About 634 million people—10 percent of the global population—live in coastal areas that are within just 33 feet (10 m) above sea level, where they are most vulnerable to sea-level rise and severe storms associated with climate change. Three-quarters of these are in the low elevation coastal zones in Asian nations on densely populated river deltas, such as India. The others most at risk are the small island nations. According to the Earth Institute at Columbia University, the 10 countries with the largest number of people living within 33 feet (10 m) of the average sea level in descending order are as follows:
China Indonesia Thailand
India Japan Philippines
Vietnam United States
The 10 countries with the largest share of their population living within 33 feet (10 m) of the average sea level are as follows:
Netherlands (74%) Djibouti (41%)
Over the last century, global sea level has risen on average of 4-10 inches (10-25 cm). Some estimates predict the rate of sea-level rise could double by the end of this century. If global warming continues unchecked, major urban areas built near sea level will see significant impacts. As an illustration, the area along the Eastern Seaboard of the United States, including New York City; Boston; Washington, D.C.; and Miami, will be at risk with an expected sea-level rise of 18-20 inches (46-51 cm) above current levels by 2100. Low-lying infrastructure in these areas, including buildings, roads, power lines, airports, train and subway systems, are all at risk of increased flooding. Popular recreational areas, such as the beaches of New Jersey, Long Island, North Carolina, and Florida's Gold Coast, will be at risk of accelerated beach erosion and loss. Wetland areas would also be threatened, endangering important habitat for shorebirds, plants, and nursery areas for fish, as well as valuable ecosystem services. Saltwater intrusion into underground water resources is also a serious issue, threatening water quality for residential and industrial users.
are often very poor and cannot afford luxuries like air-conditioning, refrigeration, or medical care. As a result, when heat waves strike-repeated days of extremely high temperatures—populations are dangerously vulnerable to health hazards.
The second reason for Gaffin's concern is climate change. Acknowledging global warming and the effects it is currently having on the environment, he believes that even if countries begin reducing their greenhouse gas emissions right now, a certain amount of further warming is inevitable because of the CO2 and other greenhouse gases that have already been released into the atmosphere. He expects a warmer climate to make future heat waves more severe. He says, "I think mitigating global warming is important, but even with mitigation, all these people living in cities are going to experience some warming. Right now, we average about 14 days each summer above 90°F (54°C) in New York. In a couple decades, we could be experiencing 30 days or more. So we have two forces—urban heat islands and global warming—that are reinforcing each other and are going to create hot, hot conditions for more than half the world's population. How do we make cities more habitable in the future?" Gaffin bases his beliefs on research conducted by the U.S. Global Change Research Program.
One of the problems with heat retention in cities, besides dark surfaces absorbing heat from car motors, hot air from air conditioners and clothes dryers, machinery, and industrial smokestacks, is that they lack enough vegetation to offset the high temperatures. Vegetation cools its surroundings by evaporating water. According to the Environmental Protection Agency (EPA), it is common for urban areas to be 10°F (6°C) hotter than nearby suburban and rural areas.
In a study Gaffin conducted in New York City—which has created an urban heat island for more than a century—he accessed satellite temperature data, city-wide land cover maps, weather data, and a regional climate model to devise a plan to cool the city. The purpose of the study was to determine how much the city could be cooled by planting trees, replacing dark surfaces with lighter ones, and installing vegetation-covered "green roofs."
Using a model called "MM5" developed by Pennsylvania State University and the National Center for Atmospheric Research, they were able to predict regional atmospheric circulation and weather phenomena and determine that a combination of urban forestry and light roofs could reduce New York City's overall temperature by 1.2°F (0.67°C) throughout the day. At 3:00 p.m., when temperatures are usually at their highest, the temperature could be reduced 1.6°F (0.89°C). Power demands increase sharply as temperatures rise above 60°F (36°C).
Although these temperatures may not seem significant, they can have a significant impact. According to Gaffin, "Power usage is very sensitive to even slight variations in temperature. If you're near the maximum power availability of your grid, which is often the case in these summer heat waves, each degree can make a significant difference in overloading the grid and leading to a blackout. So anything that would cool the city off, even a little, would ease energy demands and reduce blackout risks."
Gaffin found the model helpful, but he still had some practical reservations about it: "A lot of regional climate models were originally designed for natural land surfaces. For cities we've got to take into account the hundreds of thousands of building geometries."
He does believe that planting trees is critical. Not only does each tree cool its immediate area, but it helps shade adjacent buildings. The study estimated that 17 percent of the city's surface could be planted with trees to help cool the environment.
The last major areas of consideration in the city were the impervious surfaces such as roads and adjacent areas that cover about 64 percent of the city.
"The tar beach roof is an oven in the summer, reached 160°F (96°C)," Gaffin explained. "So cooling of this roof is like turning off an open oven."
For roofs, there are two options: a light-reflecting surface or vegetation. Of the two, he decided a vegetated roof was the better choice for a couple of reasons. First, the reflection of light off the light surface may be redirected onto another building with a darker surface and be absorbed and converted to heat. Second, the light-colored surface would still be impervious to rainwater, causing it to run off that surface and enter sewer systems, overloading them and putting raw sewage into waterways and causing health problems.
Instead, Gaffin supports the concept of the vegetation-covered or green-roof system. In the study, a roof planted with Sedum spurium was compared to a standard dark roof.
"Sedum is a desert-adapted plant with shallow root systems. The plants can tolerate long periods of drought. They're lush green, beautiful to look at, and quite pleasant to touch," says Gaffin.
He and his team found that the peak temperatures on the roofs that were planted with Sedum were 54°F (30°C) lower than the temperatures on standard roofs. They also found that the drought-resistant vegetation was able to survive without watering.
Vegetation-covered roofs can also last many decades and actually leak less than standard roofs. They absorb water like a sponge, avoid runoff problems that are typical with standard roofs, and they evaporate water, cooling the air around them. They are beginning to grow in popularity in the United States, although cost is still somewhat prohibitive; but they have been used for centuries in Europe. They are also growing in popularity in Canada and Japan. In Tokyo, the municipal government requires all structures with roof areas greater than or equal to 10,765 square feet (1,000 m2) to cultivate at least 20 percent of that roof area.
As temperatures continue to rise with global warming, however, the green roof's potential to cool cities in the United States is becoming more attractive, especially as death tolls add up from recent heat waves. The Chicago heat wave in 1995 killed more than 700 people, and the European heat wave of 2003 killed nearly 45,000 people.
Currently, planted roofs are now realizing positive benefits. In Chicago, the City Hall building is vegetated. On hot days, temperatures on that building are 25-80°F (14-44°C) cooler than the adjacent building.
According to Brad Bass with Environment Canada, "It's hard to estimate the exact amount, but green roofs can lead to at least a five percent, if not even a 15 percent, reduction in electricity in the summer."
The results of a study conducted by Environment Canada showed that if 6 percent of Toronto's roof area was converted to planted roofs, greenhouse gas emissions in the city would be reduced by 2.4 megatons a year. Most of the rain is absorbed by the plants and soil to later evaporate or transpire back to the atmosphere as water vapor, providing a cooling effect.
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