Ars Technica - How Does That Work? Rising Sea Levels 

- You've not doubt heard that sea levels are rising due to global warming. Threatening coastal communities around the world. Part of the reason for this is that ice on land is melting and flowing into the seas. And the water already in the oceans is expanding as it heats up. We have equipment called tide gauges. Most of what they measure is just the daily rise and fall of the tides. But if you focus on long term changes, what you'll find is that the ocean levels are indeed rising. 

If you look at enough tide gauges however, you'll find that it's varying. Rising at different rates in different places. Why would that be the case? Some of the factors behind these differences are pretty simple. You may have idea of the water rising everywhere like you were filling a bathtub. But that's actually wrong. 

The land itself can move for example. Plate movements can force bedrock up. While sediments will pack down below our feet over time. Either of these can change the elevation of the land surface. In Alaska for example, sea levels appear to be falling. That's because the land itself is rising, bouncing back up after having been buried in ice during the last ice age. 

In New Orleans, sea levels appear to be rising faster than they are elsewhere. That's because the land itself if falling, being compressed due to a variety of human activities. There are also places where the land is relatively stable. Wilmington, North Carolina has seen the ocean levels rise very slowly at 2.3 millimeters a year. That's about an inch a decade. While that may not sound much, we've been tracking this for over 80 years and that rate's held for the whole time. 

As a result, the floods caused by Hurricane Florence were over eight inches higher than they would've been otherwise. It's not just changes in the land that make all the difference. Ocean currents can too. Think about how blowing against your tea causes water to pile up against the opposite side of the cup. In a similar way where ocean currents run into land, they cause water to pile up against the shore. The seesaw between El Niño and La Niña conditions in the Pacific causes a sloshing of the pool of warm water across the Pacific. That's responsible for a great deal of the year to year variation between ocean levels on the West Coast. 

And here is a weird factor. Large masses of land or ice exert a gravitational pull on the seawater around them. So wrap your mind around this. If you melted all the ice around Greenland, sea levels would rise around the globe. But the sea level near Greenland would actually fall as the gravitational pull of the ice there went away. As ice melts off Antarctica and Greenland, it releases its gravitational hold on the waters nearby causing ocean levels to rise even faster everywhere else. Because North America is closer to Greenland for example, the loss of the ice there would release the gravitational pull on waters nearby. 

As a result ocean levels would rise faster in South America if the Greenland ice was lost. But if you melted the West Antarctic ice sheet, the situation would be reversed. Antarctica is closer to South America so you'd see ocean levels rise faster in North America. So while it's possible to project what the average ocean level will do as things melt in the future, it's much more difficult to project what will happen in any individual city. 

It's not enough to know how much more water is present in the oceans. You also have to know whether the land under a city is moving, whether ocean currents are changing, and whether any large bodies of ice nearby have been lost. Knowing all these factors helps us more accurately plan for the future. But they also allow us to get a better understanding of what's happening on the planet right now. 

Sea level rise data coastal sites from around the world help us better understand where the ice is melting and how much. All of this work helps confirm that we do indeed know why all the ice is melting. It's because human activities are changing the planet's temperature.