Of the many threats posed by climate change, sea level rise will certainly be among the most influential, making hundreds of thousands of square kilometers of coastline uninhabitable and potentially displacing over 100 million people worldwide by the end of the century. This threat worries national security experts the most because forced migration poses significant risks to international security and stability.
The magnitude of this threat largely depends on how much the oceans will rise in the coming decades. But due to the complex dynamics of massive ice sheets in Greenland and Antarctica, accurate estimates remain unattainable, ranging from just over one meter to several meters above current levels. That disparity is the difference between tens of millions of people forced to leave their homes or many more unruly hundreds of millions displaced.
Now, a new paper released last week warns that if global warming continues at its 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 by an average of about 10 inches globally, but the amount varies from region to region. In the last century, the greatest contribution to the rise of the ocean was made by thermal expansion; simply put, warmer water spreads. But now, mostly from , makes a larger share, and that share will only grow.
In fact, there is enough ice locked in Greenland and Antarctica, so if all the ice melted, it would cause the sea level to rise by 210 meters, a little higher than the Leaning Tower of Pisa. No scientist of this century expects anything close to that, but once the Earth crosses a certain level of warming, ice sheets become less stable and less predictable, with potential milestones coming into play.
In the latest report of the UN Intergovernmental Panel on Climate Change (IPCC AR5), mean projections of sea level rise by the end of the century range from 16 inches for the low-end warming scenario to 2 meters for the high-end scenario (relative to the average sea level of 1986-2005). Estimates also come with a high degree of uncertainty, which pushes the upper limit of likely sea level rise above 2 and a half feet.
A new article titled “Sea Level Rise in the Twenty-First Century Could Exceed the IPCC’s Predictions for Future Strong Warming,” reviews the above estimate, saying it is likely to be too low. The paper published who is who of the most famous 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 book “Tide in the Main Street” and the soon-to-be-published “Transition to Higher Ground: Rising Sea Levels and the Way Forward.” He says the paper is a reaction to “a chorus of concern in the scientific community that projections for sea level rise have been underestimated.”
He said the research team hopes to work to report on the next major IPCC report, as it is the most frequently cited document on climate change. “With the next report now being prepared for publication in 2021-22, our intention was to present the case to the IPCC leadership to better explain the reality of Antarctic potential, as this could significantly contribute to sea level rise this century.”
In an interview with Zoom for CBS News, Englander illustrated that the contribution to rising sea levels from Antarctica, by far the largest ice sheet on Earth, is not growing from a low to high warming scenario in the latest IPCC report – but in the real world. Although the footnote mentions the possibility of a significantly higher sea level rise due to Antarctica, it is by no means the front and middle side.
The reason for this, Englander explains, is because the IPCC is very careful with the data it uses in the report and includes only “numbers that meet its criteria for scientific accuracy with an acceptable degree of reliability.” 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 still not sophisticated enough.
The paper writes: “Existing ice sheet models are more likely to provide reliable projections if global warming stays below 2 ° Celsius [3.6º Fahrenheit], but a world in which the heating exceeds 4 ° C [7.2º Fahrenheit] represents a much more challenging situation. It is quite possible that this extreme situation will lead to reactions and feedback in atmosphere-ocean-ice systems that cannot be adequately modeled at the moment … ”
The figure below, compiled by Englander and based on an IPCC report, predicts various contributors to sea level rise (in inches) by the end of the century. The contribution of Antarctica is shown in turquoise blue.
Englander explains that in the high-warming scenario, the apparent melting of ice in Antarctica should contribute more to sea level rise than in the warm-warming scenario, but this is not reflected in the report. “A slight contribution of 2 inches in three scenarios, and then one inch in the highest scenario, is obviously paradoxical,” Englander says.
This paradox is something that the authors of the paper want to encourage the IPCC to clarify in an upcoming report.
A similar case is given by another paper published this week in the journal Nature, focusing on evidence from Greenland. Using the latest models used to inform the next IPCC report, the authors found that in a high-warming scenario Greenland could contribute an additional 3 inches of sea level rise by the end of the century compared to the former version of the IPCC models used. This additional rise in sea level is due to the additional 2 degrees Celsius warming predicted by the new climate models..
The great concern of Englander for our future is the nonlinear behavior of sea level rise. In recent years, the growth rate of sea levels has been accelerating. In the 1990s, the oceans rose to 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 per year.
At the current rate, we can expect at least another 15 centimeters of sea level rise by 2100. But, as has been the case for the past few decades, the pace of sea level growth is expected to continue to increase for the foreseeable future. So, 15 inches is not only the lower limit, but it is also very unlikely.
Adding confidence to the paper’s warning that IPCC forecasts for a strong warming scenario may be too low is proof that sea level rise has been going on for decades at the high end of IPCC projections. In the visual representation below, the 1990 and 2002 projections are shown in blue and green relative to the actual observations in gold and red. It is clear that the actual measurements are above the upper end of past expectations.
Because of this evidence and the possibility of “behavior towards a turning point”, the paper claims, the outcomes are above this [IPCC] ranges are far more likely than below. “
For most of us, it is human nature to assume that the height of the ocean we have observed in our lives is constant, but Englander says this perception is misleading. “Rising sea levels are easy to miss because it’s a slow effect, like dripping that fills a bucket, because the ice sheets in Greenland and Antarctica are melting,” he said.
Over the past 8,000 years – much of humanity’s modern existence – that expectation of a constant ocean height has remained true. However, the height of the ocean has always changed, sometimes dramatically.
Since the last ice age, which reached its maximum about 20,000 years ago, global temperatures have warmed by about 18 degrees Fahrenheit, and sea levels have risen 425 feet; it is greater than the length of the football field.
Historically, simple mathematics reveals that for every degree of Fahrenheit the Earth heats up, at sea level in the end it rises by an incredible 24 meters. However, there is a large delay between warming, melting and the consequent rise in sea levels.
Given that the Earth has already warmed by 2 degrees Fahrenheit since the late 1800s, we know that a significant rise in sea level has already been scorched, whether or not we stop global warming. Scientists just don’t know exactly how long it will take to see the climb or how fast it will happen. But using proxy records, glaciologists can see that as we emerge from the last ice age, sea levels have risen at an incredible rate – sometimes as high as 15 feet per century.
This suggests that the fact that there is much less ice on Earth today than 20,000 years ago means that the amount of sea level rise per degree is now likely to be less, and the maximum pace can also be mitigated. But even a pace that is half the historical maximum would still be catastrophic for an Earth with billions of people dependent on stability.
We must also remember that warming is today, due to man-made climate changethan there is in and perhaps over 100,000 years. So scientists simply do not have a directly comparable situation to measure – once again highlighting our uncertain future.
Although scientists and scientific periodicals are conservative in their public projections of sea level rise, scientists will often note that they are concerned that it could be much worse. When CBS News asked the Englishman what he thought the “real range” of sea level rise to 2100 was, he said, “With the current global temperature level and the rate of temperature rise, I believe we could cross 5 to 10 feet before the end of this century. “
Although this is only one expert opinion, if the rise in sea levels approaches those levels, the impacts would be real, dramatically and forcing hundreds of millions of people to . Englander says it is better to be prepared for the worst case scenario to reduce potential impacts.
“For that, we need to start planning and designing while there is time to adjust.”