Rising sea levels due to climate change could exceed state-of-the-art projections, scientists warn

Of the many threats posed by climate change, rising sea levels will certainly be one of the most shocking, making hundreds of thousands of square miles of coastline uninhabitable and potentially displacing more than 100 million people worldwide by the end of the century. This threat is a major concern for national security experts, as forced migration poses significant risks to international security and stability.

The magnitude of this threat depends largely on how high the oceans rise in the coming decades. But due to the complex dynamics of the massive ice sheets in Greenland and Antarctica, the exact estimates remain evasive, ranging from just over a foot to a few meters above current levels. This difference is the difference between tens of millions of people forced out of their homes or hundreds of millions of displaced people much harder to manage.

Now, a new paper published last week warns that if global warming continues at the current pace – reaching peak warming projections for 2100 – then sea level rise is likely to exceed those projections.

Since the late 1800s, sea levels have risen on average by about 10 centimeters globally, but the amount varies from region to region. The last century, the biggest factor that contributed to the rise of the oceans was thermal expansion; simply, the warmer water expands. But now melting ice sheets, mainly from Greenland and Antarctica, is a larger proportion, and that fraction will only increase.

In fact, there is enough ice stuck in Greenland and Antarctica so that if all the ice melted, it would cause a 210-foot rise in sea level, slightly higher than the Leaning Tower of Pisa. No scientist expects anything close to this century, but after the Earth exceeds a certain level of warming, the ice sheets become less stable and less predictable, with potential tipping points coming into play.

In the latest report from the UN Intergovernmental Panel on Climate Change (IPCC AR5), average projections of sea level rise by the end of the century range from 16 inches for a low-end warming scenario to 2 feet for a high-end scenario. (compared to the average sea level from 1986-2005). Estimates also come with a high degree of uncertainty, which pushes the upper limit of the likely rise in sea level above 2 and a half meters.

The new paper, entitled “Rising sea levels in the 21st century could exceed the IPCC’s projections for the future with strong warming”, challenges this higher estimate, saying it is probably too low. The paper was published by who is one of the best known glaciologists and sea level rise experts, including Martin Siegert, Richard Alley, Eric Rignot, John Englander and Robert Corell.

John Englander is the co-author of the paper and author of the books “The Great Sea on the Main Street” and soon to be released “Moving to a Higher Land: Rising Sea Level and the Way Forward.” He says the paper is a reaction to “the chorus of concern in the scientific community that projections for sea level rise have been underestimated.”

He said the research team hopes their work can inform the next major IPCC report, as it is the most widely cited document on climate change. “Once the next report is ready for launch in 2021-22, our intention was to take the case of the IPCC leadership to explain the reality of Antarctica’s potential to melt better, as it could significantly add to rising sea levels in this century. ”

In a Zoom conversation with CBS News, Englander illustrated that the contribution to rising sea levels in Antarctica, by far the largest ice sheet on Earth, does not increase from a low-end warming scenario to a high-end warming scenario. end in the latest IPCC report – but in the real world it should. While the possibility of a significantly higher sea level rise due to Antarctica is mentioned in a footnote, it is by no means front and center.

The reason for this, Englander explains, is that the IPCC is very cautious with the data it uses in the report and includes only “numbers that meet their criteria of scientific accuracy with an acceptable degree of confidence”. The level of uncertainty in the scientific community stems from the fact that glaciers can be unstable, and the computer models used to design melting are not yet sophisticated enough.

In the paper, they write: “Existing ice sheet models are more likely to provide reliable projections if global warming is kept below 2 ° Celsius [3.6º Fahrenheit], but a world where the temperature exceeds 4 ° Celsius [7.2º Fahrenheit] presents a much more challenging situation. It is very possible that this extreme situation will lead to reactions and feedback in the atmosphere-ocean-ice systems that cannot be adequately modeled at present … ”

In the chart below, made by Englander and based on the IPCC report, the various factors that contribute to sea level rise (in inches) are projected to the end of the century. Antarctica’s contribution is presented in turquoise blue.

Englander explains that in a high-end warming scenario, obviously melting ice in Antarctica should contribute more to sea level rise than in a low-end warming scenario, but this is not reflected in the report. “The slight contribution of 2 inches in three scenarios and then one inch in the highest scenario is clearly paradoxical,” says Englander.

This paradox is something that the authors of the paper aim to push the IPCC to clarify in the future report.

Another paper published in Nature this week makes a similar case, focusing on evidence from Greenland. Using the latest models used to inform the next IPCC report, the authors found that in a scenario of high warming, Greenland could contribute an additional 3-inch sea level rise by the end of the century compared to the previous version of the models used by the IPCC. This further rise in sea level is due to the additional 2 degrees Fahrenheit heating projected by the new climate models. in the Arctic.

A major concern of England for our future is the nonlinear behavior of sea level rise. In recent years, the pace of sea level rise has accelerated. In the 1990s, the oceans grew by about 2 millimeters a year. From 2000 to 2015 the average was 3.2 millimeters per year. But in recent years, the pace has accelerated to 4.8 millimeters a year.

At the current rate, we can expect at least 15 inches of sea level rise by 2100. But, as has happened in recent decades, the rate of sea level rise is expected to continue to rise for the foreseeable future. So 15 inches is not only a lower edge, but it is also extremely unlikely.

Adding confidence in the newspaper’s warning that the IPCC’s projections for a strong warming scenario could be too small is proof that sea level rise has been going on for decades. In the image below, the projections from 1990 and 2002 are shown in blue and green, compared to the actual observations in gold and red. It is clear that the actual measurements are above the upper end of past expectations.

Due to this evidence and the possibility of a “tipping-point behavior”, the paper argues, “the above results [IPCC] the range is much more likely than below it. “

For most of us, human nature is to assume that the height of the oceans we have observed in our lives is a constant, but Englander says this perception is misleading. “Sea level rise is easy to miss because it is a slow, drip-like effect that fills a bucket as the ice sheets of Greenland and Antarctica melt,” he said.

For the past 8,000 years – much of humanity’s modern existence – the expectation of a constant height of the ocean has remained true. However, the height of the oceans has always changed, sometimes dramatically.

Since the last ice age, which peaked about 20,000 years ago, global temperatures have warmed by about 18 degrees Fahrenheit, and sea levels have risen to 425 feet; that’s longer than the length of the football field.

Historically, simple mathematics shows that for every degree of Fahrenheit the Earth heats up, the sea level finally he rises with an astonishing 24 feet. There is, however, a considerable gap between warming, melting and the consequent rise in sea level.

Given that the Earth has already warmed 2 degrees Fahrenheit since the late 1800s, we know that the substantial rise in sea level is already ripe, whether we stop global warming. Scientists simply do not know exactly how long it will take to see the growth or how fast it will take place. But using proxy records, glaciologists can see that as we emerged from the last ice age, sea levels rose at remarkable rates – up to 15 feet per century, sometimes.

That being said, the fact that there is much less ice on Earth today than it was 20,000 years ago means that sea level rise would probably be slower now, and the maximum rate could be tempered as well. But even a pace that is half the all-time high would still be catastrophic for an Earth with billions of people who depend on stability.

We must also remember that today’s warming, due to man-made climate change, is it happens faster than it has in at least 2,000 years and possibly over 100,000 years. So scientists simply do not have a directly comparable situation to compare with – once again highlighting our uncertain future.

While scientists and scientific journals tend to be conservative in their public projections of sea level rise, scientists will often notice that they are worried it could be much worse. When CBS News asked Englander what he thought was a “realistic range” of sea level rise by 2100, he said: “With the current global temperature level and the rate of temperature rise, I think we could reach 5-10 feet before the end of this century. “

Although this is only the opinion of an expert, if sea level rise approaches even those levels, the impact would really be dangerous and destabilizing, dramatic reshaping the coasts of nations and forcing hundreds of millions of people to abandon their homes. Englander says that to reduce the potential impact, it is better to be prepared for a scenario in the worst case.

“We need to start planning and designing this while there is time for adaptation.”