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Analysis: Sea Levels Will Rise, But COP 26 Decisions Will Determine How Much

This story includes details about the impacts of climate change that may be difficult for some readers. If you are feeling overwhelmed by this crisis situation here is a list of resources on how to cope with fears and feelings about the scope and pace of the climate crisis.

Sea level rise is now inexorable. So is coastal flooding. And as the world’s leaders prepare for a make-or-break meeting in Glasgow, Scotland next month to confront the climate crisis, a new set of visualizations from Climate Central can help deliver the imagery of hazard in ominous detail.

Even if the world keeps its promise to contain global warming to 1.5°C, over time sea levels will rise by 2.9 metres and vast tracts of what is now valuable urban real estate in many of the world’s great coastal cities will be below the high tide line. This becomes inevitable when atmospheric carbon dioxide levels rise from the historic norm of 288 parts per million to more than 400 ppm in little more than a century.

Which means that one day—perhaps in the next 30 years—Londoners could see the waters of the River Thames lap in annual flood around the new U.S. Embassy and soak the stadium of Chelsea Football Club.

If global temperatures go on rising to 3°C by 2100—and right now, that’s where temperatures are headed—New Yorkers will one day watch the waters rise over downtown Manhattan and flood John F. Kennedy Airport, while Londoners could see the muddy waters slosh against the steps of St Paul’s Cathedral at the top of Ludgate Hill.

Sea level rises on even the most hopeful projections will be grim news for the 510 million people who right now live in those coastal districts most at risk. But if the world goes on burning fossil fuels and raising levels of atmospheric carbon at present rates, the planet will warm by 3°C to bring the menace of rising damp to 800 million urban dwellers, about one-tenth of the world’s present population.

And graphic tools, based on the latest satellite measurements and population data, and developed by climate scientists can now deliver the warning at the level of streets and buildings.

Six years ago, 195 nations agreed at the Paris climate conference to hold global warming to 1.5°C by the year 2100. Even then, there was already enough carbon dioxide in the atmosphere to go on melting the ice caps and raising global sea levels by a projected 1.9 metres. By the time the world reaches the goal of net-zero carbon dioxide emissions, the final cost in sea level rise will be 2.9 metres. 

And if it doesn’t achieve that goal—if global temperatures rise to what at the moment promises to be 3°C or even 4°  then the world’s coastal cities face threats that have no precedent, according to a companion study in the journal Environmental Research Letters.

Within decades, the world’s cities could be facing huge costs for coastal flooding. But the timetable for city floods on a for-the-moment almost unimaginable scale is protracted. In a scenario in which nations go on exploiting coal, oil, and gas as energy sources, and go on clearing the planet’s great forests, temperatures could rise to 4°C. If that happens, then there will be enough carbon in the atmosphere to raise global sea levels by 8.9 metres.

It won’t happen in a hurry. But, over an interval that could take hundreds or even thousands of years, the seas will rise, because the greenhouse gases in the atmosphere will go on raising the planetary temperature and melting the ice caps. Ice reflects solar radiation back into space: once the ice goes, then the rocks and water absorb more sunlight and warming inevitably accelerates. Once the permafrost thaws, vast reserves of once-frozen vegetation start to decompose, to release even more carbon in the form of methane and carbon dioxide, and go on raising global temperatures.

Studies of this kind are aimed at helping governments, civic authorities, and communities focus on the challenge ahead. At the COP 26 meeting in Glasgow in November, the world’s leaders will be asked to deliver plans to drastically reduce greenhouse emissions and contain global heating. The message is that a rise of 1.5°C will be expensive, but a rise of 3°C or 4°C could be catastrophic. 

It could be a slow catastrophe. Sea levels could rise over a timespan of 2,000 years. Or they could swell by seven metres in the next 180 years.  The message of the latest Environmental Research Letters study from researchers at Climate Central at Princeton, the Potsdam Institute for Climate Impact Research in Germany, and the Lamont-Doherty Earth Observatory at Columbia University is that—however swiftly or slowly—sea level rise is on the way. Protection from flooding will involve increasing expense for the wealthiest nations, and devastating loss for the poorest.

Whole nations—the Maldives, the Marshall Islands, Kiribati, the Bahamas, Tuvalu, the Cocos Islands, the Cayman Islands, and Tokelau—could watch land that is home to 90% of their population slip below the high tide line. The next question is: could levees, walls, and other engineered solutions save London, or Rotterdam in The Netherlands, or Tokyo in Japan, or Jakarta in Indonesia, where parts of both cities have already subsided by almost five metres, and where some residents already live below sea level?

”Even monumental adaptation measures will not be able eliminate all concerns. Some rivers will need to be walled off from the sea and fully pumped into it, with consequences for ecology, ports, trade, and more. Citizens living inside deep bowls of protected areas would need to agree to live with the risk of catastrophic and near-instant floods in the events of levee or pump failures, whether from natural causes, human error, or terrorist or wartime attacks,” the scientists write.

Those results “suggest that a sharp reduction in carbon emissions is in the national interest of all coastal nations.”