Explaining Rapid Climate Change: Tales from the Ice | |||
When scientists started to analyze the paleoclimate evidence in the Greenland and Antarctic ice cores, they found that the record also supported Milankovitch’s theory of when ice ages should occur. But they also found something that required additional explanation: some climate change appeared to have occurred very rapidly. Because Milankovitch’s theory tied climate change to the slow and regular variations in Earth’s orbit, the scientific community expected that climate change would also be slow and gradual. But the ice cores showed that while it took nearly 10,000 years for the Earth to totally emerge from the last ice age and warm to today’s balmy climate, one-third to one-half of the warming—about 15 degrees Fahrenheit—occurred in about 10 years, at least in Greenland. A closer look at marine sediments confirmed this finding. Although the overall timing of the ice ages was clearly tied to variations in the Earth’s orbit, other factors must have contributed to climate change as well. Something else made temperatures change very quickly, but what? |
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Greenhouse GasesScientists are now exploring a few possibilities. First, greenhouse gases probably influenced past climates. Ice cores record past greenhouse gas levels. In the past, when the climate warmed, the change was accompanied by an increase in greenhouse gases, particularly carbon dioxide. When scientists tried to build climate models, they could not get the models to simulate past climate change unless they also added changes in carbon dioxide levels. Though scientists aren’t sure why carbon dioxide levels changed, almost all believe that the shift contributed to altering the climate. Because ice cores also revealed that carbon dioxide levels are much higher today than at any time recorded in the past 750,000 years, pinning down the cause-and-effect relationship between carbon dioxide and climate change continues to be a focal point of modern climate research. Global Conveyor BeltAnother possible trigger for rapid climate change is ocean circulation. Today, warm water from the equator is carried towards the poles on ocean surface currents. Because of the arrangement of the continents, warm water is carried far into the North Atlantic, moderating the climate in Northern Europe. As the warm surface water reaches the cold air in the north, it cools. The salty Atlantic water becomes very dense as it gets cold. The cold, salty water sinks to the bottom of the ocean before it can freeze, where it is pulled southward toward the equator. More warm water from the equator flows north to replace the sinking water, setting up a global oceanic “conveyor belt.” |
Rapid changes between ice ages and warm periods (called interglacials) are recorded in the Greenland ice sheet. Occurring over one or two decades, the warming of the Earth at the end of the last ice age happened much faster than the rate of change of the Earth’s orbit. The last cool period (stadial), immediately before the current interglacial, began and ended suddenly, and was likely caused by changes in the deep ocean circulation. (Graph by Robert Simmon, based on data provided by Alley 2004.) | ||
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This pattern helps keep Northern Europe far warmer than other locations at the same latitude. The key to keeping the belt moving is the saltiness of the water, which increases the water’s density and causes it to sink. Many scientists believe that if too much fresh water enters the ocean, for example, from melting Arctic glaciers and sea ice, the water will be diluted. Fresh water freezes at a higher temperature than salty water, so the cooling surface water would freeze before it could become dense enough to sink toward the bottom. If the water in the north does not sink, the water at the equator will not move north to replace it. The currents would eventually stop moving warm water northward, leaving Northern Europe cold and dry within a single decade. This theory of rapid climate change is called the “conveyor belt theory,” and though many scientists do not yet agree with it, the paleoclimate record found in ocean sediment cores is beginning to support it. Recent paleoclimate studies have shown that when heat circulation in the North Atlantic Ocean slowed in the past, the climate changed in Northern Europe. Although the last ice age peaked about 20,000 years ago, the warming trend was interrupted at various points by cold spells. In a paper published in Nature on April 22, 2004, McManus and colleagues Roger Francois, Jeanne Gherardi, Lloyd Keigwin and Susan Brown-Leger at Woods Hole Oceanographic Institute and the Laboratoire des Sciences du Climat et de l’environnement in France showed that cold periods in Europe 17,500 and 12,700 years ago happened just after melting ice diluted the salty North Atlantic water, and the ocean “conveyor belt” slowed. The evidence, which they took from radioactive elements in ocean cores, is beginning to support the theory, but McManus cautions that there are still pieces to fill in before we fully understand what role the conveyor belt played in past climate change and what role it might play in the future.
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The large-scale movement of water through the oceans, called the thermohaline circulation, plays a large role in the duration of ice ages. Dense, very salty (saline) water sinks in the North Atlantic, pulling the “conveyor belt” of currents behind it. The conveyor belt carries heat from the equator towards the poles, and raises Arctic temperatures, discouraging the growth of ice sheets. Influxes of fresh water from the lands that surround the North Atlantic can slow or shut down the circulation, cooling the Northern Hemisphere. This map shows the general location and direction of the warm surface (red) and cold deep water (blue) currents of the thermohaline circulation. Salinity is represented by color in units of the Practical Salinity Scale. Low values (blue) are less saline, while high values (orange) are more saline. (Map by Robert Simmon, adapted from the IPCC 2001 and Rahmstorf 2002.) | ||
