Climate Change Complexity and Sea-Level Rises
“The secret of sea level rise: it will vary greatly by region”
A piece recently published by Michael D. Lemonick, senior writer at Climate Central (a non-profit American climate science organisation – http://www.climatecentral.org), highlights how the consequences of global warming are so complex to predict that even cutting-edge scientists in the field are continuously discovering new factors for consideration. One of the unexpected facts that his piece discusses is that the long-predicted increases in global sea levels due to warming, “will vary widely by region, with prevailing winds, powerful ocean currents, and even the gravitational pull of the polar ice sheets determining whether some coastal areas will be inundated while others stay dry”.
Some current projections predict that increasing global temperatures could cause a sea-level rise of around one metre by 2100, largely due to melting ice-sheets in Greenland and West Antarctic – but that figure is just an average, and while some places may be barely affected (such as Scotland, Iceland and Alaska) other areas could feel significantly greater impacts. The factors that will determine local sea-level changes are of varying importance, and scientists have been aware of some of them for decades. One such factor is that the land is actually rising in some places and falling in others. Northern Canada and Scandinavia have land that is rising at a few millimetres a year, as it slowly rebounds from the crushing compression of the Ice Age glaciers from 10,000 years ago – while areas in the north-eastern United States and southern England are correspondingly sinking at the same rate. In other areas like the Gulf of Mexico, where the already-climate-battered city of New Orleans is situated, the land is sinking at up to 10 millimetres a year as massive oil and gas extraction operations deplete the continental shelf and the land deflates like a balloon. So this will have a small affect on sea level rises in those regions.
Another factor with greater consequences may be changes in prevailing wind patterns, which literally push the oceans either towards or away from the land. For example, winds blowing westwards across the Pacific Ocean make the average sea levels in places such as the Philippines up to two feet higher than those on America’s western Pacific coast. So changing wind patterns can change local sea levels. The major ocean currents also have a significant impact, as warm water on the surface of the Atlantic Ocean – known as the Gulf Stream – flows north and east until it reaches the area between the United Kingdom and Greenland, where it cools, sinks into the deep, and flows back to the south-west. However, scientists hypothesise that the Gulf Stream could be interrupted in two main ways: a general warming of the North Atlantic could keep the surface water at a higher temperature, and decrease its tendency to sink; and/or fresh water from Greenland’s melting ice caps could dilute the water in the region, also keeping it from sinking into the saltier waters of the deep. If the water doesn’t sink as quickly, but the Gulf Stream keeps bringing it into the North Atlantic, the water could begin to pile up and raise sea levels along the North American coast even further.
However, one of the factors impacting sea-levels that scientists have begun to consider most recently is actually the gravitational pull of the polar ice sheets themselves. The impact of gravitational changes due to melting ice caps could be so profound that all of these other factors are overwhelmed by its significance. Lemonick writes that above the Earth we must imagine a landscape of gravity, called the geoid, which rather than being perfectly even is “as bumpy as the surface of the actual planet”. Perhaps counter-intuitively, because of the unevenness of Earth’s gravity: “Orbiting satellites don’t move around the Earth in perfect circles, or even perfect ellipses; their height changes when they go over the extra gravity exerted by a mountain range, and changes again when they orbit over a valley. And because water is a liquid, the surface of the sea is also warped to follow the contours of the geoid. The extra gravitational attraction of an undersea mountain range pulls water toward it, creating a literal, permanent bump on the surface of the sea, while the deficit of gravity near an undersea valley creates a depression in the water up above. The same sort of thing happens when there’s an excess of mass on land that lies near the ocean. A coastal mountain range pulls the water in its direction, raising sea level nearby. So do the massive icecaps that smother Greenland and Antarctica. Indeed, Antarctica’s polar ice sheet is so massive that it is three miles thick in places and covers an area one-and-one-half times the size of the United States, including Alaska”.
So here is the issue: at the moment the gravity of the massive polar ice caps drags water towards them, keeping the sea level higher in those regions of the world, and lower elsewhere. But as the ice sheets of Greenland and West Antarctica shrink, and their gravitational pull weakens, the water around them will flow away. Now if the ice sheets completely melted – which would be a very distant occurrence – then the average increase in sea level would be approximately five metres. But as Antarctica’s gravity currently keeps sea levels artificially low in the Northern Hemisphere, “the actual increase along the US mid-Atlantic coast would be more like 6.3 metres”. So the melting of Antarctic ice may effectively transfer ocean water from the Southern Hemisphere into the Northern Hemisphere, with results not yet fully ascertained.
While climate scientists are currently working to understand the effects of all these factors in greater detail, the message that these initial studies are sending is that the process of sea-level changes will be the result of an extremely complex mixture of countervailing elements, and that the final results will vary dramatically from region to region, and even locality to locality. So when climate sceptics point to different rates of sea-level rise in different regions (or different trends in temperature), in an attempt to prove that there is “no global trend, and thus no global warming”, they are merely exposing their lack of understanding of the science. Unfortunately, the right-wing shock-jocks and conservative politicians who support the status quo often have an easier time making their arguments – based on what they call ‘common sense’, but what is actually anti-intellectual populism. While climate science has to be approached with the same care and scepticism that is embodied at the heart of all proper scientific theory, these new areas of investigation remind us once again to look towards available evidence and well-reasoned logic, and to eschew the simplistic demagoguery of reactionary commentators.