The relentless rise of the waters

There’s a lot of uncertainty about what a warming climate will deliver, but one thing is certain, locked into the story of our future. The seas will rise. 

Or rather, they’ll continue a 170-year rising trend that today averages 4 mm a year. If by a miracle we held warming below 1.5C, or even if all the world’s fossil fuel emissions stopped today, the extra heat now embedded in oceans and ice sheets will keep pushing up sea level, at a gradually slowing rate, for well over 1000 years.

But we won’t be that lucky. In its 2021 report, the Intergovernmental Panel on Climate Change (IPCC) identifies 3.5C of warming by 2100 as our most likely fate. On that basis NASA has posted data showing that the seas lapping Tasmania will be over 20 cm higher by 2050, up 60 cm by 2100, and 70 cm higher in 100 years’ time.

Scientists have a rule of thumb that the risk of coastal flooding triples for every 10 cm rise in sea level. If emissions don’t decline Tasmania can expect what today is a once-in-a-decade coastal flood to happen about once a year by 2050 and every day or two by 2100. Our new waterfront stadium will need a saltwater-tolerant playing surface and players attuned to, let’s say, damp conditions.

Another rule of thumb is that the vertical rise should be multiplied by 100 to get the impact horizontally, so we can expect that by 2050 coasts will have retreated an average of two metres. But not all coastlines are equal. Tasmania has many resilient rocky coasts, and many beaches – though not all – retain enough sand offshore to ensure they can be replenished by storms for some time yet.

If you think that’s good news, it ends there. In its latest report the IPCC warns of a high chance of abrupt change with significant ice loss from Greenland and Antarctic ice sheets over a short period of time and an increasing chance that by 2100 sea levels will have risen by around two metres.

It has happened before. The most recent large-scale ice sheet decay started 12,000 years ago, long after humans had walked across low-lying plains to occupy this southern part of Australia. There’s strong geological evidence that the final breach to create Bass Strait happened within one or two human lifetimes.

Earlier this century sea level rise was a big topic here and everywhere, and the Tasmanian government and some coastal councils were moved to consult people in the know. Those early studies called for more refined modelling of offshore sand – how much there is and how it moves about – and other conditions affecting coastal erosion. 

But despite the obvious need to protect vulnerable coastal areas and in some cases to prepare for evacuation, the practical response since then has been limited to occasional ad hoc repairs to damaged coasts.

One of those early consultants was Chris Sharples, a geologist with a lifetime’s accumulated knowledge of Tasmania’s coastal landforms and processes. That knowledge, extending back over many decades and now being further refined with each passing year, is surely pure gold to authorities needing to understand what rising seas will do to their domain, and how those seas might affect their coastal ratepayers. But it seems not.

Sharples is the quintessential scientist, driven to know how his world works, building on his science education in countless field trips, where he’s gathered mountains of data so that he can compare present shorelines with past aerial images.

Having finally decided to put his accumulated knowedge into a written thesis a few years ago, he can now call himself a doctor. But honorifics and public acclaim aren’t what drives him and self-promotion is not one of his strengths. 

Sharples is not a fixer, like a coastal engineer who’ll build you a sea wall or a breakwater if you can pay for it. That might fix your problem until you’ve been able to sell your property, but it won’t tell you what drives all that – the natural forces that shape our coastlines, how that reshaping will roll out in decades to come, and why today’s coastal defences may not be the long-term solution needed. That knowledge above all is what authorities need to pursue.

One of the fundamental lessons from studying how climate changes is that it’s never neat and tidy. Coming to grips with this untidiness requires sustained, disciplined, independent mental effort, the sort of thing we used to expect from universities. That kind of science rarely delivers living wages, but it’s long past time that it did.

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