Warming Could Bring Some Cold Surprises
By William K. Stevens
The New York Times
Sept. 9, 1998
The big wild card in the game of climate change is the possibility of uwelcome surprises that would suddenly occur if some threshold of warming caused the climate system to shift to a new state.
For example, some scientists say, the warming could melt more snow and ice, flooding the North Atlantic with fresh water and upsetting the workings of large ocean currents. If that happened, the world might suddenly find itself with serious disruptions in currents like the Gulf Stream that transport heat around the globe, keep Europe warm and maintain favorable conditions for marine life.
Just such an event may have been responsible for plunging the world back into glacial conditions for several centuries, starting about 12,000 years ago, after it had temporarily warmed near the end of the last ice age.
A study reported last month in the journal Nature suggested that the critical ocean currents could be shut down altogether should atmospheric concentrations of heat-trapping carbon dioxide double within 100 years, as mainstream scientists expect to happen if emissions of the gas from power plants, industries and motor vehicles are not reduced. The study, by Thomas F. Stocker and Andreas Schmittner of the University of Bern in Switzerland, was based on computer simulations of the ocean-atmosphere system.
Moreover, they found, the currents slow but do not stop if the atmospheric concentration of carbon dioxide grows more slowly. This, they wrote in Nature, has "potentially important implications" in deciding how much and how fast to reduce emissions.
Earlier computer modeling studies, by scientists at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory at Princeton University, had suggested that the ocean currents would shut down irreversibly if atmospheric carbon dioxide were to reach four times today's levels. Most studies have stopped at a doubling of carbon dioxide, and have not carried the analysis of global warming beyond 2100.
Dr. Jerry Mahlman, the director of the Princeton laboratory, says that if no action is taken to reduce emissions, concentrations of carbon dioxide could quadruple before 2200. According to the computer simulations, the quadrupling would result in a rise of about 12 degrees Fahrenheit in the average global temperature within 500 years. In much of the Northern Hemisphere, the rise would be around 20 degrees. In the Southeastern United States, according to the computer model, the typical July heat index (a combination of temperature and humidity) would rise to about 110 degrees, compared with 85 to 90 degrees now.
That is a long time away, of course, and the nightmare scenario may or may not come to pass. But to Dr. Mahlman, it is real enough to inject yet another concern into the policy debate over greenhouse emissions: if too little is done too late, the world may in time regret the consequence. "If you get to where you don't like, there's not a lot you can do about it -- you're wired in for a long time," Mahlman said.
Copyright 1997 The New York Times Company
NATURE 28 August 1997
Summaries of some of this week's stories
Nature 388, 860 (1997) Influence of CO2 emission rates on the stability of the thermohaline circulation
Present estimates of the future oceanic uptake of anthropogenic CO2 and calculations of CO2-emission scenarios are based on the assumption that the natural carbon cycle is in steady state. But it is well known from palaeoclimate records and modelling studies that the climate system has more than one equilibrium state, and that perturbations can trigger transitions between them. Anticipated future changes in todays climate system due to human activities have the potential to weaken the thermohaline circulation of the North Atlantic Ocean, which would greatly modify estimates of future oceanic CO2 uptake. Here the authors use a simple coupled atmosphere--ocean climate model to show that the Atlantic thermohaline circulation is not only sensitive to the final atmospheric CO2 concentration attained, but also depends on the rate of change of the CO2 concentration in the atmosphere.
A modelled increase to 750 parts per million by volume (p.p.m.v.) CO2 within 100 years (corresponding approximately to a continuation of today's growth rate) leads to a permanent shut-down of the thermohaline circulation. If the final atmospheric concentration of 750p.p.m.v. CO2 is attained more slowly, the thermohaline circulation simply slows down. The reason for this rate-sensitive response of the climate system lies with the transfer of buoyancy in the form of heat and fresh water from the uppermost layers of the ocean into the deep waters below. This sensitivity of the simulated thermohaline circulation to the rate of change of atmospheric CO2 concentration has potentially important implications for the choice of future CO2-emission scenarios.
T F Stocker & A Schmittner Influence of CO2 emission rates on the stability of the thermohaline circulation (Letter to Nature)
GLOBAL WARMING MIGHT CAUSE BIG SURPRISE, STUDY WARNS.
By Maggie Fox, Health and Science Correspondent
Nov. 29, 1997
WASHINGTON, (Reuters) - Global warming may bring more than a slow rise in the Earth's oceans -- it could also bring some abrupt, and unpleasant, surprises, an expert warned on Thursday.
Severe winters could wipe out crops and fisheries in Northern Europe, causing millions to starve, Wallace Broecker of the Lamont-Doherty Earth Observatory at New York's Columbia University said.
The key lies in the ocean's currents which, as anyone watching the effects of El Nino can testify, can have dramatic and wide-ranging effects. El Nino, a periodic current off South America's Pacific coast, is causing floods, drought and other serious effects around the world.
Climate models predict that a large buildup of greenhouse gases -- such as the carbon dioxide often blamed for helping global warming -- could cause a collapse in current circulation patterns, Broecker wrote in a paper in the journal Science.
We must also take seriously the possibility that such a collapse would have profound consequences to atmospheric operation and hence global climate," Broecker wrote.
Right now the ocean's currents act like a conveyor belt, taking tropical heat to the north Atlantic. This makes winter in Northern Europe relatively mild -- as opposed to bitter climes at similar latitudes in Canada and Asia.
But it has not always been so.
"During the last glacial period, Earth's climate underwent frequent large and abrupt global changes," Broecker wrote. "The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times."
The results were clear, even hundreds of thousands of years later. Tree pollen disappeared, meaning that forests were wiped out, and there were profound changes in the kinds of fish in the sea.
"Should this occur when 11 to 16 billion people occupy our planet, it could lead to widespread starvation, for in order to feed these masses it will be necessary to produce two to three times as much food per acre of arable land than we do now," Broecker predicted.
Broecker said scientists currently had an "embarrassing" lack of knowledge about deep-water currents. "To me, it is the Achilles heel of the climate system," he wrote.
A better understanding of such currents was vital. But he also recommended that people stop their dependence on fossil fuels, blamed in large part for causing the buildup of carbon dioxide that is contributing to global warming. "To this end I see a ray of hope," Broecker concluded. "The idea is to separate the hydrogen atoms contained in fossil fuels by reacting them with steam."
He said the hydrogen could be used in fuel cells and leftover carbon dioxide injected far below the Earth's surface or sea floor.
"We must get off to a running start to put into place this insurance policy," he urged.
(c) Reuters Limited 1997
Climatology: Threat to Atlantic's Conveyor Belt'
By Curt Suplee
The Washington Post
Sept. 1, 1997
If the present rate of increase in atmospheric carbon dioxide continues, the results could completely shut down the Atlantic Ocean circulation system -- the essential marine "conveyor belt" that dominates weather patterns in America and Europe. That is the ominous conclusion of a study in the Aug. 28, 1997 issue of Nature by physicists at the University of Bern, Switzerland.
Normally, warm and very salty surface water from the equatorial Atlantic flows north until it hits a mass of cold, less saline water around Greenland. Then it cools, sinks, and returns toward the equator far beneath the surface. The incoming warm water has some big advantages. It sheds its heat into the air, keeping Europe's weather comfortable. And as it cools, it absorbs atmospheric CO2. (The cooler a liquid, the more dissolved gas it can hold.)
If CO2 emissions continue to increase at the present rate (1 percent per year) for several more decades, the Swiss scientists' computer model predicts, it would lead to "irreversible changes" as the water-mixing stopped. Europe's weather would become unstable and much less CO2 would be trapped in deep ocean water. But reducing the rate of CO2 build-up, they calculated, would merely slow the conveyor belt, not turn if off. So policymakers should "take into account critical limits on the rate of greenhouse gas increase" to guard against climatic catastrophe, they conclude.
© Copyright 1997 The Washington Post Company