No two ways about it, sea level is rising and climate change is the primary culprit. But just how much sea level has risen – and will rise – depends on where you are. Sea level has risen an average of just over seven inches in the past century and is expected to rise at least another 2-3 feet by 2100. But when it comes to sea level rise, New England is above average. Here, the rise has been closer to a foot. And some scientists think the northeast is poised to experience among the most extreme rises in sea level. Understanding why requires digging into the multiple distinct processes, both global and local, that fall under the general heading of sea level rise.
- Rising ocean temperatures - Water expands as it heats up. A pot of water that boils over is a crude demonstration of the concept. And while the ocean certainly hasn’t hit the boiling point, it is getting warmer. The ocean has absorbed the vast majority- 80% or more – of the excess heat trapped by carbon dioxide emissions from human activities, primarily fossil fuel burning. The increase in the average temperature of the ocean has been small in absolute terms (the IPCC’s 2007 assessment estimated it to be 1-2°F), but even small changes can have big impacts when multiplied by 350 million trillion gallons – the approximate volume of the ocean. Scientists estimate that so-called thermal expansion has accounted for around half of the global sea level rise we’ve seen in the past century.
- Melting ice – From the polar ice caps, to Greenland’s massive ice sheets and the myriad smaller glaciers around the world, there’s a lot of water locked up in ice – about 1.7% of all water on Earth. As land-based ice melts, it adds to the volume of the ocean and global sea level rises. Just how quickly land-based ice is melting and how much water it’s adding to the ocean is one of the biggest areas of uncertainty in climate change predictions, but recent research indicates that melting has accelerated in recent decades. As a result, even conservative predictions of sea level have risen such that one meter, or three feet, by 2100 is now considered a reasonable, middle-of-the-road expectation. (Quick note: floating ice is a different story for a different day. Since it’s already in the water, it doesn’t add volume when it melts. In the case of very large floating ice sheets, melting may impact local sea levels but the science here is very young.)
- Changing circulation – We take it for granted that land has hills and valleys. Well, so does the ocean – and I’m talking about the water’s surface, not the seafloor. Winds and ocean currents push water around ocean basins, piling it up in some places and leaving behind low spots. Differences in the temperature and saltiness of different parts of the ocean are major driving forces for ocean currents, ones that are changing as the sea surface warms and the input of cold, fresh glacial meltwater increases in some places. Some of the most dire climate change scenarios involve major rearrangements in ocean circulation patterns, including weakening or even reversal of the Gulf Stream. The implications of such a change for northern Europe, where the climate is made livable by the warm waters the Gulf Stream carries, have received a lot of a attention. But a recent study suggests that changes in the Gulf Stream could also impact sea levels here by allowing water – as much as several inches of it – to pile up along the northeast coast of the U.S..
- Shifting land - Not all sea level rise is driven by climate change; the tectonic plates upon which all land sits are constantly moving. Earthquakes are high-profile movers and shakers, but there are lots of other forces at work. Here, in southern New England, we’re still feeling the effects of the last Ice Age. As the glaciers retreated some 15,000-18,000 years ago, the newly revealed land rebounded – rising up from the release of the heavy load. While much of Canada is still rising, we’ve started to sink again. It’s like being on a see-saw when the big kid hops off the other end. Of course, as land sinks, local sea level relative to land rises. That’s why New England’s sea level has risen so much more than the global average in the past century.
Looking ahead, some degree of sea level rise is inevitable – particularly here in sinking New England. Even if we stopped carbon dioxide emissions cold-turkey today, we’d still be facing decades, possibly centuries, of rising temperatures – and seas – thanks to the greenhouse gases already accumulated in the atmosphere. In the near-term, learning to adapt to rising sea level will be key … but we’ll get to that tomorrow.