Report: Flooded Future: Global vulnerability to sea level rise worse than previously understood

Flooding in Jakarta, Indonesia, February 2017. Source: World Meteorological Organization / Flickr

New elevation data show that by midcentury frequent coastal flooding will rise higher than areas currently home to hundreds of millions of people

EXECUTIVE SUMMARY

As a result of heat-trapping pollution from human activities, rising sea levels could within three decades push chronic floods higher than land currently home to 300 million people

By 2100, areas now home to 200 million people could fall permanently below the high tide line

The new figures are the result of an improved global elevation dataset produced by Climate Central using machine learning, and revealing that coastal elevations are significantly lower than previously understood across wide areas

The threat is concentrated in coastal Asia and could have profound economic and political consequences within the lifetimes of people alive today

Findings are documented in a new peer-reviewed paper in the journal Nature Communications

Sea level rise is one of the best known of climate change’s many dangers. As humanity pollutes the atmosphere with greenhouse gases, the planet warms. And as it does so, ice sheets and glaciers melt and warming sea water expands, increasing the volume of the world’s oceans. The consequences range from near-term increases in coastal flooding that can damage infrastructure and crops to the permanent displacement of coastal communities.

Over the course of the twenty-first century, global sea levels are projected to rise between about 2 and 7 feet, and possibly more. The key variables will be how much warming pollution humanity dumps into the atmosphere and how quickly the land-based ice sheets in Greenland and especially Antarctica destabilize. Projecting where and when that rise could translate into increased flooding and permanent inundation is profoundly important for coastal planning and for reckoning the costs of humanity’s emissions.

Projecting flood risk involves not only estimating future sea level rise but also comparing it against land elevations. However, sufficiently accurate elevation data are either unavailable or inaccessible to the public, or prohibitively expensive in most of the world outside the United States, Australia, and parts of Europe. This clouds understanding of where and when sea level rise could affect coastal communities in the most vulnerable parts of the world.

A new digital elevation model produced by Climate Central helps fill the gap. That model, CoastalDEM, shows that many of the world’s coastlines are far lower than has been generally known and that sea level rise could affect hundreds of millions of more people in the coming decades than previously understood.

Based on sea level projections for 2050, land currently home to 300 million people will fall below the elevation of an average annual coastal flood. By 2100, land now home to 200 million people could sit permanently below the high tide line.

Adaptive measures such as construction of levees and other defenses or relocation to higher ground could lessen these threats. In fact, based on CoastalDEM, roughly 110 million people currently live on land below high tide line. This population is almost certainly protected to some degree by existing coastal defenses, which may or may not be adequate for future sea levels.

Despite these existing defenses, increasing ocean flooding, permanent submergence, and coastal defense costs are likely to deliver profound humanitarian, economic, and political consequences. This will happen not just in the distant future, but also within the lifetimes of most people alive today.

GROUND TRUTH

Scientists have long worked to project how quickly various amounts of global warming could raise the level of the world’s oceans—a question about which much uncertainty remains, given the challenges of understanding how ice sheets will respond to the extreme pace of warming they are now experiencing. Yet as researchers have labored over sea level rise models, another factor critical to understanding the world’s vulnerability to rising waters has been largely overlooked. That factor is coastal elevation. In the absence of coastal defenses such as levees, elevation determines the extent to which ocean floods can wash over the land.

Box 2. Visualizing CoastalDEM’s improvements In Bangkok, Thailand, CoastalDEM (first image) reveals significant increases in areas below projected average annual flood heights in 2050.

*Maps do not factor in potential coastal defenses, such as seawalls or levees, and are based on elevation, rather than flood models. Emissions pathway: moderate emissions cuts (RCP 4.5) roughly consistent with the Paris climate agreement’s two-degree celsius target. Sea level rise model: Kopp et al. 2014, median climate sensitivity.

Accurately measuring coastal elevation over large areas is neither easy nor cheap. Some countries, such as the United States, use a remote-sensing technology called lidar to reliably map the heights of their coastlines, and publicly release the results. Lidar is relatively expensive, however, typically requiring plane, helicopter, or drone overflights, as well as laser-based equipment. Where lidar data are not available, researchers and analysts rely on one of several global datasets, most typically data sensed from Earth’s orbit through a NASA project known as the Shuttle Radar Topography Mission, or SRTM.

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 8.2 feet (2.5 meters). Globally, the average overestimate appears to be roughly six feet (two 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 machine learning working with more than 51 million data samples (see methodology), 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 square mile, such as parts of Boston, Miami, and New York City, SRTM overestimates elevation by 15.5 feet on average. CoastalDEM cuts the average error to less than 2.5 inches.

Combining CoastalDEM with sea-level-rise and 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). In fact, using CoastalDEM’s improved data on coastal elevation makes a bigger difference in projected exposure to ocean flooding than does switching from a low-end to a high-end sea-level-rise scenario when SRTM data are used.

THREE DECADES FROM TODAY

Sea level rise is a global story, and it affects every coastal nation. But in the coming decades, the greatest effects will be felt in Asia, thanks to the number of people living in the continent’s low-lying coastal areas. 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 roughly 75 percent of the 300 million people on land facing the same vulnerability at midcentury.

Box 3 (below) details caveats and limitations for this report’s findings.

Chart 1. Current population below the elevation of an average annual flood in 2050, top six countries

Moderate emissions cuts

Country SRTM CoastalDEM Change 1. China (mainland) 29 million people 93 million people +67 million people 2. Bangladesh 5 million people 42 million people +37 million people 3. India 5 million people 36 million people +31 million people 4. Vietnam 9 million people 31 million people +22 million people 5. Indonesia 5 million people 23 million people +18 million people 6. Thailand 1 million people 12 million people +11 million people Total, global 79 million people 300 million people +221 million people Moderate emissions cuts (RCP 4.5), Kopp et al. 2014, median climate sensitivity. Population exposure estimates do not factor in potential coastal defenses, such as seawalls or levees.