January 10, 2002
Deep convection in far north could be key in global climate change
Greenhouse warming and other human effects on the environment may increase the possibility of large, abrupt changes in global climate, according to a recent National Research Council report issued by a panel of 11 scientists that includes the UW’s Peter Rhines and John Wallace.
Until recently, many scientists have assumed that climate change would occur gradually, with annual temperatures slowly increasing during the next century. But new evidence shows that periods of gradual change in Earth’s past were punctuated by episodes of abrupt change, including temperature changes of 18 degrees F, severe flooding and droughts.
Examples from the past century include the rapid warming of the North Atlantic from 1920 to 1930 and the Dust Bowl drought of the 1930s. Data suggest that at one point, about 11,500 years ago, global temperatures fell by as much as 16 degrees and rainfall was halved in just a decade. Those conditions then persisted for 1,000 years.
Scientists do not foresee any such imminent changes and point out that societies usually learn to adapt in time. Still the panel said coping will be easier if more is known about the mechanisms that might cause sudden changes and the ways oceans and the atmosphere interact.
Rhines, some of his co-authors on the report and various other colleagues think one priority is to better understand how frigid water from the poles and water heated in the tropics drive each other around the world. Warmer water that is drawn to the poles radiates its heat into the atmosphere and outer space. If that process were to slow or stop, huge amounts of heat would remain trapped on Earth.
“Mathematical models and computer models of climate predict a slowdown, by up to 50 percent, of this global circulation in coming decades,” Rhines says. “Such changes can be called abrupt in the great scheme of things.”
Rhines described work conducted in the far north since 1981 during a public lecture sponsored by the College of Ocean and Fishery Sciences and the School of Oceanography just a week before the National Research Council report came out. He and his colleagues are trying to pin down details of “deep convection” taking place in the Labrador and Greenland seas, two of only four places on Earth where scientists know this process occurs.
The first time Rhines saw Greenland he thought the ship was approaching a deep bank of clouds stretching across the horizon, when he actually was seeing the 2-mile thick Greenland ice sheet towering over the shoreline mountains. Air sitting over northeastern Canada can be absolutely frigid even before blowing over the Greenland ice sheet. When the wind reaches the waters off Labrador and Greenland it causes what Rhines terms a “ferocious cooling” of the seas.
Normally surface water would freeze solid but the Gulf Stream delivers water from the tropics so loaded with salt that it doesn’t freeze. Cold-and-salty makes for a mass of dense water that sinks through the less-dense layers beneath it. That water then spreads across the ocean depths to any place where the water is less dense. That’s the deep convection Rhines and others are so interested in.
Eventually all that water revisits the tropics where it is warmed by the sun and carried north again as part of the Gulf Stream.
As well as being part of the system that drives warm water to polar regions where it vents its heat, this cold deep water is crucial in forcing nutrients that have drifted down into the depths back to the surface layers of the ocean. They act as plant fertilizers for photosynthesis in waters where the sun penetrates. Carbon dioxide from the atmosphere is absorbed in the process.
Deep convection also is known to occur in the Weddell and Ross seas of Antarctica and in the northwestern Mediterranean. At neither site are the depths as great as places in the Labrador and Greenland seas where the ocean is nearly 4 miles, deep.
“This is very powerful in getting the world’s oceans moving,” Rhines said.
The deep convection in the northwestern Mediterranean, where evaporation concentrates the salt and winter winds from the Alps and the Rhone Valley cool the waters, is the least dominant in global terms but is the most studied. It’s readily observable, Rhines explains, whereas “we scarcely have a single sustained observation program in the far north.”
Rhines is enthused that new classes of moorings, drifters and sensors may finally help scientists establish the long-term monitoring that is needed. It is in the far north that Rhines says he and his colleagues already see some of the greatest evidence of global warming.
“Global warming skeptics should look to the far north to convince themselves that climate change is under way,” he says. He noted that:
- A global average rise of 0.5 degrees C in the last 30 years has been detected, yet in the far north the average rise has been several degrees C.
- He and his colleagues have documented a “massive” decrease in the salinity of the Labrador Sea in the last 30 years. This is caused by such things as melting ice, more rain and more river water flowing into the sea. “That flood of fresh water is like a lid keeping cold water from sinking and removing the driving force for global ocean circulation,” he says.
- Both the area covered by ice and the thickness of ice have decreased.
- Even before recorded temperatures in the 1900s started warming, measurements of glaciers around the world showed a retreat starting as early as the mid-1800s, leading Rhines to speculate that, perhaps, global warming started 50 years earlier than most people have been saying.
For 300 years explorers sought the “Northwest Passage” – a much-desired way to sail from the Atlantic to the Pacific, and the Orient, without traveling all the way around South America. Such a passage was found in 1905 through the Canadian Archipelago but it was so choked with ice to be virtually unpassable until the age of modern icebreakers.
“Now that we don’t really need the ‘Northwest Passage,'” Rhines says, “we appear to be looking forward to seeing its ice free.”
The “Oceans to Stars” Lecture Series sponsored by the College of Ocean and Fishery Sciences and the School of Oceanography will be Jan. 23, 7 p.m., Kane 210 with UW’s John Delaney’s presentation “Volcanoes, Oceans and Life in the Solar System.”
