Researchers have just figured out which cities across the globe face the highest risk from coastal flooding. 
 
 To do so, they compiled data on 136 coastal cities with more than 1 million residents, looking at the elevation of the cities, the population distribution and the types of flood protection they had, such as levees or storm-surge barriers. 
 
 They then combined that data with forecasts of sea level rise, ground sinking due to groundwater depletion, as well as population growth projections and economic forecasts of gross domestic product (GDP). From there, they used the depth of water flooding a city to estimate the cost of the damage. [Read full story on coastal flooding] 
 
 In both their best- and worst-case projections of sea level rise, the yearly global cost reached higher than $1 trillion. The most vulnerable city was Guangzhou, China, followed by Mumbai and Kolkata in India, Guayaquil, Ecuador and Shenzen, China. Almost all cities at the highest risk of flooding damage were in North America or Asia. 
 
 Here are the top 20 most vulnerable cities: 
 
 1. Guangzhou, China 
 
 2. Mumbai, India 
 
 3. Kolkata, India 
 
 4. Guayaquil, Ecuador 
 
 5. Shenzen, China 
 
 6. Miami, Fla. 
 
 7. Tianjin, China 
 
 8. New York, N.Y.—Newark, N.J. 
 
 9. Ho Chi Minh City, Vietnam 
 
 10. New Orleans, La. 
 
 11. Jakarta, Indonesia 
 
 12. Abidjan, Ivory Coast 
 
 13. Chennai, India 
 
 14. Surat, India 
 
 15. Zhanjiang, China 
 
 16. Tampa—St. Petersburg, Fla. 
 
 17. Boston, Mass. 
 
 18. Bangkok, Thailand 
 
 19. Xiamen, China 
 
 20. Nagoya, Japan 
 
 Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. ||||| Coastal flooding in cities around the world could cause damage totaling $1 trillion annually by the year 2050 if no mitigating steps are taken, new research suggests. 
 
 Almost all cities facing the worst damage are in Asia and North America, the study showed. Three American cities — New York, New Orleans and Miami — are at particularly high risk of damage, according to the study, published today (Aug. 18) in the journal Nature Climate Change. 
 
 "If we did nothing about the risk, the flood damages in coastal cities would grow to huge amounts. So that's really not an option," said study co-author Robert Nicholls, a coastal engineering professor at the University of Southampton in England. 
 
 Damaging storms 
 
 Climate change models generally predict that storms will grow more frequent and fierce over the next several decades. We may already be seeing its impact: Many scientists believe that climate change worsened the toll of Hurricane Sandy. [How Weird!: 7 Rare Weather Events] 
 
 After Hurricane Katrina ravaged the Gulf Coast in 2005, Nicholls and his colleagues realized that scientists had little idea which cities around the world were most vulnerable to flooding. 
 
 So the team compiled data on 136 coastal cities with more than 1 million residents, looking at the elevation of the cities, the population distribution and the types of flood protection they had, such as levees or storm-surge barriers. 
 
 They then combined that data with forecasts of sea level rise, ground sinking due to groundwater depletion, as well as population growth projections and economic forecasts of gross domestic product (GDP). From there, they used the depth of water flooding a city to estimate the cost of the damage. 
 
 Two vulnerable continents 
 
 The researchers found that in both their best- and worst-case projections of sea level rise, the yearly global cost reached higher than $1 trillion. The most vulnerable city was Guangzhou, China, followed by Mumbai and Kolkata in India, Guayaquil, Ecuador and Shenzen, China. Almost all cities at the highest risk of flooding damage were in North America or Asia. [See The 20 Cities Most Vulnerable to Flooding] 
 
 If cities take steps to prevent damage — by increasing the height of levees, erecting storm surge barriers, making buildings flood-resistant or converting flood-prone, low-lying areas to parks or football fields — the cost of the damage could be brought down to about $50 billion annually. 
 
 It would be cheaper and more sensible to make these changes before Katrina-like storm surges become the norm, Nicholls said. 
 
 "The bottom line is it shows that flood risk is rising today — it's happening" Nicholls told LiveScience. "All these cities need to be preparing for that." 
 
 Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.
– It's a gloomy day in the world of flood-related studies. New research indicates that by 2050, coastal flooding could cause $1 trillion in damage—per year. That is if no steps to safeguard these areas are taken. LiveScience notes the $1 trillion figure was reached in both the best- and worst-case scenario projections. On the flip side, preventative measures could drop the damage figure to $50 billion a year. In any case, now might be the time to move to Europe: The list of most at-risk cities is filled with mainly Asian and North American locales. LiveScience presents the top 10: Guangzhou, China Mumbai, India Kolkata, India Guayaquil, Ecuador Shenzen, China Miami Tianjin, China New York, NY-Newark, NJ Ho Chi Minh City, Vietnam New Orleans Click for the next 10 cities on the list, which include two more American locations.