Rising seas have swamped five small Pacific islands and six more have eroded significantly according to a new study in Environmental Research Letters. Researchers looked at 33 low-lying coral islands in the Solomon Islands, an area that has been hit particularly hard by rising seas, and found that two villages would soon have to be relocated.

Sea level rise may seem to be the most predictable impact of global climate change. If the ocean is coming up at a rate of an inch a year, say, and a particular island is no higher than three feet above sea level, it would seem logical to conclude that 36 years from now the whole thing will be underwater.

Except it’s way, way more complicated than that. One recent study found that some coral atolls can grow even as sea levels rise. What this means is that every island community will have to react in its own unique way to changing environmental conditions. And while there are rays of hope, the news is mostly pretty awful.

Global Versus Relative Sea Level Rise

On the whole, the world’s oceans are definitely rising as the planet gets warmer. Three factors contribute to this — melting sea ice, glacier calving, and warmer ocean temperatures (water expands to take up more room as it warms). But the sea level relative to a particular stretch of coast? That could be going up or down. The relative position of a beach changes based on the movement of the tectonic plate that supports it. A piece of coast can experience sudden subsidence (sinking) or uplift in the event of an earthquake, or go through long periods of more gradual change.

Coral reef islands, including atolls like Tuvalu, are particularly vulnerable to rising seas.

The authors of the new study picked the Solomon Islands in part because the relative sea level rise in that region is higher than the global average. Over the last two decades, the ocean has risen about a third of an inch on average, compared with a worldwide average of about a tenth of an inch. For this reason, the authors suggest that the Solomon Islands could provide an interesting test case for what might happen in other places in the future, since sea level rise is expected to speed up in coming decades.

Waves, Storms, and Coastal Erosion

The most immediate threat to islands often isn’t sea level rise, but erosion. Erosion is driven by wave action, and by big storm events in particular. The researchers describe the village of Nuatambu, where half the homes have been washed into the ocean by wave action that has gradually but steadily caused massive shoreline erosion.

In the bigger picture, how well a particular island fares in the face of climate change depends not only on sea level rise, but on waves, storms, and currents. All of these factors are themselves susceptible to change in a warming world.

Islands That Grow and Change

Waves and storms that take away can give, too. As waves crash over the face of an island, they can deposit sand and rocks on the backside, actually raising the profile of the island overall. It is action that allows for some islands to elevate and grow even in the face of rising seas.

The remote island of Tikopia, of the Solomon Islands, after a tropical storm in 2002.

Of course, for this new material to stick around, it has to land on some sort of foundation. And the corals that form the base of many of these low-lying islands are under threat from climate change too, as they bleach out and die from too-warm temperatures and increasingly acidic water.

Existence Doesn’t Guarantee Survival

Let’s say you live on an island where the rising sea level is offset by the deposit of new material on land. Does that mean you’re safe? Not necessarily. Climate change poses threats to residents of low-lying islands even if those islands go on existing forever. The rising seas and growing storms may not wipe out an island completely, but they will certainly change it in significant ways.

Sand-laden salt water will smother existing vegetation, possibly resulting in mass die-offs. If the new landscape isn’t left alone long enough to recover, the land could become a desert, no longer able to offer food and shelter to human and animal inhabitants. Some parts of the island will erode even as others grow. Depending on where you built your house, it may either become washed out to sea or buried in sand. Fresh water sources could be contaminated by salt.

Continued survival depends on changes coming gradually enough that plants, animals, and the islands themselves can adapt to new conditions. For some humans, this may mean relocating communities uphill or to an entirely new location. In the long run, islands will continue to exist and be colonized by some form of life. It’s what’s going to happen over the next century or two that’s particularly hard to predict, and particularly disconcerting.