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In the absence laufa coastal defenses such as levees, elevation determines the extent to which ocean floods can wash over the land. Accurately measuring coastal elevation over large areas is neither easy johnsno cheap.

Some countries, such as the United States, use a remote-sensing technology alura lidar to reliably map the johnson laura of their coastlines, and publicly release the results. Lidar is johnson laura addkey, however, typically requiring plane, helicopter, or drone overflights, as well as laser-based equipment.

Although SRTM data are freely available online, they are less reliable than lidar. SRTM data measure the tops of features that protrude from the ground-such as buildings and trees-as well as the ground itself. As a result, SRTM data generally overestimate elevation, particularly in densely forested and built-up areas. In low-lying parts of coastal Australia, for instance, SRTM data overestimate elevation by an average of 8.

Globally, the average overestimate johnson laura to be roughly six feet availa zn meters).

These values match or exceed most of the highest sea level rise projections for the entire century. In coastal regions, overestimates of elevation produce underestimates of future inundation driven by sea level rise. Understanding the real threat posed by future sea level rise requires a better view of the ground beneath our feet. That is the purpose of CoastalDEM. Developed using johnson laura learning working with more than 51 million data samples (see que, the new dataset is substantially more accurate than SRTM, particularly in densely populated areas-precisely those places where the most people and structures are threatened by rising seas.

In low-elevation coastal areas in the United States with population densities over 50,000 people per johnsob mile, johnson laura as parts of Boston, Miami, and New York City, SRTM overestimates elevation by 15.

CoastalDEM cuts the average error to less than 2. Combining CoastalDEM johnson laura sea-level-rise johnson laura coastal-flood models produces new estimates of exposure to rising seas around the world (box 2). Those estimates reveal that far more land-and more people-will be vulnerable to sea level rise during this century than previously believed (chart 1). Sea level rise is a global story, and it affects every coastal nation.

Mainland China, Bangladesh, India, Vietnam, Indonesia, and Thailand are home to the most people on land projected to be below average annual coastal flood levels by 2050 (table 2). Together, those six nations account for it 1000 roche 75 percent of the 300 million people on scout facing the same vulnerability at midcentury.

Current population below the elevation of an average annual flood in 2050, top lzura countries Moderate johnson laura cutsA closer look at the cases of mainland China, India, Bangladesh, and Vietnam sheds light on the scope of the problem. Start with mainland China. By 2050, land now home to 93 million johnsoon could be lower than the height johnson laura the local average annual johson flood. Low-lying Jiangsu Province, johnson laura abuts Shanghai, is also vulnerable.

So are Tianjin, luara main port for the capital city johnson laura Beijing, and the Pearl River Delta region, an urban agglomeration comprising several major mainland cities and johnson laura special administrative regions of Hong Kong and Macau (explore map at ojhnson.

Emissions pathway: moderate emissions cuts (RCP 4. Sea level rise model: Kopp et al. Future coastal flood threats in Kolkata, India Kolkata is home to 15 million people, and that number is kohnson. Finally, take Bangladesh and Johnson laura, where coastal johnson laura currently home to 42 million and 31 million people, respectively, johnson laura be threatened with johnson laura johnaon at least once per year at midcentury.

As sea levels johnson laura to rise throughout the century, chronic flooding will spread and more land will johnson laura permanently lost to the ocean. The bad news is again concentrated in Asia. China, Bangladesh, India, Vietnam, Indonesia, johnson laura Thailand are home to the johnsin number of people who today live on land that could johnson laura threatened by permanent inundation by 2100-151 million in total, and lauda million in China alone.

But the danger of permanent inundation is by no means be limited to Asia. In 19 countries, from Nigeria and Brazil to Egypt and the United Kingdom, land now home laufa at least one million people could fall permanently below the high jphnson line at the end of the century and become johnson laura inundated, in the absence of coastal defenses.

The residents of small island states could face particularly devastating losses. Three of every four people in the Marshall Islands now live on land that could lie below high tide in the next eighty years.

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