Storms of My Grandchildren (39 page)

Remarkably precise measurements of Earth’s gravitational field by the Gravity Recovery and Climate Experiment (GRACE) reveal that the Greenland ice sheet has been losing mass for the past few years at a rate of about 100 cubic kilometers per year. West Antarctica is losing mass at a comparable, although somewhat smaller rate. These precise satellite gravity data go back only to 2002, but other, less accurate data suggest that even as recently as the 1990s these ice sheets were much closer to mass balance, i.e., they were neither gaining nor losing mass at a substantial rate.

Thus it seems that a disintegration of the ice sheets has begun, but so far the effect on sea level is moderate. The rate of sea level rise in the past decade, including the effects of mountain glacier melt and the thermal expansion of warming ocean water, has been 3.4 centimeters per decade, i.e. a rate of 34 centimeters (about 14 inches) per century. This rate of sea level rise will grow as global warming increases. Ice shelves that buttress the West Antarctic ice sheet and some portions of the Greenland ice sheet are melting. As the ice shelves disappear, the rate of discharge of icebergs to the ocean is expected to increase. I have already pointed out another process that may hasten the beginning of rapid ice sheet disintegration: heavier summer rainfall, which may occur over portions of the ice sheets because of the existence of warmer, more moisture-laden air.

Ice sheets eventually begin to disintegrate at rates of several meters of sea level per century, even with the slow pace at which natural climate forcings change. But predicting when ice sheet mass loss will accelerate in the twenty-first century is a notoriously difficult “nonlinear” problem. We could “lock in” disastrous sea level rise very soon, that is, create conditions that guarantee its occurrence, but it is likely to be several decades before a rapid sea level rise begins. On the other hand, we have been surprised by how fast some other climate changes have occurred—such as disappearance of Arctic sea ice, expansion of the area of subtropical climate, and melting of mountain glaciers. If methane hydrates released from the deep oceans and tundra begin to contribute substantially to atmospheric methane, if human-made sulfate aerosols decrease rapidly because we clean up pollution, if solar irradiance bounces back soon from its current low point…such factors may accelerate climate change. For the moment, the best estimate I can make of when large sea level change will begin is during the lifetime of my grandchildren—or perhaps your children.

With the combination of a higher sea level, even of only a meter or so, and increased storm strength, the consequences of future storms will be horrendous to contemplate. The problems will not be restricted to those places commonly subjected to tropical storms. Other storms with comparable power will affect populations that are one or two orders of magnitude greater than the number of people displaced by Hurricane Katrina, which struck New Orleans and the American Gulf Coast in 2005.

Consider a storm such as the 1991 Halloween Nor’easter. It began as a low-pressure area over Indiana, which moved to the east-northeast into the Atlantic off Canada. There, the low deepened and moved to the east-southeast, but, encountering a blocking ridge in the northern Atlantic, it curved to the west, where it met northward-moving Hurricane Grace. The hurricane, swept aloft by the cold front and absorbed into the circulation of the deep cyclone, added energy to the cyclonic storm. The minimum pressure fell to 972 mbar (millibar is an old unit for atmospheric surface pressure still employed in weather forecasting—the global average for atmospheric pressure at sea level is about 1011 mbar), with sustained winds of 75 miles per hour, making this extratropical system a Category 1 hurricane. A Canadian buoy at 42N, 62W, about two hundred miles off the coast of Nova Scotia, reported wave heights as great as 31 meters (101 feet). Fortunately, the strongest forces remained offshore, although the northeastern United States was hit with a storm tide of 4 meters (13 feet) with an added storm surge of 1.5 meters (5 feet).

Now consider the situation when sea level is even 1 to 2 meters higher, storms are stronger, and atmospheric moisture content is greater. More powerful Nor’easters and hurricanes will hit the East Coast cities along with higher sea levels—it is not a question of whether, only a question of when. Social and economic devastation could be unprecedented. It is not necessary to put the entire island of Manhattan under water to make the city dysfunctional and, given prospects for continuing sea level rise, unsuitable for redevelopment.

Other parts of the world are as vulnerable, if not more so. Consider the North Sea flood of 1953, which affected the coastlines of the Netherlands and England, and to a lesser extent Belgium, Denmark, and France. The flood was caused by the combination of a high spring tide and a storm tide due to a severe European windstorm. The combined tidal surge in the North Sea exceeded 5 meters above mean sea level. About 1,400 square kilometers were flooded in the Netherlands and 1,000 square kilometers in the U.K. In response, the Dutch have built an ambitious flood defense system. The British, too, built improved flood defense systems, including the Thames Barrier to secure central London against a future storm.

When sea level rise reaches a level of meters—and note that there is no “if” about this sea level rise, only a “when,” assuming that politicians are allowed to continue their business-as-usual game—these enhanced barriers will eventually prove futile. Indeed, when the barriers are breached, the area and extent of devastation will be unprecedented. Sea level rise will make a mockery of Dutch plans to build floating houses—unless they plan to live on the open sea. The lowlands of northern Europe will no longer be inhabitable.

What about the effect of sea level rise on developing nations? The consequences for a nation such as Bangladesh, with 100 million people living within several meters of sea level, are too overwhelming, so I leave it to your imagination. No doubt you have seen images of the effects of tropical storms on Bangladesh with today’s sea level and today’s storms. You can imagine too the consequences for island nations that are near sea level. We can only hope that those nations responsible for the changing atmosphere and climate will provide immigration rights and property for the people displaced by the resulting chaos.

The timing of the third ratcheting effect of global warming, the melting of methane hydrates, is as unpredictable as the others. Warning signs are beginning to appear already, with bubbling of methane from melting tundra and from the seafloor on continental shelves. So far the amounts of methane released in this way have been small. The methane hydrates of greatest concern are those in sediments on the ocean floor, because of their great volume. Although estimates of the current amount of methane hydrates range widely, the long cooling trend of the past 50 million years surely has resulted in an accumulation exceeding that which drove the sudden 5- to 9-degree-Celsius global warming that occurred during the Paleocene-Eocene thermal maximum (PETM) about 54 million years ago.

Global ocean circulation reorganized during the PETM, with deep water formation occurring in the Pacific Ocean rather than the North Atlantic, where it occurs today. The flooding of the ocean floor with warmer Pacific Ocean water may have been a key factor in the melting of methane hydrates during the PETM. Could a change of ocean circulation happen again in the near future? Global models of today’s climate sometimes have a problem with spurious formation of deep water in the Pacific Ocean, which suggests that it would not take much change in the densities of ocean surface waters to alter the location of deep water formation. The instigation for such a change could be the freshwater additions to both the North Atlantic and Antarctic oceans, after the rate of ice sheet disintegration in both hemispheres has reached high levels. This freshwater, because it is less dense than salty ocean water, would tend to shut off the usual sinking of surface water in both the North Atlantic and circum-Antarctic Oceans, that is, it could stop the formation of both North Atlantic Deep Water and Antarctic Bottom Water.

When deep water formation begins in the Pacific Ocean, the inertia of the climate system, specifically ocean circulation, will be far too great for humans to stop, even if social systems are still in order. Once large sea level rise begins to devastate coastal cities around the world, creating hundreds of millions of refugees, there may be a breakdown of global governance. But regardless of that, if ocean circulation changes, such that warmer Pacific Ocean water begins sinking to the ocean floor and melting methane hydrates, there will be no plausible way for humans to reverse that change of ocean circulation.

While we can’t predict the details of short-term human history, changes will be momentous. China, despite its growing economic power, will have great difficulties as hundreds of millions of Chinese are displaced by rising seas. With the submersion of Florida and coastal cities, the United States may be equally stressed. Other nations will face greater or lesser impacts. Given global interdependencies, there may be a threat of collapse of economic and social systems.

Physical science is easier to foresee. While the timing of the three ratcheting effects is difficult to predict, their effects are not. With methane hydrate emissions added on top of those from conventional and unconventional fossil fuels, the future is clear. Diminishing feedbacks that help to keep the magnitude of natural long-term climate changes within bounds, such as the ability of the long-term carbon cycle to limit atmospheric carbon dioxide, will have no time to counter amplifying feedbacks. The huge planetary energy imbalance caused by the high levels of atmospheric carbon dioxide and methane will take care of any remaining ice in a hurry. The planet will quickly get on the Venus Express.

When the global warming topic emerged publicly in the 1980s, I assumed that policies would begin to move in a direction to protect the public and future generations. Unfolding reality paints a different picture. Politicians pretend understanding, while ignoring discomfiting implications of the science.

Is it really conceivable that the world will allow squeezing of oil from tar sands, from oil shale, from coal—and go after every last drop of oil in the ground? The popularity of the slogan “drill, baby, drill” in the last election campaign in the United States made me shudder, as it must have other scientists who recognize the threat of all-out fossil fuel exploitation.

What will the world be like if we do go down this route? The science tells us exactly what we could expect to happen on Earth if we continue our business-as-usual exploitation of fossil fuels. I’ve referred to it earlier: the Venus Syndrome. But how to portray the horror of that devastation in a way beyond graphs and numbers and phrases we have heard before, like “climate disaster”? Even though science fiction isn’t my area of expertise, I use the following scenario as a clarion call. I must try to make clear the ultimate consequences, if we push the climate system beyond tipping points, beyond the point of no return.