The rising tide

Average sea level is rising, but averages can be misleading (9 minute read)

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As a child, I was both fascinated and terrified by the BBC television series The Day of the Triffids. In the series, based on the book of the same name by John Wyndham, the world was taken over by semi-intelligent, walking plants, which were able to kill a human with a single swipe of their stings. It was an intriguing scenario for a future botanist, and it encouraged me to read the book when I got older, and then the other novels of John Wyndham.

Most of Wyndham’s tales were about the end of the world, in one way or another. In The Midwich Cuckoos, it was the threat of the terrifying alien children. In The Chrysalids, much of the world had already been destroyed by nuclear war. But the story which remains most relevant today is The Kraken Wakes. This story featured aliens which arrived on earth largely unnoticed and settled in the deepest parts of the ocean. Their attacks on humanity were all the more frightening because we never saw them or knew what they looked like. And the most successful way that they attacked humans was to melt the ice caps of Greenland and Antarctica, causing catastrophic sea level rise.

I often think back to that novel when I think about sea level rise caused by climate change. Wyndham had the sea level rising visibly over a period of months, not the situation we face, but it gives a chilling vision of the future. The main characters observed the rising tides from a multi-storey building in London, from where an orderly evacuation was planned. Of course, that wasn’t what happened. Two thirds of the population of England was forced to flee for higher ground and much of the country descended into lawlessness. Since it was published in 1953, the only other countries to receive much attention in the novel were in western Europe, but it was the same picture there – millions of refugees fleeing low-lying areas, battling with those who had possession of the higher ground. Although the amount that the sea level rose was never stated exactly, it was somewhere over ten metres.

The sea level rise described in The Kraken Wakes was massive in comparison to what we are currently facing. So far, since 1880, the global average sea level has risen by just over 20 centimetres, which doesn’t sound much. But scientists from the Intergovernmental Panel on Climate Change (IPCC) are predicting a sea level rise from 0.3-1 metre between now and 2100.

Image credit: Gary Webber/ Getty Images

Why is the sea level rising, and what does it really mean, especially for those of us living in nations with a large amount of coastline?

The first reason that sea levels are rising may be surprising – it has nothing to with melting ice caps. It relates quite directly to the temperature of the ocean. You may remember learning at school that objects expand as they get warmer and contract as they get cooler (I’ve linked here to a very low-tech video which explains how the process works). With climate change the ocean is warming, and so it expands.

Understanding how much the ocean has warmed is not a simple matter, because until very recently the only way to measure the temperature was to drop a thermometer over the side of a ship, and that method didn’t cover much of the ocean. There is a lot of variation –  different areas are warming at different rates, and some areas, such as parts of the north Atlantic, have even cooled. On average, though, it is only a degree or two warmer, so far.

That degree or two of warming, however, is important. As well as contributing to sea level rise through expansion, warmer waters also contribute to sea level rise by helping to melt ice shelves. As I discussed in my article two weeks ago, melting ice shelves accelerate the melting of Antarctica’s giant glaciers, putting more water into the ocean.

It’s not just Antarctica though. A more immediate concern is the ice sheet which covers Greenland, because the Arctic is warming much faster than almost anywhere else. The Greenland ice sheet is currently contributing more to global sea level rise than the whole of Antarctica, which is much larger and has more ice. The actual amount of sea level rise from Greenland is currently an average of 0.7 millimetres per year. That sounds tiny, but melting is accelerating. By 2100, the Greenland ice sheet will have added 5-33 cm to global average sea levels.

But global averages can be misleading. The surface is not, as you might think, all at the same level. Believe it or not, the surface of the ocean has its high points and low points, just as the land does.

When I first found out about this fact, my brain struggled to make sense of it. The behaviour of water is so obvious – when we pour it into a container, it spreads itself so that the surface is flat. If two bodies of water are connected, water will flow from the higher to the lower. There’s even a common expression describing this behaviour – water seeks its own level.

But when it comes to the ocean, the situation becomes more complicated. Under the influence of the moon, the water level on the sea shore appears to rise up and down (in fact it’s not the water that’s rising and falling, but the earth rotating through the bulge of water pulled by the moon). Wind and ocean currents also influence sea level, something I learned about when I wrote about El Niño and La Niña weather patterns, back in 2020. The prevailing winds which blow from east to west across the tropical Pacific Ocean result in sea levels up to half a metre higher in south-east Asia than on the South American coast. Variations in the earth’s crust can also affect sea level. Areas where the crust is thicker and denser have higher gravity, pulling in more water and causing higher sea levels. And sea levels are higher than average around Greenland and Antarctica because of the gravitational pull created by all the ice.

The total variation in the ocean surface is not huge. The hills and valleys on the ocean caused by currents and gravity only vary by about two metres. But that’s a lot if you live near the coast.

Now, this is where things get really complicated. I’m going to pause for a moment and say that this next bit blew my mind when I first learned about it, only two weeks ago when one of my readers pointed it out to me. That was when I decided that I wanted to learn more about sea level so I could share it with you.

Climate change isn’t just going to add from 0.3-1 metre of water to whatever the local sea level is. Some areas are going to see much greater rises, and some may even see the sea level fall.

What happens on each stretch of coastline depends on a number of different factors. As the Antarctic and Greenland ice sheets lose mass, they have less gravitational pull and so sea levels will drop around Greenland and Antarctica, and rise in more distant areas. Then there’s the potential slowing of the Gulf Stream, the current that pulls water from warm parts of the Atlantic northwards. A slowing Gulf Stream could see water banking up behind it, resulting in an extra 25 cm of sea level rise along the eastern coast of North America. Changes in other winds and currents will also affect sea level, meaning it’s very difficult to predict exactly what is going to happen where. However, in general, the areas most affected are closer to the equator, particularly in the Pacific Ocean. This is bad news for our Pacific neighbours on low lying island groups such as Tonga.

But there’s another factor affecting global sea levels that we need to take into account. The level of the land itself is changing in different ways in different parts of the world – and these changes are fast enough to make a real difference.

During the last ice age, much of North America and northern Europe was covered with ice sheets. These ice sheets were heavy, and squashed the land beneath them down. Around the edges of the ice sheets, however, the land bulged up slightly. When the ice melted, the land adjusted – the squashed-down land began to rise and the bulges began to sink. This adjustment hasn’t yet finished. Scandinavia, once crushed under the weight of ice, is rising at a startling rate – 80 centimetres in the last century. But southern England and Wales, once on a bulge at the edge of an ice sheet, are sinking, although fortunately not at the rate that Scandinavia is rising. Southern England and Wales will sink by around 5 cm over the next century.

There’s another reason, too, why some land is sinking, and it’s a reason that gives New Zealanders cause to worry. Many of the world’s cities are subsiding, and at a faster rate than sea levels are rising. Some of this subsidence, especially in places like the east coast of North America, is because they were once on land bulges at the edge of ice caps. But there are other reasons too – the extraction of ground water, oil and gas, and even the sheer weight of the buildings we are adding to the land, all contribute.

A recent study, which looked at the rate of subsidence in 99 coastal cities, found that some cities were subsiding at a rate of over 2 centimetres per year. Among the worst-affected was Jakarta in Indonesia – so much so that the government is planning to create a new capital city on the island of Borneo. Although not as badly-affected as Indonesia, New Zealand is affected too. A large part of Auckland is sinking by 2 millimetres a year, and although that doesn’t sound like much, it amounts to 16cm by the end of this century, on top of sea level rise.

If we add up all the changes in the sea level, both from land sinking and the sea rising, it’s clear that most of us aren’t going to be living underwater by 2100. But that isn’t the immediate problem. The real problem is that rising sea levels increase our vulnerability to flooding caused by extreme weather. Flooding is on my mind at the moment, because members of my family in both Brisbane and Auckland were affected by the recent extreme rainfall there. And, because of climate change, extreme weather events are increasing, both in frequency and intensity. That is a problem I will return to in future articles.

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