Black water rising: the growing global threat of rising seas and bigger hurricanes.

New Orleans is a sort of American Venice, built on a watery site ideal for commerce and surrounded by a huge delta assembled over millennia by the Mississippi River. Levees intended to contain the river's floods have coincidentally eliminated the annual silt deposits; as a result, along with the effects of oil and gas extraction, the area is subsiding. In New Orleans the problem has been compounded by groundwater use and the immediate pumping of rainwater after storms. Without rainwater's slow percolation into the soil, aquifers are never recharged, and the land compacts still further, especially under heavy structures.

The coastal parishes of Louisiana are sinking, on average, about 11 millimeters each year. At the same time, global sea level has been slowly rising over the last century, at 1.0-2.5 millimeters per year. The two trends have consigned half a million hectares of South Louisiana to the sea since 1932; the ongoing rate is about one and a half football fields every hour. Much of the lost land was coastal wetlands, which historically protected New Orleans by slowing and soaking up storm surges (the sudden rises in sea level that occur as hurricanes come ashore). The remaining wetlands are increasingly fragile, fragmented by canals built for navigation and access to the area's oil and gas.

All these factors make New Orleans particularly vulnerable to hurricanes, but within a few decades many more low-lying coastal areas around the world may be equally threatened. Global warming threatens to sharply accelerate the rate of global sea-level rise by melting glaciers and ice sheets in Greenland and Antarctica. Rising tropical sea surface temperatures also appear to be causing an increase in the average strength of hurricanes and tropical storms. A study by researchers at the Georgia Institute of Technology and the National Center for Atmospheric Research shows that the number of Category 4 and 5 hurricanes, the strongest storms, which are often accompanied by huge storm surges, has gone up by 80 percent in the last 30 years. Satellite measurements are increasingly revealing melting at both poles. Seismographs and laser and radar studies show the ice moving more quickly to the sea. In that ice are huge stores of fresh water, enough to raise global sea levels by catastrophic amounts if they are released. Driving the melt are warmer oceans and a warmer atmosphere, attributed by most scientists to human-caused emissions of greenhouse gases.

Climate, Oceans, and Ice

Sea level is almost completely determined by global temperatures, which control how much of the Earth's water is frozen on land as snow and ice. It has risen by about 120 meters since the peak of the last ice age 21,000 years ago. During the period of the most rapid melt of the giant ice sheets, about 14,000 years ago, sea level rose approximately 20 meters in only 400 years. After stabilizing 3-4,000 years ago, it began to increase again in the middle of the 19th century, rising since then between 1.0 and 2.5 millimeters per year.

In 2001, the Intergovernmental Panel on Climate Change (IPCC) estimated that sea levels would rise by .09-.88 meter by 2100. This estimate assumed that the slow expansion of warming seawater would be the major force driving the rise. Since 2001, however, evidence of strong warming at both poles has mounted, and a 2004 study argued that melting ice is much more important than thermal expansion, probably accounting for two-thirds to three-quarters of sea level rise in recent decades. The study implied a rise over the last century of two millimeters per year or more. A research report in late 2005 showed a slowing of the deep southerly flow of cold water in the North Atlantic thermohaline circulation, which many scientists expect would accompany a significant increase in fresh meltwater in the Arctic Ocean. Most disturbing, studies in early 2006 indicated that both the Greenland and Antarctic ice sheets may be losing mass.

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Other scientific findings raise the question of how soon elevated greenhouse gas levels will lock in unacceptable levels of future warming, and of sea level rise. Because of human activities such as burning fossil fuels, there is about two-thirds again as much carbon dioxide in the atmosphere now as the average over the last 400,000 years, and perhaps four times as much methane. The last time the atmosphere contained this much carbon dioxide was about 10 million years ago, when Greenland had no significant ice sheets, sea level was several meters higher, and temperatures were several degrees above today's. Today's higher levels mean the Earth is absorbing significantly more energy from the Sun than it is radiating back into space. James Hansen, head of the Goddard Institute at the National Aeronautic and Space Administration (NASA), estimates this imbalance at about 1 watt per square meter (W/m2) of the planet's surface, roughly equivalent to a small Christmas-tree light bulb on every square meter.

Small as this additional amount of solar energy may seem, study of past climate change indicates that the climate should warm between one-half and one degree Celsius for every additional W/[m.sup.2]. In other words, the Earth is likely to warm by 0.5-1.0 degree C with current greenhouse gas levels, and by more as they continue to increase. Climate modeling has regularly shown that global increases in temperature are likely to show up first at the poles, especially the Arctic. However, recent analysis of deep core samples from the Arctic Ocean bed suggests that the additional heat may make it to the poles--melting the ice there--much faster than previously thought. The samples show that, 55 million years ago, the Arctic had a subtropical climate like modern-day Florida's. Computer models did not predict these remarkably high temperatures in simulations of past climates. The same models now may be substantially underestimating future polar warming, and the threat of major sea level rise.

For some time, climate scientists were mystified by the relatively slow warming of the Earth's atmosphere. Climate models said that the planet should have been absorbing much more energy than was showing up in air temperatures. According to Hansen and his colleagues, the missing heat has been "hiding" in the oceans. In 2000, scientists reported that, from the mid-1950s to the mid-1990s, the oceans had warmed at an average rate of 0.3 W/[m.sup.2]. Studies in 2005 concluded that 84 percent of the planetary warming over the previous 40 years had gone into the ocean--and that the warming could only be explained by human modification of the atmosphere.

Since oceans take a long time to warm up and release that energy to the atmosphere, global warming takes a long time to start and a very long time to stop or reverse. Once the heat is in the oceans, it will eventually go somewhere: into the atmosphere, or into melting of floating ice, ice sheets, glaciers, and/or permafrost--and probably into making stronger storms and hurricanes.

The Warming Arctic

According to a major, multi-year study commissioned by the Arctic Council (which represents the eight Arctic countries and the region's indigenous peoples), the Arctic is warming about twice as fast as the rest of the world, with roughly a one-degree Celsius average increase over the last 30 years.

The study found that permafrost is warming and thawing across much of the region, with the southern limit of permafrost projected to move several hundred kilometers northward this century. Arctic precipitation is increasing, mostly as rain. Snow cover has declined by about 10 percent over the past 30 years. Lake and river ice is declining and glaciers are melting and retreating. River discharges to the ocean are rising, and spring peak flows are occurring earlier. Perhaps most remarkable is the shrinkage of the Arctic Ocean ice pack (see images above). Summer sea ice has declined 15-20 percent in the last 30 years and researchers now predict ice-free Arctic Ocean summers within a century.

Sea ice is already floating, so it does not contribute to sea level rise when it melts. The Arctic's potential contribution to sea level rise will come from snow and ice on land, most of which is in Greenland, a large, mountainous island almost completely covered by glaciers. Greenland is significantly colder than much of the Arctic, but if the warming trend takes hold there, the consequences could be enormous. Scientists believe that as little as a three-degree average temperature increase there will be enough to melt the island's glaciers. They contain enough water to raise global sea level by about seven meters.

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Greenland may have warmed enough already for some of its glaciers to melt. During the brief northern summer, surface melting occurs on portions of the island's ice sheet, forming huge, temporary lakes. In 2002, and again in 2005, surface melting was greater than at any time since monitoring began in 1979. Last year Jay Zwally of NASA showed that surface water could reach under glaciers through the formation of "moulins," vertical tunnels in the ice. Liquid water at the glacier base could then, in theory, act as a lubricant, speeding glacial flow. Then, early this year, analysis of satellite-laser observations by a Jet Propulsion Laboratory/University of Kansas team of scientists showed that two major Greenland glaciers have roughly doubled their speed, one over a five-year period, the other over nine. The same scientists believe that snowfall is not making up for the increased ice loss, and have calculated that the island's net loss of ice doubled between 1996 and 2005. That would mean Greenland is contributing 183-265 cubic kilometers of additional water to the oceans every year. (By comparison, Los Angeles uses one cubic kilometer per year.) University of Colorado researchers used satellite measurements of local gravity variations to estimate a smaller loss, about 82 cubic kilometers per year, between 2002 and 2004. A third study, by Jay Zwally and his colleagues, found a slight annual increase in mass from 1992-2002. However, Zwally believes that, based on all the evidence, Greenland now is probably losing mass.

Whatever the current rate of loss, it is clear that the Arctic is still warming, and positive feedback loops make it likely that the current warming will accelerate. Open water reflects far less sunlight than ice or snow, so when sea ice turns to water during the Arctic summer, the amount of energy absorbed goes up by a factor of nine. Similar, though smaller, differences exist between snow and bare land, and between tundra and forested land. Soot from burning of wood and fossil fuels, which can be carried aloft for great distances, also decreases the albedo (reflectivity) of snow and ice. As snow and ice cover decreases and gets dirtier, as the tundra retreats northward, and as the ice-free months lengthen, the entire region absorbs much more solar energy, spurring still more warming.

Antarctica: Colder, Bigger, Also Warming

The Arctic is largely ocean, but Antarctica is a vast, mostly ice-covered continent, and is therefore much colder. It does not undergo the large-scale summer surface melting characteristic of Greenland, and is generally assumed to respond more slowly than its northern counterpart to global climate trends. At the same time, the long-term consequences of substantial Antarctic warming would be much graver, because the region contains vastly more ice. Its most-vulnerable-to-melting area, West Antarctica, holds enough ice for about seven meters of global sea level rise. Much larger and colder East Antarctica has enough ice for about 65 meters of sea-level rise.

Scientists long believed Antarctica to be too cold for the effects of global warming to show up any time soon. But in 1995 Larsen-A, an 11,000-year-old, 2,400-square-kilometer section of a floating ice shelf off the Antarctic Peninsula, collapsed and disintegrated in just a few weeks. Seven years later, the 3,400-square-kilometer Larsen-B shelf collapsed as well (see images above). Subsequent studies have indicated that warmer ocean water--about half a degree above freezing-is mostly responsible, melting the shelves from below.

The collapses did not affect sea level directly, since both shelves were floating before they disintegrated. But such shelves usually buttress adjacent land-based glaciers or sheets of ice grounded on the sea floor, and their removal can accelerate glacial ice discharge. By 2003, studies showed that some of the glaciers behind Larsen-A had sped up dramatically after its collapse and that their fronts had also retreated. Other research showed widespread melting beneath Antarctic glaciers at their "grounding lines"--the point where their ice begins to float rather than resting on the sea bottom.

East Antarctic glaciers have been growing slightly because of increases in annual snowfall, but two studies early this year suggested that West Antarctic losses are outpacing East Antarctic growth. Jay Zwally's group estimated that the continent is thereby losing 19-43 cubic kilometers of water per year; University of Colorado researchers, using gravity data, estimated a loss of 72-232 cubic kilometers per year, mostly from West Antarctica. In March 2006, another study revealed that winter temperatures in the middle troposphere over Antarctica have gone up 0.5-0.7 degrees C per decade over the past 30 years--the largest such warming identified anywhere.

The growing evidence of warming and melting at both poles does not yet indicate catastrophe--but it makes catastrophic developments conceivable. Until now, global climate models have generally assumed that melting of ice in polar regions would take thousands of years. New scientific developments suggest that sea levels will rise more quickly--perhaps much more quickly-than the IPCC predicted five years ago.

Warmer Water, Bigger Storms

The 2005 Atlantic hurricane season was remarkable in every way. It was very long, and included more storms and hurricanes than ever before; three of the hurricanes were among the six strongest ever observed in the region. However, this cannot necessarily be blamed on global warming. Hurricanes are regionally cyclical and there is no global upward trend in hurricane or tropical cyclone numbers.

However, global warming is likely to produce bigger hurricanes on average, and in fact may already have done so. Kerry Emanuel of the Massachusetts Institute of Technology has described such storms as giant heat engines: apply more heat and their theoretical potential strength increases. And indeed a 2004 study comparing the results from many computer models of the global climate concluded that continued global warming will have strengthened hurricanes by half a category, on average, in 80 years. A study published by Emanuel in August 2005 analyzed data on storms and ocean temperatures going back to the 1950s, and found that the total destructive power (a measure integrating storm intensity and duration) of tropical cyclones in the North Atlantic and the western North Pacific has doubled over the past 30 years. Emanuel has since stated that the energy released by the average hurricane increased by around 70 percent over the same period, which corresponds to a 15-percent increase in maximum wind speed and a 60-percent increase in storm lifetime. Another paper from a team led by Peter Webster of the Georgia Institute of Technology found that the number of Category 4 and 5 hurricanes worldwide had nearly doubled since the 1970s. A newer study, also from Georgia Tech, showed the increase in large storms to be tied closely to increases in sea surface temperatures.

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The total economic damage done by a hurricane does not increase in a linear way with wind speed; it increases with the cube of the maximum wind speed. More intense hurricanes thus can wreak drastically more havoc than their weaker counterparts. Stronger hurricanes also carry with them much greater storm surges, which are usually responsible for most hurricane damage and for most of the loss of life.

Coasts at Risk

The looming possibility of both accelerated sea-level rise and more frequent large hurricanes poses an unprecedented threat and multiplies the unpredictability of coastal risks. As of 1990, about 200 million people lived in areas susceptible to coastal flooding and about 10 million people per year, on average, actually experienced such flooding. A 2003 study by British analyst Robert Nicholls estimated that, with no increase in sea level, the population in areas subject to coastal flooding would double by the 2020s. Under Nicholls's highest-rise scenario--a 0.8-meter increase by the 2080s--the number of people annually flooded would grow to 30 million in the 2020s, 176 million in the 2050s, and 353 million in the 2080s. An interactive mapping system developed by researchers at the University of Arizona shows the land that would be flooded in sea-level-rise scenarios ranging from one to six meters (see images, p. 29).

Wealthy cities such as Hamburg, London, Miami, and New York City (and of course New Orleans) would all be seriously threatened by a meter of sea-level rise. Most of the people at risk, however, are in the developing world. Bangladesh, for instance, would lose 17.5 percent of its territory to a one-meter rise in sea level, according to a national study published in 1995. At least 13 million people would be inundated by this rise. Egypt and Vietnam, two other developing countries with large, unprotected river deltas, each have 8 to 10 million people living within one meter of high tide. A 1995 study of China estimated that 72 million people would be threatened with increased coastal flooding by a one-meter sea level rise. In fact, of the 33 largest cities in the world (those projected to have over 8 million people in 2015), 21 are coastal, and 16 of those are in developing countries. Also highly imperiled are the half a million people living in low-lying island nations, whose very existence may cease if sea level rises sharply. Needless to say, natural resources and agriculture are at risk as well. A 1992 study estimated that about 9 percent of world rice production would be threatened by a one-meter rise in sea level.

A one-meter gradual rise in sea level is bad enough, but there is also a plausible catastrophic scenario: the sudden collapse of the West Antarctic Ice Sheet (WAIS), which could bring a five- or six-meter increase in sea level within a few decades, or even more quickly. Such abrupt change would reshape coastlines so radically as to make them unrecognizable. For example, a six-meter rise would inundate roughly a third of the U.S. state of Florida. In the Netherlands, the harbor of Rotterdam--one of the most important in Europe--might well be abandoned with a five-meter rise, and the country's economy would be radically affected. Well over 400 million people around the world would be immediately threatened by the collapse of the WAIS. Such a collapse is unlikely, according to scientists who have studied it, but cannot be ruled out. Scientists did not predict the abrupt collapse of the Larsen-A Ice Shelf--and they have learned much about ice-sheet dynamics in recent years, nearly all of which points to more instability, not less.

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Can the world adapt to rapid sea-level rise? Yes--but there are two likely, and sharply differing, models. One prevails in Hong Kong and the Netherlands, both places that specialize in long-term planning and careful development because they are densely populated and short of undeveloped land. In Hong Kong, recent land reclamation projects have actually been built as much as half a meter higher, with future sea-level rise in mind. The Netherlands, where land reclamation is a centuries-old practice, meticulously plans and constructs coastal defenses to a 10,000-year-storm standard and carefully restricts development.

The other model is New Orleans: a calamity, followed by an uncertain national response that doesn't necessarily take into account, or plan for, underlying trends in sea level and storms. Ignoring the question of whether it is even possible to protect southern Louisiana from the sea in the long run, it is clear that New Orleans will remain highly vulnerable to large hurricanes unless huge investments are made in carefully designed and far more protective storm defenses.

For the developing world, the New Orleans scenario seems far more likely than the well-funded, well-organized Hong Kong/Netherlands model. Most at risk are the poor, who lack the resources to plan for or adapt to rapid change, or to escape the more frequent calamities that will likely be the combined consequence of rising water and bigger storms. If sea level rises rapidly, what will it take for the citizens of Bangladesh to realize that they must pull back from the sea? And where will they go? Neighboring areas, some across national borders, most of them just as poor, will find it equally hard to accept the need to host a huge refugee population. The same question applies to cities across the world, and especially those in tropical areas: Mumbai, Manila, Jakarta, Lagos. The death toll in poor countries from increasing coastal hazards could be staggering, even as wealthy countries build sea walls to protect cities and reluctantly abandon vacation homes on fragile coasts.

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In the end, the most obvious implication of the threats from sea-level rise and stronger hurricanes is that the costs of global warming are real, huge, and unpredictable. By not acting to avoid future climate change, we are making explicit sacrifices of our coastal assets and putting residents of such areas at major long-term risk. Climate change may mean saying goodbye to New Orleans, one of America's--and the world's--greatest cultural treasures. There, as everywhere, the greatest loss will not be the city's buildings--though they are beautiful, unique, and historic--but of a community that will never be replicated elsewhere. The residents of the industrial countries that have pumped most of the climate-altering gases into the air may finally have to come to terms with the fact that other countries will face similar sacrifices, without having reaped the benefits of nearly unlimited fossil-fuel use.

John Young, a former senior researcher at Worldwatch, is an independent writer and consultant on global environmental issues.