Next five years will shape sea levels for next 300

The world is far off course from its goals in cutting greenhouse gas emissions — and research published Tuesday illustrates one of the most striking implications of this.

Namely, it finds that for every five years in the present that we continue to put off strong action on climate change, the ocean could rise an additional eight inches by the year 2300 — a dramatic illustration of just how much decisions in the present will affect distant future generations.

“One important point was to reveal that sea level [rise] is not in the far future, it’s now, and because the system is so slow, we just can’t see it at the moment,” said Matthias Mengel of the Potsdam Institute for Climate Impact Research, the lead author of the study, which was published in Nature Communications. “But we cause it now.”

The study, which also included scientists at institutions in Australia, Austria and Germany, takes as its premise that the world will work to achieve the Paris climate agreement’s sharp cuts in greenhouse gas emissions. The goal is to limit warming below 2 degrees Celsius (3.6 degrees Fahrenheit), which requires that the world essentially cease adding any more greenhouse gases to the atmosphere by 2050 or so.

This, in turn, means that global greenhouse gas emissions must reach a peak by either 2020, 2025 or 2030 (or possibly 2035) and then begin to decline rapidly. The longer the wait, the faster the necessary decline after the peak. If we delay longer still, it simply becomes too difficult to bring emissions down fast enough to avoid dangerous levels of global warming.

All of this has major implications for the global sealevel-rise commitment, the study finds. Because emissions today impel rising seas over centuries, the researchers were able to calculate a surprisingly large impact of present choices on sea levels in the distant future.

For every five-year delay in the peaking of emissions, the middle-range sealevel projection for 2300 increased by 20 centimeters, or about eight inches, the study found.

“The more cumulative emissions, the more warming, the more sea level,” Mengel explained.

And that’s just the central estimate in the study. At the extreme end of what’s relatively unlikely but still certainly possible, the research found that each five-year delay could mean as much as an additional meter, or over three feet, of sea-level rise. That is because of the evergrowing chance of major destabilization of the Antarctic ice sheet. In some small number of scenarios, even with the sharp emissions reductions contemplated by the Paris agreement, the Antarctic ice sheet still gives up meters worth of sea-level rise.

“It tells us that we are really uncertain, we don’t know enough about the ice sheets,” Mengel said.

Eight inches of sea-level rise is about what the planet experienced during the entirety of the 20th century, Mengel said. That itself highlights just how much worse sea-level rise could get in the future, as global warming increasingly warms the oceans (causing them to expand) and melts glaciers, the Greenland ice sheet and Antarctica.

It is important to note that Mengel’s study considers only sea-level rise scenarios that are consistent with the very ambitious goals of the Paris agreement. The world is not at all on a pathway to achieve those goals right now. And even in these rosy scenarios, the study finds that seas could rise by between 2.3 and 4 feet by 2300, or about .75 to 1.33 feet per century.