published last week, researchers in the United States reported that
“large increases in heavy precipitation have occurred in the Northeast,
Midwest and Great Plains, where heavy downpours have frequently led to
runoff that exceeded the capacity of storm drains and levees, and caused
flooding events and accelerated erosion.”
The future, it would seem, has arrived.
Climate
is a difficult branch of science, full of ambiguities and
uncertainties. But scientists can justly claim to have demonstrated some
predictive skill about many of the potential implications of the human
release of greenhouse gases.
Their track record actually goes back to 1896, when a Swede named Svante Arrhenius first predicted
that emissions of carbon dioxide would cause the planet to warm. It
took more than 80 years to be sure he was right. At roughly the same
time that realization was taking hold, climate scientists running
computer models of the atmosphere began to focus on the likelihood of
heavier rains in a future climate.
Many
people are still catching up with the science, but it is hard to miss
the ubiquity of these heavy rainstorms in recent years.
People in the Florida Panhandle recently had to dodge flash floods after two feet of rain fell in 26 hours. Torrential rains caused a Washington State hillside to collapse and bury a community earlier this year. Tumultuous rainstorms and floods overwhelmed Colorado last year, and sudden floods swept through Nashville in 2010, and Atlanta in 2009.
We’re seeing a pattern here.
In
the National Climate Assessment, the experts reported huge increases
since the mid-20th century in the amount of precipitation falling in
very heavy rainstorms: up 71% in the Northeast, 37% in the
Midwest and 27% in the Southeast. The effect was seen on a
smaller scale west of the Mississippi River, too, even in parts of the
country where the climate is drying out over all.
What led the researchers to expect this long before it actually happened?
Initially, the forecast was based on simple physics
from the 19th century. As we pour carbon dioxide into the air, the
lower atmosphere has to warm. As it does, it is able to hold more
moisture, and as the surface of the ocean also warms, more moisture
tends to evaporate from it.
In
the United States, the increase in water vapor has been on the order of
3 or 4 percent since the 1970s (most of the human-caused global
warming has occurred since then). That may not sound like a big jump,
but the effect is enormous.
Two leading scientists, Kevin E. Trenberth at the National Center for Atmospheric Research and David R. Easterling at the National Oceanic and Atmospheric Administration,
ran some calculations and agreed that the warming has, on average, put
more than a trillion gallons of extra water into the air over the
contiguous 48 states, probably closer to two trillion.
That
extra water has to fall as rain or snow. But from the elementary
physics, it was long unclear whether this would mean more rainy days
over all, or more intense rains, or both.
It
was the computer models of the climate that suggested, starting in the
late 1980s, that the answer would be the latter, and so it has turned
out. One way to think of it is that even with a lot of moisture in the
air, conditions are not always right for rain, but when they are right,
the skies have a lot more water to dump.
“It rains harder than it used to,” said Dr. Trenberth, who could not resist adding: “When it rains, it pours.”
Researchers
sponsored by the Australian government were the first to really drill
into the implications of the finding. In their 1995 overview paper,
A. M. Fowler of the University of Auckland in New Zealand and K. J.
Hennessy of Australia’s national research program warned that society
needed to start thinking about the risks. They suggested toughening
standards for the designs of levees and dams, and hardening roads and
culverts against the possibility of more flash floods.
Society
responded by ignoring them. For someone sitting in Pensacola, Fla.,
wondering why the roads were washed out the other day, that longstanding
refusal to confront reality might be a good part of the answer.
The warming of the planet has slowed
in recent years, but scientists think that is likely temporary. They
expect it to get much, much warmer as this century progresses, and that
can only mean that the rains will fall harder still.
So
if you are still a little amazed at what these heavy downpours have
been doing to communities around the country, the message from science
is pretty blunt: Get used to it.
The New York Times article focuses on the U.S., which is understandable. But there have been several very bad landslides in other countries very recently, due to heavy rains, that I think they should have mentioned.
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