Even if a radical reduction in greenhouse gas emissions could be rapidly achieved, temperatures would continue to rise due to green house gases that have already been emitted and the thermal inertia in the climate system. How much temperatures rise depends on what mitigation strategies are developed and when they are begun. In the meantime, people, communities, and nations can respond to environ mental changes after they happen, or they can anticipate and prepare for the changes.

Communities and nations differ greatly in the resources they have to protect their people from the impacts of climate change. Poor com munities already rarely have enough resources to deal with immediate problems, such as poverty. Poor people lack the access to financial and natural resources and social services and, as a result, are often unable to rebuild their lives after a disaster. For adaptation to climate change to work, wealthier communities will have to assist poorer communities in developing their economies while reducing their greenhouse gas emissions and learning to use alternative technologies.

Adaptation before the predicted changes occur has a large cost benefit. Preparing for a disaster is less expensive and less disrup tive to people's lives than mopping up after one. Hurricane Katrina is a tragic example of how planning could have decreased economic costs, the number of lives lost, and the number of those whose lives were disrupted. For decades, climatologists and coastal scientists had warned that a very powerful hurricane could break the levees that protected New Orleans from surrounding water. The levees were designed only to withstand a Category 3 storm. (In the meantime, for a variety of reasons, the city had sunk to 20 feet [6 m] below sea level in some areas.) Despite these warnings, the recommended improvements to the levees were never made. Hurricane Katrina reached Category 5 as it traveled through the Gulf of Mexico but had dwindled to a Category 3 at landfall and was only a Category 1 or 2 as it passed over New Orleans. Although initially people thought that the city had been spared, the storm's slow passage over the region was enough to break the levees. The resulting flood left 1,800 people dead while displacing about one million others from their homes. The economic impact is estimated at as much as $150 billion. By upgrading the levees so that they could have protected against a Category 5 hurricane, New Orleans would have been ready for this inevitable storm and the storm surge that accompanied it. This preparation would have been expensive, but compared to the cost of the damage caused when the levees broke, the cost would have been minor.

Other regions can adapt to climate change by recognizing and preparing for their own potential problems. Healthy ecosystems can protect coastal regions, and the original wetlands that once thrived on the Louisiana coast might have spared New Orleans some of the brunt of the hurricane. Hard structures, such as seawalls, are better used sparingly; but soft protection, such as beach nourishment, is wise, although it is very expensive. Increasing the capacity of rainwater storage systems may reduce the number of times a city floods and can be used to save water for drier times. It is necessary for evacuation plans for residents of storm - prone areas to be well thought out, easy to implement, and understandable by all who need them.

Although this is unlikely to happen until the effects of global warming are even more pronounced, coastal scientists recommend that communities retreat from the shoreline, and that new building takes place farther inland. Insurance companies can help to reduce coastal development by increasing rates for those who live in dangerous areas, as they are beginning to do in the hurricane vulnerable regions of Florida. Federal insurance, which has allowed coastal development to thrive, can also be eliminated (with some compensation for those who own vulnerable property).

London is the first major world city to recognize the need for a detailed climate change adaptation plan. This old but vital city is mostly built on the floodplain of the River Thames, which is a tidal river. Adaptation to higher water levels began in the early 1970s when a movable barrier was built along the Thames to stop flooding from storm surges. In the early years, the barrier was closed no more than two times per year. In most of the years since 1986, the barriers have been closed between 3 and 19 times. The barrier was designed to mitigate sea level rise until 2030. The city is working on plans for what will come next for flood protection and also on plans for other impacts of climate change, including positive ones. For example, planners anticipate an increase in tourism and recreational activities as weather becomes more favorable in the United Kingdom and less attractive in the Mediterranean region. Some small communities are facing inevi table climate change with similar foresight.

To adapt agricultural systems to a warmer world, agriculturalists may need to develop crop strains that require less water and less soil moisture. Farms may need to be moved to more climatically hospitable areas. Changing the timing of farming events, such as planting crops earlier, will need to continue. In southern Africa, where droughts have become longer, farmers are making changes such as seeking out crops that are better adapted to the current climate, planting trees to protect the soil, and diversifying their livelihoods.

Adaptation will be an effective response to warming only to a cer tain extent. If no changes in emissions are made, at some point the environmental changes will likely become too overwhelming, and the costs too great. One example is what could happen to South Florida, where a small increase in sea level would flood some of the lowest lying areas and make the coast more vulnerable to damage from hur ricanes and other storms. While people may be able to prepare for these changes or at least mop up after large climatic events, this is very draining when it occurs on the scale of a major city, as has been seen in New Orleans. As sea level rises even higher, the entire southern portion of the state of Florida could flood. If this scenario comes true, at some point Floridians will need to give up trying to patch up the damage and relocate. The economic and social costs of doing this for an entire region are unfathomable.

Flood barrier gates on the Thames River are closed when there is a high risk of tidal flooding. Closing the barrier seals off part of the upper Thames with a continuous steel wall. More engineering projects like the Thames barriers will be needed to protect low-lying cities from sea level rise. (Skyscan /Photo Researchers, Inc.)

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