Wednesday, November 09, 2016

Why Scientists Are So Worried About Sea-Level Rise in the Second Half of This Century

The rising sun hits the Statue of Liberty in New York Harbor in New York. (Credit: Lucas Jackson/Reuters) Click to Enlarge.
Even as negotiators meet in Marrakech, Morocco to take the next steps to avert dangerous human-caused climate change — and, even as the U.S. decides whether or not to elect a president who is skeptical 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 degrees Celsius (3.6 degrees Fahrenheit) increase above what it was in pre-industrial times.  We’ve already seen about a 1 degree Celsius (1.8 degrees Fahrenheit) 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 institutionalize, then we could hit 2 degrees Celsius by 2040 or so. For the planet’s sea level, this would mean over 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 U.S. East Coast, where it could be over 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 degree increase in temperatures — to 4 degrees Celsius — 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 degrees, and then we will do it in 40 years time, 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 Center in the United Kingdom, in describing the scenario presented in the study.  Jevrejeva completed the work with researchers at institutions in the U.K., the Netherlands, Denmark, China, and Finland.

The key punchline of the study, then, is that going beyond the Paris barrier of 2 degrees Celsius 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 meter 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 and examined 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 meters (0.65 feet), but “more than 90 percent 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 millimeters 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 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 degrees as early as 2040 and as much as 5 degrees by 2100, is not necessarily the one that will be realized.  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 degrees of warming by 2100, recent research has suggested.  And if they’re tightened further, the world may still settle at 2 degrees C or even somewhat below that.

Read more at Why Scientists Are So Worried About Sea-Level Rise in the Second Half of This Century

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