Sea-level rise from climate change could be worse than projected

Of the many threats posed by climate change, sea level rise is sure to be one of the greatest, rendering hundreds of thousands of square kilometers of coast uninhabitable and potentially displacing more than 100 million people worldwide by the end of the century. This threat is a major concern of national security professionals as forced migration poses a significant risk to international security and stability.
The extent of this threat depends heavily on how much the oceans rise in the coming decades. However, due to the complex dynamics of massive ice sheets in Greenland and Antarctica, accurate estimates are difficult to make and range from just over a foot to several feet above current levels. This inequality is the difference between tens of millions of people displaced from their homes or the far more unmanageable hundreds of millions of displaced persons.
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A new paper released last week warns that if global warming continues at the current rate and reaches high-end warming projections for 2100 high-end warming projections, sea level rise is likely to exceed these projections.
Since the late 19th century, sea levels have risen by an average of 10 inches worldwide, but the amount varies from region to region. In the last century, thermal expansion contributed most to the rise of the oceans. Simply put, warmer water expands. But now the melting of ice sheets, mostly from Greenland and Antarctica, is a bigger proportion, and that proportion is only going to grow.
In fact, enough ice is trapped in Greenland and Antarctica that if all the ice were to melt, sea levels would rise 210 feet, slightly higher than the Leaning Tower of Pisa. No scientist expects anything near this century this century, but after the Earth passes a certain level of warming, the ice sheets become less stable and less predictable, and potential turning points come into play.
In the most recent report from the United Nations Intergovernmental Panel on Climate Change (IPCC AR5), mean projections for sea level rise through the end of the century range from 16 inches for a low warming scenario to 2 feet for a high-end scenario (compared to the average sea level from 1986-2005). The estimates are also accompanied by a high level of uncertainty, adding to the upper limit of likely sea level rise above two and a half feet.
The new paper, titled "Sea Level Rise in the 21st Century Could Beat IPCC Projections for Highly Warming Futures," challenges this upper estimate and says it is likely too low. The paper was published by a who's who 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 co-author of the paper and author of the books "High Tide on Main Street" and the upcoming book "Moving to Higher Ground: Rising Sea Levels and the Way Forward". He says this paper is in response to "a 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 big IPCC report, as this is the most cited document on climate change. "With the next report, which is now being prepared for publication in 2021-22, we wanted to challenge the IPCC leadership to better explain the reality of the melting potential of Antarctica as it will significantly improve sea level rise this century could."
In a Zoom interview with CBS News, Englander showed in the latest IPCC report that the contribution of sea level rise from Antarctica, by far the largest ice sheet on earth, does not increase from a low-warming scenario to a high-warming scenario 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 in front and in the middle.
The reason for this, explains Englander, is that the IPCC is very careful with the data used in the report and only "includes numbers that meet their criteria for scientific accuracy with an acceptable level of confidence". The level of uncertainty in the scientific community stems from the fact that glaciers can be unstable and the computer models that are used to project the melting are still not high enough.
In the paper, they write: "Existing ice sheet models provide more reliable projections when global warming is kept below 2 ° Celsius, but a world where warming exceeds 4 ° Celsius presents a much more difficult situation. It is entirely possible that this extreme situation leads to reactions and feedbacks in the atmospheric-ocean-ice systems that cannot currently be adequately modeled ... "
In the graph below, compiled 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. The contribution of the Antarctic is shown in turquoise blue.
Sea level rise projections in four warming scenarios using IPCC data, broken down by input from various sources. / Photo credit: John Englander
Englander explains that in a high-end warming scenario, the Antarctic ice melt should obviously contribute more to sea-level rise than in a low-end warming scenario, but this is not reflected in the report. "The small contribution of 2 inches in three scenarios and one inch in the highest scenario is clearly paradoxical," says Englander.
This paradox is what the authors of the paper want to get the IPCC to clarify in the forthcoming report.
Another paper published in Nature this week makes a similar case and focuses on the evidence from Greenland. Using the latest models used to inform the next IPCC report, the authors found that in a high-warming scenario, Greenland would see an additional 3 sea-level rise by the end of the century compared to the earlier version of the models used Customs could effect from the IPCC. This additional rise in sea level is due to an additional 2 degrees Fahrenheit warming projected by the new climate models in the Arctic.
A major concern for Englander for our future is the non-linear behavior of sea level rise. In recent years, the pace of sea level rise has accelerated. In the 1990s, the oceans rose about 2 millimeters per year. From 2000 to 2015 the average was 3.2 millimeters per year. In recent years, however, the pace has accelerated to 4.8 millimeters per year.
The pace of sea level rise is accelerating / Credit: John Englander
At the current rate, we can expect sea levels to rise by at least 15 inches by 2100. However, as in the past few decades, the pace of sea level rise is expected to continue to accelerate for the foreseeable future. So 15 inches is not only a lower limit, but also extremely unlikely.
Confidence in the paper's warning that the IPCC projections may be too low for a severe warming scenario is evidence that sea level rise has been at the high end of the IPCC projections for decades. In the following figure, the 1990 and 2002 projections 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 high end of past expectations.
Actual sea level rise is at the high end of previous projections by IPCC / Credit: John Englander
Because of this evidence and the possibility of "turning point behavior," the paper argues, "results above this [IPCC] range are far more likely than below this range."
For most of us, it is human nature to assume that the height of the oceans we have observed in our lives is a constant, but Englander says that perception is misleading. "Sea level rise is easy to miss as it has a slow effect, like a drop filling a bucket when the ice sheets on Greenland and Antarctica melt," he said.
For the past 8,000 years - much of modern human existence - that expectation of constant sea level 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 about 18 degrees Fahrenheit and sea levels have risen 425 feet. That is greater than the length of the football field.
Historically, a simple math shows that with every degree of warming the earth warms, sea levels rise an astonishing 24 feet. However, there is a considerable lag time between warming, melting and the resulting rise in sea levels.
Given that the earth has warmed by 2 degrees Celsius since the late 19th century, we know that there is already a significant rise in sea level regardless of whether we stop global warming. Scientists just don't know exactly how long it will be before Ascension begins or how quickly it will occur. However, using proxy records, glaciologists can tell that sea levels rose at remarkable rates during the last ice age - at times up to 15 feet per century.
Since the end of the last ice age 20,000 years ago, sea levels have risen dramatically, sometimes very quickly. / Photo credit: John Englander
However, the fact that there is much less ice on Earth today than it was 20,000 years ago means that sea level rise per degree is now likely to be less and the maximum pace can also be moderated. But even a pace half the historical maximum would still be catastrophic for an earth with billions of people who depend on stability.
We must also remember that, due to human-made climate change, warming is happening faster today than it will be in at least 2,000 years and possibly over 100,000 years. Scientists therefore have no directly comparable situation to measure themselves against - and they are once again underlining our uncertain future.
While scientists and scientific journals tend to be conservative in their public projections of sea level rise, scientists will often note that they fear this could be much worse. When CBS News asked Englander what it thought was a "realistic range" of sea level rise by 2100, he said, "Given the current global temperature level and the rate of temperature rise, I believe we will reach 5 to 10 feet before the end could this century. "
While this is only the opinion of an expert, if sea level rise came even close to these levels, the effects would be really dangerous and destabilizing, would dramatically change the coasts of nations, forcing hundreds of millions of people to leave their homes. Englander says in order to reduce the possible impact, it is better to be prepared for a worst-case scenario.
"We have to start planning and designing this while we have time to adapt."
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