Will the greenhouse effect lead to a new ice age?
The influence of global warming on currents in the ocean could be reflected in fatal climate changes: According to leading climate experts, a change in the Gulf Stream and other currents in the world's oceans could lead to a new ice age in Europe. Dublin would then have the same frigid climate as Spitsbergen. The third international climate conference will open in Kyoto, Japan on December 1st. Ahead of this event, one of the world's leading climate experts warned against underestimating the dangers of the greenhouse effect. It could lead to a collapse of the current system in the ocean. The consequence would be that temperatures across Europe would drop dramatically in just 10 years. If the system collapsed today, winter temperatures in the regions around the North Atlantic would drop by 10 or more degrees Celsius. Dublin would have a climate like Svalbard, 1000 kilometers north of the Arctic Circle.
"The consequences would be devastating," said Wallace S. Broecker, professor of earth and environmental sciences at Columbia University's Lamont-Doherty Earth Observatory and author of the new research, published Nov. 28 in Science.
The Achilles heel of the world climate
Our climate is determined by a complex of globally interconnected ocean currents, also called conveyors, that transport heat and moisture around the globe. But this conveyor is delicately balanced and very sensitive. According to Broecker, it has often failed or changed direction over the course of Earth's history. Each time the conveyor has changed course, it has resulted in marked global temperature changes over decades, large changes in wind conditions, dramatic fluctuations in the amount of dust in the atmosphere, an advance or retreat of glaciers, and other drastic changes in many regions of the world, he said.
The conveyor “is the Achilles' heel of the climate system,” wrote Broecker in Science. “The data… indicate that this flow was not constant, but jumped from one mode of operation to another, so to speak. The climate changes triggered by these jumps have now proven to be significant, abrupt and global.”
The progressive accumulation of heat-accumulating industrial gases, which envelop the earth like a blanket, threatens to cause global temperatures to rise. But Broecker says such an increase would only happen gradually. Much more troubling is the possibility that the climate system will be pressurized beyond a critical threshold. This could result in the conveyor being disrupted and the rapid reorganization of Earth's climate predicted by today's computer simulations.
Scientists generally agree that periodic changes in the Earth's orbit and the amount of solar radiation that hits the Earth have led to fundamental changes in the Earth's climate over a period of millions of years. But the short-term global climate shifts were caused by sudden changes in the conveyor, Broecker said.
Water's journey around the world
Nowadays, the conveyor's driving force is the cold, s alty waters of the North Atlantic. This water is denser than warm, saline water and therefore sinks to the bottom of the ocean, pushing the water through the world's oceans like a giant pump. The volume of this deep undersea current is 16 times the volume of all the world's rivers combined, Broecker said. It flows south to the southern tip of Africa, where it joins a current that circles Antarctica. There the conveyor is charged with cold, saline water created by the formation of ice in the sea: the s alt is left behind when the water freezes. This renewed subsidence drives the water north again, where it is gradually warmed and rises to the surface in the Pacific and Indian Oceans.
In the equatorial Indian Ocean, the surface water is too warm to sink. The water in the North Pacific is cold but not s alty enough to sink to the depths. This is because the winds blowing around the planet bump against and dump their wetness on the high mountains of the western United States and Canada. The resulting snow and rain flows into the Pacific, bringing fresh water that dilutes its s alt water, Broecker said.
The surface waters of the North Atlantic are only about 7 percent more saline than the waters of the North Pacific, but just enough to exceed the limit at which depth sinks. If the water in the North Atlantic warms up by just a few degrees, or is diluted by more fresh water from melting glaciers and sea ice, or more precipitation, then this threshold will not be reached and the water could no longer sink. Computer models simulating the Earth's climate system show that the ocean's so-called thermohaline circulation is sensitive to small changes of this kind. The entire conveyor could disappear in the short term and rearrange itself in a different form. This would have serious implications for the global climate.
Currently, the conveyor is going through the full cycle, eventually returning warm surface water, including the Gulf Stream, back to the North Atlantic. During the winter months, this warm water transfers its heat to the overlying icy air masses from ice-covered Canada, Greenland, and Iceland. Due to the air masses warmed up in this way, moving eastwards, it is noticeably warmer in Europe in winter than in comparable latitudes in North America. Without the Gulf Stream, nothing would ease the arctic air and Europe would freeze.
Climate change in the past
In recent years, evidence has accumulated that the Earth has often experienced rapid, large-scale climate changes. Samples from the Greenland ice sheet show that during the last ice age, Earth's climate oscillated between periods of intense and moderate cold every few thousand years. The transitions happened in periods ranging from a few decades to a few years. In each interval of intense cold, an entire armada of icebergs broke off and floated in the North Atlantic, as shown by ocean sediment samples. There was also a large increase in dust in Earth's atmosphere, indicating a marked change in wind and storm patterns. Tropical wetlands and mountain glaciers in Chile and New Zealand each expanded and contracted in sync with changes in the North Atlantic.
Glaciers in tropical latitudes also show strong evidence that the water vapor content of the Earth's atmosphere can also change. Water vapor is the most abundant “greenhouse gas” in the earth's atmosphere. A significant decrease would cause air and ocean temperatures to fall drastically.
"Although the exact circumstances causing these climate changes are not yet known, one can theorize that their roots lie in the thermohaline circulation of the ocean," said Broecker. The most important clue is that the boundaries marking the climate changes in continuous sediment or ice samples are clearly and not gradually delineated. This even applies to climate changes that extend over millions of years and whose rhythm is therefore controlled by the Earth's orbit. Broecker believes that sudden shifts in the thermohaline circulation can trigger ice ages and other large-scale climatic cycles. He noted that abrupt climate changes occurred not only during ice ages, but also during warmer periods, such as the epoch in which we live. The Eemian Period - the last major warm period before the most recent Ice Age, which began about 115,000 years ago - ended with a brief but intense cold period. A similar cold period also occurred about 8,000 years ago-about 2,500 years after the end of the last Ice Age-when conditions were similar to those of today, or even slightly warmer.
Danger for the future
"The finds in the Greenland ice sheet reveal a disturbing feature of the Earth's climate system: the ability to undergo abrupt climate changes. I say worrying because it's certainly possible that the ongoing increase in greenhouse gases is triggering a renewed reorganization of ocean currents and associated changes in the atmosphere," Broecker said. “If this happened 100 years from now, at a time when we are struggling to produce enough food for a projected population of 12 to 18 billion people – the consequences would be devastating.”
Article on the subject in Spectrum of Science: Wallace S. Broecker: Sudden Climate Changes, Spectrum of Science, January 1996, page 86
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