With a significant portion of the world's population living within close proximity to the oceans, often in large cities, rising sea levels bring the potential for devastating consequences.

But scientists are still unable to make predictions precise enough for people to plan how to handle the loss of land and threat to coastal communities expected over this century, two researchers point out in a commentary this week in the May 4 issue of the journal Science.

"We know sea level is going to rise, but how much, and how fast, and where, we really still don't know," co-author Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory, told LiveScience.

The complex seas

It turns out the ocean isn't like water in a bathtub; it doesn't rise uniformly as more water pours in. As global warming raises sea levels, some places are expected to see higher-than-average increases, and a few places may even see decreases.

Currently, projections suggest that over the course of this century, sea levels will rise between 8 inches and 6.6 feet (20 centimeters and 2 meters) around the planet. Scientists know this increase will be driven by the expansion of water as it warms (warmer water takes up more space) and the melting of ice, most importantly, ice stored in the massive ice sheets that cover Greenland and Antarctica. [Stunning Photos of Antarctic Ice]

But the effects of warming water and melting ice on sea-level rise are expected to vary depending on location. What's more, some of the dynamics involved aren't well represented in models used to make projections for the future, write Willis and his co-author, John Church of the Centre for Australian Weather and Climate Research in their essay.

The melting of the Antarctic and Greeland ice sheets presents the largest uncertainty for the future, but air, land and water also play roles in changes to sea level, they write. For instance, sea level (which is measured relative to land) in the vicinity of the ice melt actually decreases, because the ground underneath the melting ice rebounds as the heavy ice disappears.

Planet-scale effects also influence regional changes. Mass that starts out locked up as ice in the high latitudes spreads around the planet once it has melted and flowed into the ocean. This re-arrangement of mass can tweak the tilt of the Earth's axis. In turn, a slight change in the tilt of axis also redistributes the oceans because the forces of the Earth's rotation help shape the surface of the ocean, Willis said. [50 Amazing Facts About Earth]

Likewise, thermal expansion doesn't play out uniformly across the oceans. For example, during an El Niño event, which is associated with warmer waters in the equatorial Pacific, the arrival of warmer waters off the California coast lifts sea levels. During La Niña, when waters are cooler, sea levels tend to subside, Willis said.

Climate change is expected to change ocean currents and the winds that help drive ocean currents. These changes will affect the distribution of heat within the oceans, and, as a result, affect changes in sea level.

Future sea-level rise

Scientists use two types of models to make projections about the future of sea levels, but the two don't agree, Willis and Church point out. Earth system models are simulations that include the atmosphere, ocean and ice, but while these models include decent representations of the ocean and the atmosphere, the behavior of ice sheets is not as well represented, he said.

These models make predictions on the lower end of the spectrum for 2100 sea levels. Meanwhile, the other class of models, called semi-empirical models, base projections on the relationship between warming and the rate of global sea-level rise. These semi-empirical models rely heavily on historical observation, and tend to give higher estimates of future sea-level rise.

The simplest projection, based on the observed rate of sea-level increase, is a continued 0.1 inches (3 millimeters) rise in sea level per year. But it's clear that much greater rates are possible.

"We know from geologic records that ice sheets are capable of causing very rapid sea-level rise three to four times what we see today," Willis said.

If scientists can't accurately project sea-level increase for the coming decades, the least we can do is measure what is happening today, Willis and Church say. Scientists' ability to do this, however, is threatened by delays in the launch of a new satellite, Jason-3, Willis said. The current satellite responsible for measuring ocean height, Jason-2, is reaching the end of its planned operation life.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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