The study finds that by 2100, New York could see a sea level rise of more than 3.5 feet. Image Credit: AP

Even as negotiators met in Marrakesh, Morocco, to take the next steps to avert dangerous human-caused climate change — and, even as the United States elected a president who is sceptical it is happening — a new study has highlighted the sharp stakes involved, particularly when it comes to the ongoing rise in global sea level and the dramatic but uneven way in which it could affect the world’s coastlines.

The goal of the Paris climate agreement is to hold the planet’s temperature rise to “well below” a 2°C increase above what it was in pre-industrial times. We’ve already seen about a 1°C increase since then.

But the new research just published in the “Proceedings of the National Academy of Sciences” notes that if we stay on a current, high-emissions pathway and do not achieve the cuts that the Paris agreement seeks to institutionalise, then we could hit 2°C by 2040 or so. For the planet’s sea level, this would mean more than a half-foot rise averaged around the globe, in comparison with average sea levels from 1986 to 2005. The sea-level increase, however, would be far worse in certain places, such as the US East Coast, where it could be more than a foot.

And that’s just the beginning. Assuming we still don’t reform our ways, the 40 years after 2040 could then see another sharp 2°C increase in temperatures — to 4°C — and another dramatic surge in sea level, culminating in a rise of 2 feet averaged across the globe, or more if we’re unlucky. The study finds that by 2100, New York could see a sea level rise of more than 3.5 feet.

“Basically we spent 200 years to warm our planet by 2°, and then we will do it in 40 years, this shows a completely different scale of what’s going on,” said Svetlana Jevrejeva, the lead author of the study and a researcher at the National Oceanography Centre in the United Kingdom, in describing the scenario presented in the study. Jevrejeva completed the work with researchers at institutions in the UK, the Netherlands, Denmark, China and Finland.

The key punchline of the study, then, is that going beyond the Paris barrier of 2°C condemns the world to increasingly dire sea level rise — especially for certain locations.

The new work goes well beyond an increasingly dated consensus finding of the international scientific community on sea level, which stated that it could increase by nearly 1 metre by the year 2100, under a worst-case scenario version of global warming. More recent research focusing on the instability of Antarctica has nearly doubled that projection.

The new paper is part of a growing wave of research that attempts to get beyond such broad estimates for the whole planet, to try to assess more precisely what kind of sea level rise will strike in different locations across the world. For while a steady rise is expected, it is likely to vary greatly in particular locations, as a result of factors ranging from the circulation of the oceans to the redistribution of mass on the planet.

To this end, the study used a probabilistic approach to examine how sea levels would vary across the world’s coasts under a worst-case warming scenario by the year 2100. The researchers used the results of 33 climate-change models to examine factors ranging from how much ice Greenland and Antarctica could lose to how the global ocean circulation could change.

Sure enough, global sea-level rise was highly variable. The study noted, for instance, that most coastal locations were prone to experience a worse sea-level rise outcome than the global average. For instance, it found that for the year 2040 on the worst-case emissions pathway, the global average sea-level rise would be 0.2 metre (0.65 foot), but “more than 90 per cent of coastal areas will experience sea level rise exceeding the global estimate”.

One key subtext to these findings is that scientists expect the rise in sea levels to accelerate in coming years, beyond the current estimated rate of 3.4 millimetres per year. The melting of Greenland, Antarctica, mountain glaciers around the globe, and the corresponding expansion of the volume of the ocean as it warms, are expected to increase their pace.

The study found that by 2040, the rate of sea-level rise could actually hit 6 millimetres per year. By the end of the century with extreme warming, it could hit 10 millimetres (or 1 centimetre) per year, or even higher.

“The high end of the unchecked pollution scenario would threaten the homes of hundreds of millions with chronic flooding or permanent submergence this century,” said Ben Strauss, a sea-level rise expert with Climate Central. “This research adds to the evidence that strong cuts in carbon pollution could strongly curtail the danger to global coasts and their cities.”

As Strauss’s words suggest, this worst-case scenario for global warming, which envisions 2° as early as 2040 and as much as 5° by 2100, is not necessarily the one that will be realised. That still depends on choices made in the present and near future, in Morocco and beyond. If the current Paris climate pledges are achieved, the world might settle closer to 3° of warming by 2100, recent research has suggested. And if they’re tightened further, the world may still settle at 2°C or even somewhat below that.

These changes would certainly help stave off some of the worst-case scenarios for sea-level rise, but researchers have also found that that rise is unlikely to end by the year 2100. It involves a slow discovery of a new equilibrium between the planet’s temperature and its ice and oceans that would play out over hundreds to thousands of years, meaning that we are committed to it for a long time, although the rate and extent of rise will depend on the rate and extent of warming itself.

“Even if temperature will be constant, sea level will continue to rise for a few hundred years, that’s what we know,” said Jevrejeva.

In some places in the world, the study points to the possibility of a particularly devastating combination — sharp sea-level rise combined with major land subsidence because people are drawing so much drinking water out of the ground. Jakarta, Indonesia’s capital, is actually forecast to subside by more than 6 feet just by the year 2025, an effect that would dramatically compound the consequences of the sea level rising all around it.

One outside researcher who was familiar with the work said it seemed to fit our current understanding well. “The numbers look reasonable,” said Rutgers University sea-level rise expert Robert Kopp, who was not involved in the current study. Kopp said other research, for instance examining sea-level risk facing the coast of New Jersey, finds something similar: for instance, a 50 per cent chance of a 1.4 foot or greater rise by 2050.

And even small rises in sea level can translate into more damage, said Jevrejeva. “Small changes in sea-level rise will significantly change what happens during extremes, during hurricanes, during tropical cyclones, during storm surges,” she said.

–Washington Post