Climate Change
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Edwin Schiele The interplay between ocean and atmosphere is one of the keys to understanding Earth’s climate. Winds redistribute heat around the world and help drive ocean surface currents. Currents in turn redistribute the heat the ocean carries and play a major role in shaping global temperature and rainfall patterns. As phenomenon such as El Niño demonstrate, even subtle shifts in winds and surface currents can profoundly impact temperature and rainfall patterns both locally and globally. The image below, derived from AVHRR data, shows the frozen continent of Antarctica and its surrounding sea ice. |
Warming temperatures due to rising levels of greenhouse gases will likely rearrange things further. Scientists are developing models to determine how the wind and ocean currents might respond to global warming and how such changes may in turn alter climate patterns. Now there is another potential impact that is grabbing scientists’ attention. Scientists are exploring the small but real possibility that even small shifts in ocean currents, possibly set in motion by global warming, may trigger the catastrophic melting of the world’s two great ice sheets. |
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The World’s Two Great Ice Sheets |
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Robert Bindschadler, the chief scientist of the Hydrospheric and Biospheric Sciences Laboratory at NASA’s Goddard Space Flight Center, has been monitoring the Antarctica and Greenland ice sheets. He uses words such as “awestruck” to describe scientists’ reactions to what has been happening. In 2002, satellites documented the dramatic 34-day collapse of a Rhode Island-sized section of the Larsen-B Ice Shelf on the eastern coast of the Antarctic Peninsula. Other ice shelves along the rapidly warming Antarctic Peninsula, including the Wordie, Müller and Jones Ice Shelves, have also disintegrated. Then in April of 2009, the final narrow bridge of ice anchoring Antarctica’s Wilkins Ice Shelf in place broke apart. Now that massive ice shelf is on the verge of collapse. In 2002, a gigantic section of the Larsen B Ice Shelf in the Antarctic Peninsula shattered and crashed into the ocean. It disintegrated in a mere 35 days, losing 1,255 square miles - an area somewhat larger than Rhode Island. Captured from space using a variety of orbiting instruments, these pictures illustrate the dramatic changes that may occur as a function of a changing climate. (1.7 Mb – no audio). Credit: NASA Meanwhile, glaciers on Greenland and Antarctica are accelerating at astonishing rates, disgorging increasing amounts of ice into the ocean. The possible implications of this increased ice loss are dire. The Greenland ice sheet contains enough water to raise the sea level seven meters. The Antarctic ice sheet contains enough water to raise the sea level 57 meters. Few scientists think that either ice sheet will completely disappear anytime soon. But if present trends continue, the sea level could rise to submerge low-lying islands and devastate coastal populations during this century. |
Scientists are also exploring the possibility that if the Greenland Ice Sheet (pictured above) collapses quickly, the infusion of fresh melt water could short-circuit the ocean’s conveyor belt known as the meridional overturning circulation. Such a change could alter both ocean and atmospheric circulation patterns around the world.
To determine whether this increased melting of the ice sheets is part of a longer-term trend, Bindschadler and other scientists have set out to answer two daunting questions. What are the mechanisms that trigger the rapid loss of ice? And how might the complex interplay between atmosphere and ocean set these mechanisms into motion? |
What are the Melting Mechanisms? |
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The Greenland and Antarctic ice sheets are built from compacted snow. Ice streams or glaciers ferry the ice from the center of each ice sheet to the ocean. An ice sheet is at equilibrium when the amount of accumulating snow is matched by the amount of ice lost in the ocean. In Antarctica, the ice extends far out onto the ocean, forming enormous floating ice shelves. The largest ice shelf, the Ross Ice Shelf off of West Antarctica, covers an area roughly the size of France. In Greenland, where the glaciers empty onto narrow fjords, the ice shelves, also known as ice tongues, are far less extensive. Ice shelves are the most vulnerable part of the ice sheets. Waves and currents buffet the edges. Large chunks calve off, creating icebergs. But according to Bindschadler, ice shelves also help stabilize the ice sheet. Because the edges are often partially grounded in shallow water, ice shelves hold back the glaciers that feed them. When an ice shelf retreats or collapses, as some are doing now, the breaks are removed. The glaciers behind it speed up and drain more ice into the ocean. The loss of ice now outpaces the creation of new ice, and the ice sheet shrinks. When the Larsen-B Ice Shelf collapsed, some of the liberated inland glaciers increased their speed five fold. Bindschadler says an ice sheet’s greatest enemy is water. Warming temperatures on the margins of Greenland and on the Antarctic Peninsula, are creating pools and rivers of melt water on top of the ice. Scientists have found that on the Greenland Ice Sheet, the melt water penetrates through cracks in the ice down to the bedrock. There it acts as a lubricant, speeding up the glaciers as they slide towards the ocean. The effect of melt water on the ice shelves of Antarctic Peninsula is even more dramatic. There the penetrating water opens up networks of cracks and carves the ice shelves into thin vertical slabs. Eventually these slabs topple like dominos, and the ice shelves collapse in spectacular fashion. These events doomed the Larsen-B Ice Shelf and other ice shelves along the West Antarctic Peninsula. Water from the ocean, however, may pose the greatest threat of all. Normally the Antarctic ice shelves and the ends of the Greenland outlet glaciers are bathed in cold water. But Bindschadler and his colleagues are now seeing unusually warm water penetrate beneath the ice. Although this water is only warm in a relative sense, just a couple of degrees above freezing, it can melt away the ice at an astonishing rate—up to 100 meters per year. Bindschadler says his intuition tells him that this infusion of warm water is the dominant mechanism responsible for accelerating the loss of ice. It is also the mechanism that scientists know the least about.
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Scientists have found evidence that warm water is eroding the floating ends of two of the world’s important outlet glaciers: the Jakobshavn Isbræ in western Greenland and the Pine Island Glacier in West Antarctica. The responses of these glaciers have been dramatic. Satellites show that the Jakobshavn Isbræ, which drains ice from seven percent of the Greenland Ice Sheet, has retreated rapidly and doubled in speed during the past decade, making it the fastest glacier on the planet. The base of the Pine Island Glacier, which drains ice from ten percent of the West Antarctic ice sheet into the Amundsen Sea, has also thinned significantly and accelerated 60 percent in the past 35 years. Scientists have also found evidence that a similar infusion of warm water may have helped bring the Antarctica’s Wilkins Ice Sheet to the verge of collapse. David Holland, the director of the Center for Atmosphere-Ocean Science at New York University’s Courant Institute, is one of the scientists trying to find the source of this warm water. He says the stakes are enormous. Warm water can rapidly melt exposed ice in areas where the climate remains extremely cold. If, for example, the warm water were to reach Antarctica’s Ross Ice Shelf, the damage it might inflict could unleash some of West Antarctica’s largest glaciers. More ominously, according to Holland, the bulk of the West Antarctica ice sheet sits below sea level. If currents carried the warm water beneath this ice, it potentially could trigger the collapse of an ice sheet that scientists estimate holds enough water to raise the sea level five meters. It is clear that global warming did not create this warm water. Holland says it has always been present below the surface. Surface currents carry warm salty water from the tropics towards the poles. There the tropical water encounters the far colder and fresher polar water. Because the salty surface water is denser, it submerges to between 200 and 1,000 meters. Normally this warm intermediate water stays in the deeper ocean below the continental shelves and clear of the ice sheets. But shifts in winds and surface currents have redirected it. |
The North Atlantic Oscillation (NAO) |
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| Holland and his colleagues have proposed that the influx of warm intermediate water around Greenland may be tied to a complex climate phenomenon called the North Atlantic Oscillation (NAO). Among other things, the NAO influences the tracks of winter storms across the North Atlantic and the severity of winter weather over eastern North America and western Europe. In the North Atlantic, there is a permanent high-pressure system over the Azores and a permanent low-pressure system over Iceland. The strengths of the high and low-pressure systems vary, sometimes from day to day. However there can be periods ranging from months to decades where the average pressure differences between the two systems are either high (positive phase) or low (negative phase). | It remains to be seen whether the influx of warm water to both ice sheets is part of a natural cycle or reflects longer-term trends. Holland has found circumstantial evidence that the last prolonged negative phase in the 1920s also sent warm intermediate water towards western Greenland. Fishery data recorded the presence of fish off of Greenland’s west coast that are more typically found in warmer waters further south. Their presence suggests that the fish had followed the warm water towards Greenland. There are now signs that the North Atlantic Oscillation is exiting its present negative phase, so Holland and his colleagues will be watching whether the supply of warm water off the western Greenland coast slackens and the rate of melting slows. |
During a prolonged positive phase, the enormous pressure gradient drives powerful winds northeast across the North Atlantic. If sustained, these winds strengthen the subpolar gyre that circles counterclockwise in the far North Atlantic. Cold water in the gyre then extends east and keeps warm salty surface currents from spreading northward towards Greenland. During a prolonged negative phase, however, the winds slacken due to the lower pressure gradient. The subpolar gyre shrinks and retreats westward. More warm and salty subtropical surface water then can move northward into the eastern part of the North Atlantic basin. One of these warm pathways is the Irminger Current, which carries warm salty water north towards Greenland. Upon encountering the polar water, the water submerges, and then circles west around Greenland, reaching Greenland’s outlet glaciers. The North Atlantic Oscillation has been in a negative phase since 1997. According to Holland, this period coincides with the presence of warm water around western Greenland and the rapid retreat and acceleration of the Jakobshavn Isbræ glacier. Holland says that increased pressure gradients and strengthened westerly winds may also account for the influx of warm water under the West Antarctic ice shelves. Stronger westerly winds accelerate the surface currents that circle Antarctica. As the waters curve north away from Antarctica due to the Coriolis effect, the warm water below the surface rises up to replace it. The trajectory of this upwelling warm water likely swept it up onto the continental shelf and beneath the ice shelves. One hypothesis links the strengthening of the west winds to the ozone hole over Antarctica. The presence of the ozone hole has prevented Antarctica from warming as quickly as the rest of the world. The resulting temperature gradient from south to north likely increased the pressure gradient that drives the winds. |
Down south, the eventual erosion of the ozone hole over Antarctica could shut off the supply of warm water that is infiltrating the ice shelves. Or like in the North Atlantic, the increase in westerly winds may simply be part of a cyclical process that periodically sends warm intermediate water towards Antarctica. Even if scientists can decipher these cycles, predicting the future remains a daunting task. Scientists know too little about how climate change will alter wind patterns and surface currents. But it is becoming increasingly clear that future changes in the ocean currents will help determine the future survival of the ice sheets. |
