Singular Abrupt Events

With the assumptions of scenario 3, the probability and consequences of abrupt events move beyond the bounds of the assumptions of the IPCC projections. This departure is necessary as the potential consequences of large-scale abrupt events are of particular concern, yet the science for projecting and assessing them remains significantly underdeveloped.47 To assess the consequences of such events, therefore, we draw upon the author's own assessment of a few particularly informative but uncertain studies.

Collapse of the North Atlantic meridional overturning circulation. The Gulf Stream is part of the North Atlantic meridional overturning circulation (MOC; also known as the thermohaline circulation or the Atlantic conveyor belt). The Gulf Stream transports warm tropical surface water from the equatorial North Atlantic Ocean northward along the East Coast of North America and then eastward toward northern Europe. From here the water flows north toward southern Greenland and the North Sea. Throughout this journey, the surface water cools and consequently becomes denser, eventually causing it to sink in the far North Atlantic near Greenland and flow south ward at depth; this sinking drives the overturning circulation and sustains the continued transport of heat from the equator northward. This ocean transport of heat may warm the climate of northwestern Europe by several degrees. Global warming is thought to present a risk of shutting down the MOC by warming and freshening northern North Atlantic surface water as a result of Arctic ice melt, increased Arctic river runoff, and increased precipitation over the North Atlantic. Because warm water is less dense than cold water and freshwater is less dense than saltwater, global warming could slow or stop the MOC by reducing the tendency for water to sink in the far North Atlantic.48

Collapse of the MOC has often been described as a low-probability, high-impact event. In fact, however, there is tremendous variation among models and in experts' judgments regarding the probability of such an event.49 There is even some disagreement about whether the MOC actually warms Europe significantly.50 Moreover, there has been little investigation of the potential consequences of such an event, and it remains unclear whether it would indeed be of great consequence.51 It is therefore all the more important not to regard the scenario outlined here as a prediction. Our purpose is to explore the possibility that collapse of the MOC could have a large impact, as such an outcome is widely considered plausible, if improbable.52

According to the IPCC, models that accurately represent past and current climate project a slowing of the Atlantic MOC of up to 60 percent, but none indicates a complete shutdown during the twenty-first century. As a result, the IPCC places the likelihood of a shutdown of the MOC during the twenty-first century at not more than 10 percent.53 In the IPCC models, slowing of the MOC of up to 60 percent does not produce a cooling of Europe, as the warming effect of increasing atmospheric greenhouse gases offsets the cooling effect of the slower MOC. If, however, the rate of warming and loss of polar ice has been underestimated by the models, as assumed in scenario 3, then the chance of a collapse of the MOC during this century could be considerably higher. Should an abrupt collapse occur, a cooling of the North Atlantic region, including northwestern Europe, is more likely.54 We therefore consider the potential consequences of North Atlantic MOC collapse in scenario 3.

As it is not possible to estimate the timing of MOC collapse for a given degree of warming, we arbitrarily assume a collapse during the 2050s, with attendant impacts occurring in subsequent decades of the twenty-first century (and beyond). This approach mirrors that of Nigel W. Arnell, who sim-

Figure 3-3. Model-Projected Changes in Annual Water Runoff for 2050, Percentage Change Relative to the Average over 1900-70

Figure 3-3. Model-Projected Changes in Annual Water Runoff for 2050, Percentage Change Relative to the Average over 1900-70

Source: Updated from P. C. D. Milly, K. A. Dunne, and A. V. Vecchia, "Global Pattern of Trends in Streamflow and Water Availability in a Changing Climate," Nature 438 (2005): 347-50.

a. Any color indicates that more than 66 percent of models agree on sign of change.

Figure 3-4. Distribution of Current Global Population Density

Figure 3-4. Distribution of Current Global Population Density

Source: Gridded Population of the World, version 3 (New York: Columbia University, Earth Institute, Center for International Earth Science Information Network (CI ES IN)) (http://sedac.ciesin.columbia.edu/ gpw/).

Figure 3-5. Index of Regional Sensitivity to Projected Changes in Temperature and Precipitation

Figure 3-5. Index of Regional Sensitivity to Projected Changes in Temperature and Precipitation

180°W 150°W 120°W 90 °W 60°W 30°W 0° 30"E 60°E 90'E 120°E 15CTE 180°E

Source: The aggregated CCI in figure 2 of M. B. Baettig, M. Wild, and D. M. Imboden, "Climate Change Index: Where Climate Change May Be Most Prominent in the 21st Century," Geophysical Research Letters 34 (2007):L01705, doi:10.1029/2006GL028159.

Figure 3-6. San Francisco Bay and the Sacramento-San Joaquin River Delta, Inundated by One Meter of Future Sea Level Rise Shown in Red

Figure 3-6. San Francisco Bay and the Sacramento-San Joaquin River Delta, Inundated by One Meter of Future Sea Level Rise Shown in Red

Source for figures 3-6, 3-7, 3-8, 3-9: J. L. Weiss and J. T. Overpeck, Climate Change and Sea Level: Maps of Susceptible Areas. Department of Geosciences Environmental Studies Research Laboratory, University of Arizona, 1997 (www.geo.arizona.edu/dgesl/research/other/climate_change_and_sea_level/sea_level_rise/sea_ level_rise.htm).

Figure 3-7. Middle Atlantic Coast of the United States, Inundated by One Meter of Future Sea Level Rise Shown in Red

Figure 3-7. Middle Atlantic Coast of the United States, Inundated by One Meter of Future Sea Level Rise Shown in Red

Figure 3-8. Southeastern Coastline of Australia,

Inundated by One Meter of Future Sea Level Rise Shown in Red

Figure 3-8. Southeastern Coastline of Australia,

Inundated by One Meter of Future Sea Level Rise Shown in Red

Figure 3-9. Mekong Delta, Vietnam, Inundated by One Meter (Bright Red) and Two Meters (Dark Red) of Future Sea Level Rise
Figure 3-10. Mekong Delta, Vietnam, with Population Density in the Low-Elevation Coastal Zone (LECZ)a
femn ikt lq km -23 an UJi 20.U BIOH HB

Z 1

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1 i

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Source for figure 3-10: G. McGranahan, D. Balk, and B. Anderson, "The Rising Tide: Assessing the Risks of Climate Change and Human Settlements in Low Elevation Coastal Zones," Environment & Urbanization 19 (1997): 17-37 (adapted and used with permission based on Creative Commons 2.5 attribution. License: http:// creativecommons.org/licenses/by/2.5).

a. The LECZ indicates an area below 10 meters elevation and shown in shades of red, with the population density outside the LECZ shown in shades of green.

ulated a shutdown of the North Atlantic MOC in a global circulation model in the year 2055 and followed its subsequent effects on water resources, energy use, human health, agriculture, and settlement and infrastructure.55 Because there are few studies of this nature, we base the effects of an MOC collapse in scenario 3 on the results of that study. Arnell forced a global climate model with SRES scenario A2 and separately forced a shutdown of the MOC by imposing an artificial freshwater pulse in the North Atlantic.56 Temperature change from the A2 scenario is similar to that of the A1B scenario until late in the twenty-first century, when A2 produces more warming than A1B. The impact of shutting down the MOC was compared to impacts of the A2 scenario without the freshwater pulse to shut down the MOC. It is important to understand that the MOC would not have shut down in the model if not for this artificially imposed freshwater pulse, an experimental manipulation applied solely to assess the potential impacts of an MOC collapse. If, however, the model underestimates the loss of polar ice as assumed in scenario 3, then it is reasonable to compensate for the underestimation by imposing additional freshwater input.

In general, MOC collapse resulted in cooler temperatures around the northern North Atlantic, with the largest effect centered south of Greenland and decreasing with distance from this central area. Areas of northwestern Europe cooled by as much as 3°C (5.4°F), with broader areas of Europe and northeastern North America cooling by 1 to 2°C (1.8 to 3.6°F). Many other parts of the world warmed because of a redistribution of heat from changes in ocean currents. Precipitation changes were more widespread than cooling, with attendant changes in runoff, drought, and flooding. The largest decreases in precipitation occurred in North Africa, the Middle East, Central America, the Caribbean, and northeast South America, including Amazonia. Intermediate decreases in precipitation were more widespread, including central North America, southern Greenland, central and southern Europe, central and southeast South America, Central and South Asia, western and southern Africa, and Australia. The largest increase in precipitation was centered on the southwestern United States, providing a net reduction in the number of people in the country under water stress. Increased precipitation also occurred in the eastern United States, Canada, East Africa, and northern, eastern, and Southeast Asia.

Several of the world's major grain-exporting regions, particularly in North America and South Asia, were affected by increased drought as a result of reduced precipitation after MOC collapse. In Europe, this trend would be exacerbated by lower temperatures and shorter growing seasons. Hence, global food markets would likely be affected by short supply and high prices. In Europe and northeastern North America, demand for heating fuel would increase because of colder winters. Although demand for cooling fuel would decrease in these regions, most other regions of the world would experience increased demand for cooling fuel. The cost of maintaining and adapting transportation infrastructure and demand for heating fuel would increase in northern Europe and northeastern North America, resulting in a southward shift of economic activity and population.57

Another consequence of a complete MOC collapse is likely to be an increase in sea level in the North Atlantic region, in addition to global mean sea level rise.58 Model results and expert opinion suggest that this effect could add up to 1 meter (3.3 feet) of sea level rise in the Atlantic north of 45 degrees N, bringing total sea level rise for this region to 3 meters (10 feet) in our catastrophic scenario 3, with attendant coastal impacts (see discussion on abrupt sea level rise below).59

In general, the effects of accelerated global warming without MOC collapse are larger than the effects of MOC collapse. Broadly, however, accelerated climate change is expected to intensify current precipitation patterns, offering some degree of predictability and maintaining current geographic patterns of large-scale food production. By reorganizing precipitation patterns, MOC collapse may threaten major crop-producing regions with decreased precipitation, raising the possibility of major disruptions in global food supply.60 It also appears to amplify the decrease of precipitation in Central America and Amazonia, threatening tropical forests and their dependent species with extinction and adding additional carbon to the atmosphere through large-scale forest dieback, amplifying the global greenhouse warming trend. Although water stress increases in parts of Africa and Asia, increased precipitation in East Africa and East and Southeast Asia results in a net of 1 billion fewer people under water stress with MOC collapse, but adds to flood hazards in these regions.

Abrupt sea level rise. The AR4 projects sea level rise in the range of 0.18 to 0.59 meter (0.6 to 1.9 feet) by the end of the century. As discussed earlier, however, this projection excludes an estimate of accelerated ice loss from the Greenland and Antarctic ice sheets (ice loss raises sea level even if the ice has not yet melted) and therefore cannot be considered either a best estimate or an upper bound for future sea level rise.61 Moreover, the IPCC projections depict a gradual change in sea level over the next century, whereas abrupt and intermittent rises may be more likely, particularly for individual regions (see box 3-1). In the climate impacts scenarios outlined here, we assume that sea level rises 0.23 meter (0.75 foot) (scenario 1) or 0.52 meter (1.7 feet) (scenario 2) relative to 1990 by 2040, or 2 meters (6.6 feet) (scenario 3) relative to 1990 by 2100 (see table 3-1). As noted, under scenario 3, additional sea level rise of up to 1 meter (3.3 feet) would occur in the northern North Atlantic as a consequence of North Atlantic MOC collapse.62 The possibility that extreme sea level rise could occur abruptly and unpredictably in this region (or others) should be considered in risk assessments.

Although it is safe to assume that greater sea level rise leads to relatively more severe impacts, studies of potential sea level rise impacts have not been conducted for most parts of the globe, and those that have been typically examine only one aspect of sea level impacts, such as beach erosion or storm surge height.63 Sea level rise varies regionally, but future regional patterns are unpredictable at present.64 Moreover, a lack of highly resolved global demographic data for coastal areas has hampered systematic assessment of coastal hazards.65 Recent geographic population estimates indicate that about one-tenth of the world's population lives in coastal regions within 10 meters (33 feet) of sea level, and the global population continues to migrate coast-ward.66 This estimate offers a general sense of the relative susceptibility of different regions to sea level rise impacts but cannot tell us how many people are likely to be directly impacted by sea level rise of the magnitude assumed in our scenarios (0.23 to 2.0 meters). In summary, it is currently extremely difficult to quantify future damage to humanity from sea level rise, although damage from a rise of 2 meters (6.6 feet) during the current century would clearly be catastrophic for many regions, including key areas within the United States.67

Sea level rise causes or contributes to several distinct types of impacts, including inundation, increased flooding from coastal storms, coastal erosion, saltwater intrusion into coastal water supplies, rising water tables, and coastal and upstream wetland loss with attendant impacts on fisheries and other ecosystem services.68 Current distribution of natural and human coastal systems has been adapted to past extreme high tides and storm surges. Future sea level rise will inundate additional land not so adapted. Only the lowest-lying, unprotected areas will be extremely vulnerable to inundation within the time frame of our thirty-year scenarios. There are dozens of coastal cities worldwide in both industrialized and developing nations that lie at least partly below 1 to 2 meters (3.3 to 6.6 feet) elevation, but most of them have flood protection. Hence, inundation from extreme high tides alone might not rise to crisis proportions for most of these cities within the com ing century, although enhanced defenses will be required to avoid increasing damages.

Inundation is a serious issue nonetheless for unprotected low-lying areas, including coastal wetlands that serve as natural nurseries for important fisheries, and productive agricultural lands situated on river deltas, a particularly sensitive problem for coastal aquifers and Asian mega-deltas.69 Because of their inherently low elevations, proximity to the open sea, and general lack of flood protection, coastal wetlands are probably the most vulnerable of all natural systems to inundation and are also of underappreciated importance to society.70 For example, about 75 percent of the commercial fish catch and 90 percent of recreational fish catch in the United States depends on wetlands that serve as nurseries and feeding grounds for fish and shellfish. Habitat loss and modification are the dominant causes of the worldwide decline in ocean fish catch during the past two decades.71 One meter of sea level rise could eliminate or damage half of coastal wetlands globally, with the most vulnerable wetlands located along the Mediterranean and Baltic coasts and the Atlantic coasts of Central and North America, including the Gulf of Mex-ico.72 Chronic saltwater inundation would devastate agricultural production as well, and the situation is similar for coastal groundwater supplies, which cannot be protected by levees or other surface-level devices.

Beyond the twenty-first century, sea level rise could far exceed 2 meters (6.6 feet), such that inundation eventually redraws coastlines altogether.73 For the near term, however, more frequent and more severe flooding from coastal storms is likely to be the largest impact of sea level rise along low-lying coastlines.74 Existing flood protection systems built to withstand extreme storm surges will be overcome much more frequently as local sea levels rise.75 For example, levees around New Orleans were designed to withstand storm surges associated with category 3 hurricanes,76 which historically attained heights of 2.8 to 3.7 meters (9.1 to 12.1 feet). Such defenses would be reduced effectively to category 2-level protection with 1 meter (3.3 feet) of sea level rise and category 1-level protection with 2 meters (6.6 feet) of sea level rise. Because weaker storms occur more frequently than the most intense storms, sea level rise portends a nonlinear increase in flood risk for protected areas in the absence of defense enhancement.77 As another example, current flood defenses in New York City were designed to protect against a hundred-year flood—that is, the highest floodwaters expected to occur in a hundred-year period, on the basis of average past climate. However, 1 meter (3.3 feet) of sea level rise would lower the return interval of such a flood to as little as five years.78 This estimate does not account for storm intensification, which would raise maximum storm surge and wave heights even further and is a predicted consequence of global warming.79 The most critical areas of low-lying coastlines are cities and farmed deltas. Dozens of the world's most populous and culturally and economically important cities—New York, Miami, London, Copenhagen, Dublin, Sydney, Auckland, Shanghai, Bangkok, Calcutta, Dhaka, Alexandria, Casablanca, Lagos, Dakar, Dar es Salaam—are susceptible to sea level rise, as are some of the most important agricultural sites, such as the Sacramento, Ganges, Mekong, Yangtze, and Nile deltas.

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