|
Why the global deep freeze
By Paul R. Epstein and James J. McCarthy
Op-ed Column, The Boston Globe, Jan. 28, 2003
Why is it so frigid when the globe is warming?
As an Arctic frost chills two-thirds the nation and kills hundreds in
Bangladesh, some answers may come from changes in the Arctic itself.
First, if you had any doubts, we are in an unusually deep cold spell, with
snow and records falling across the South. Driving conditions are
hazardous (and sometimes tragic) as pedestrians and the homeless face
bitter winds and icy ''orthopedic weather.'' Ice dams are blocking Latvian
ports, winds and storms are battering Europe, Portugal is freezing,
Vietnam has lost one-third its rice crop, and the cold has caused close to
2,000 deaths in usually temperate South Asia.
As several scientists have warned, global warming will be full of
surprises. Warming over the past half-century has already brought more
erratic and extreme weather. Some climatologists are increasingly
concerned about the stability of the climate system itself and the
potential for abrupt shifts - to warmer or even much colder states. Can we
make sense of the present cold snap?
Part of the explanation comes from changes to our north.
Warming causes ice to melt, forming cold fresh water. And increased input
of cold fresh water to the ocean can affect weather patterns as well as
global ocean circulation.
Recent warming in the Northern Hemisphere has melted a lot of North Polar
ice. Since the 1970s the floating North Polar ice cap has thinned by
almost half.
A second source of cold fresh water comes from Greenland, where
continental ice is now melting at higher elevations each year. Some melt
water is trickling down through crevasses; lubricating the base,
accelerating ice ''rivers,'' and increasing the potential for sudden
slippage.
A third source of cold fresh water is rain at high latitudes. Overall
ocean warming speeds up the water cycle, increasing evaporation. The
warmed atmosphere can also hold and transport more water vapor from low to
high latitudes. Water falling over land is enhancing discharge from five
major Siberian rivers into the Arctic, and water falling directly over the
ocean adds even more fresh water to the surface.
The cold, freshened waters of the North Atlantic accelerate transatlantic
winds, and this may be one factor driving frigid fronts down the eastern
US seaboard and across to Europe and Asia.
It is too early to know how long the current cold spell will last, and
time and hindsight will be needed to reveal all the factors contributing
to the present chill. But the ice itself and pollen and marine fossils
reveal that cold reversals have interrupted warming trends in the past.
The North Atlantic Ocean can freshen to a point where the North Atlantic
deep water pump - driven by sinking cold, salty water that is, in turn,
replaced by warm Gulf Stream waters - can suddenly slow down. Some 13,000
years ago, when the globe was emerging from the last Glacial Maximum and
continental ice sheets were thawing, the Gulf Stream abruptly changed
course and shot straight across to France. The Northern Hemisphere refroze
- for the next 1,300 years - before temperatures jumped again in just
several years, warming the world to its present state.
In the past few years the northern North Atlantic has freshened, and since
the 1950s the deep overflow between Iceland and Scotland has slowed by 20
percent.
Since cold reversals occurred naturally, one may ask whether humans can
influence these cycles as well.
Natural variability and human influence together explain the observed
changes in the North Atlantic. Calculations (of orbital cycles) indicate
that our hospitable climate regime was not likely to end due to natural
causes any time soon. But due to the burning of fossil fuels, atmospheric
levels of carbon dioxide are now greater than at any time in the last half
million years. The recent buildup of heat-trapping greenhouse gases is
forcing the climate system in new ways and into uncharted seas.
The hopeful news is that unstable systems can be restabilized. But
substantial reductions in greenhouse gas emissions will be needed in the
next few years to stabilize the atmospheric concentrations and reduce the
human-generated forces pushing on the climate system.
In order to change directions, however, we must first slow down. And
slowing down means burning much less coal, oil, and gas and cutting far
fewer trees.
Significant incentives will be needed to simultaneously solve the problems
of energy, the environment, and the economy. But clean development through
renewable energy can become the engine of growth for this 21st century and
provide the best insurance for a healthier, more stable, and more secure
future.
Dr. Paul R. Epstein is associate director of the Center for Health and the
Global Environment at Harvard Medical School. James J. McCarthy is
professor of oceanography at Harvard University and was co-chair of the
IPCC 2001 Assessment, Working Group II.
This story ran on page A15 of the Boston Globe on 1/28/2003.
| 
