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Date posted: 18 October 2019
This post is written by our guest blogger Bill McGuire. Bill McGuire is Professor Emeritus of Geophysical & Climate Hazards at UCL and author of Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanic Eruptions. He was a contributor to the IPCC 2012 report on Climate Change & Extreme Events and Disasters.
The battle against climate breakdown is being fought across the planet, from the wildfire-ravaged bush of Australia and California, to the hurricane-lashed Bahamas and monsoon-swamped India. But the front line in this intensifying battle is, without doubt, to be found where the land meets the sea. Here, in the coastal zone, are located many – if not most – of the world’s great cities and countless smaller communities. Close to 700 million people in all.
Global sea levels are climbing, relentlessly; the oceans encroaching steadily upon the land, bringing increased flooding and the threat of permanent inundation.
At present, sea levels are rising incrementally, but the rate is increasing fast. From less than a millimetre a year in the 1920s, annual sea level rise is now half a centimetre. Much of the recent rise has happened in just the last few years, adding to concerns that we may be on the verge of a large and rapid hike in sea level that is far beyond current forecasts.
Over the course of much of the last century, rising sea level was largely a consequence of the thermal expansion of the oceans, as they absorbed the heat associated with global heating. Now, year-on-year, melting ice sheets in the polar regions are adding more and more to the total, and this contribution is becoming increasingly obvious. As melting rates accelerate, so sea level is following suit.
Worryingly, the West Antarctic Ice Sheet is showing signs of massively accelerated melting, with the rate of ice loss tripling in just the five years between 2012 and 2017 – from 76 to a colossal 219 billion tonnes. Should the rate of melting continue to triple every five years, then – by the mid 2040s – the annual rate of sea level rise would be a staggering 5cm. And that’s without adding in any rise associated with melting of the Greenland and East Antarctic ice sheets. We will have to wait a while to find out if this becomes reality, but with global heating accelerating, it would be a little odd if the rate of ice melting slowed.
The recent disturbing behaviour of the West Antarctic Ice Sheet is not at all a surprise in light of a number of predictions for how much higher global sea level will be by the end of the century. While the new IPCC special report on The ocean and the cryosphere in a changing climate1 imagines a worst case sea-level rise of 110cm by 2100, others think it will be much higher. In fact, peer-reviewed research published over the last few years forecasts rises of two, three or even more metres by this date. Rises on this scale would see the coastal zone swamped and hundreds of millions of people displaced.
Looking ahead, the picture may actually be even worse, with some scientists considering the possibility that the so-called ‘doubling time’ of sea level rise may become as short as a decade. This would mean that the current rise of half a centimetre a year would become a full centimetre a year in 2030, 2cm a year in 2040, 4cm in 2050 and so on. Such a progression is said to be exponential, so that although the initial rise is small, the constant growing rate (doubling time) ensures that increases quickly become very large. A doubling rate of a decade, therefore, would see sea-level climbing by nearly 1.3m a year in 2100, with every coastal town and city drowned long before this.
Like many, I find it difficult to imagine that such a scenario will actually play out, but this is more a consequence of a failure of the imagination than my having any issues with the science. Studies of the rates of past sea-level rise during times of rapid global heating reveal that such rapid rises are perfectly possible and have happened before, most notably at the end of the last Ice Age. The corollary of this is that – in barely eight decades time – the world’s great coastal cities could be submerged. If not for ever, then likely for many centuries.
1IPCC Special Report on the Ocean and Cryosphere in a Changing Climate