Climate: 9 questions on rising seas

Story highlights

  • John Sutter talks with CNN meteorologist Brandon Miller about sea level rise
  • This is part of the CNN series "2 degrees," which focuses on climate change

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(CNN)You've probably heard about some of the impacts of rising seas: Nearly a fifth of Bangladesh, in South Asia, could be submerged; some low-lying islands in the Pacific may disappear; coastal cities like Miami and New York are expected to see more floods.

But exactly how and why are the seas rising?
The underlying answer, of course, is that humans are warming the planet by burning fossil fuels, cutting down forests and engaging in other activities that release heat-trapping gases into the atmosphere. Policy makers have concluded that the warming would be especially dangerous if it crosses a threshold of 2 degrees Celsius higher than Earth's temperature at the start of the industrial revolution. But for details on how climate change is raising the height of the ocean, I turned to CNN International meteorologist and supervising weather producer Brandon Miller.
Below you'll find an edited transcript of our email conversation.
Brandon Miller
I'll be exploring sea-level rise and what happens to people whose homes are flooded this month as part of CNN's "2 degrees" series on climate change. Let me know in the comments at the bottom of this post if there are other sea-level-related questions you'd like to see answered. And consider signing up for the "2 degrees" newsletter if you'd like to follow along.

1. Why are sea levels rising?

Global sea level rise occurs because of two factors. The biggest reason is because of something called "thermal expansion," which is simply that water expands as it warms, just like how the liquid in a thermometer expands as the temperature increases. Therefore, as global temperatures continue to rise, the oceans get warmer and they literally expand, making the level of the sea rise. The other main contributor to sea level rise is the loss of glaciers and the polar ice caps. When the glaciers and ice caps melt, that water flows into the ocean and increases its volume.

2. Is this already happening -- or is it a future thing?

Yes it is already happening, and yes it will continue to happen in the future, with the rate of sea level rise likely increasing as well. Since 1900 we have seen seas rise, on average, about 20 cm (8 inches). While ocean levels varied over the past 2,000 years, they did so much more slowly. It would have taken several hundred years to change the ocean level by the amount we have witnessed over the past century, with a bulk of that increase coming in the last 25 years.
The rate of change in the past 20 years is about double the rate of change over the past 100 years, indicating that the rate of sea level rise is increasing.

3. What will happen to sea levels if the climate warms 2 degrees?

Sea levels will certainly continue to rise if the climate warms 2 degrees Celsius (3.6 Fahrenheit). Sea level rise will not stop, even if the warming ends at 2 degrees. Instead, it will continue for many centuries due to the lag in response times of the world's oceans and massive ice sheets.
If we look at the distant past, 120,000 years ago, when only very early humans existed, the temperature was 2 degrees warmer than it is now. Sea levels were about 5 meters (16 feet) higher than they are today. Nearly 500 million people worldwide currently live below 5 meters elevation.

4. How much are sea levels expected to rise by, say, 2100?

According to the latest projections from the Intergovernmental Panel on Climate Change (IPCC), a 2 degree increase in temperature by the end of this century would result in about a half a meter (1.5 feet) of sea level rise. If we drastically and immediately reduce the amount of greenhouse gasses we are currently emitting, the sea level will still continue to rise as the result of the warming we have already seen, but it should be less, likely around 40 cm (15 inches). If we continue increasing greenhouse gas emissions at the current rate, the increase in sea level could be up to double that amount, or 80 cm (31 inches) by 2100.

5. What about in the really, really long term?

One of the big questions is how much warming will it take to melt the Greenland Ice Sheet, which contains enough water to raise sea levels by 7 meters (23 feet). This is an incredibly difficult question to answer, but current research shows it is likely between 1 degree and 4 degrees warming, compared to pre-industrial averages. If 2 degrees is enough to melt the Greenland Ice Sheet, for instance, it could mean sea levels would rise much higher and faster than current projections show. A total melting of Greenland's ice likely would take thousands of years, though.

6. Are lots of the Pacific islands actually going to disappear?

Likely not disappear, at least not in the short term (through the end of this century), but they will be dramatically impacted. Many low-lying regions will see drastic changes to their coastal geography from beach erosion, significant infrastructure challenges from things like saltwater intrusion into the water system, and population displacement from inundation. There are some extreme examples, such as the Marshall Islands, where 99% of the country lies below 5 meters in elevation, which could literally be "wiped off the map," but the problem is far more wide-reaching than a few tiny Pacific islands. Coastal communities around the world will face higher risks for storm-induced tidal surges, frequent inundation from high tides, etc. Many of the world's largest and most influential cities are located on the coast and must make significant preparations to combat rising sea levels.

7. What about Bangladesh? Isn't it very high risk?

Yes, absolutely. Bangladesh is in a very precarious situation in that it is vulnerable to flooding from all sides as a result of climate change. Increasing sea levels will continuously shrink the low-level coastal communities, while the major rivers that flow through Bangladesh (Ganges and Brahmaputra) will face increasing water levels from melting glaciers in the Himalayas. Bangladesh has the eighth largest population of any country -- at 158 million -- and a majority of these people live in the areas where these rising rivers meet a rising sea.

8. What's happening to U.S. cities -- like New York, NOLA and Miami?

The World Bank recently led a study that ranked cities around the globe in terms of their risk of coastal flooding resulting from climate change. When looking at overall costs of damage, five of the top 10 cities were in the United States: Miami (2), New York (3), New Orleans (4), Tampa (7), Boston (8). This is largely due to the massive amounts of infrastructure, economic assets and population that are located directly on the coast and at low elevation.
Based on the 2014 U.S. National Climate Assessment, New York City would see a local sea level rise of nearly 4 feet by 2100. A 4-foot sea level rise for New York would result in nearly 100,000 people being underwater in New York City alone, with a property value of $16.5 billion.

9. How many people will be displaced by rising seas?

In a 2-degree world, hundreds of millions of people will be affected by coastal flooding as a result of climate change. Asia, namely East, Southeast, and South Asia will be particularly affected. Unfortunately, some of these regions which will face the largest and most immediate impacts from rising seas have the fewest resources to combat them. In many cases, the cost of coastal adaptation may be too much, flooding from storms pushing higher water levels into homes will become too frequent, or the intrusion of saltwater into the local water table will make growing crops impossible -- and people in all regions of the world will be forced to leave their homes and cities.
Email questions to: climate@cnn.demo.robotrailer.com.
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