Sea level rise (SLR) is one of the most severe impacts of climate change, with rising waters threatening to inundate small-island nations and coastal regions by the end of the century.
At the same time, SLR is one of the impacts with the largest uncertainties, with different studies projecting widely different ranges over the 21st century.
The Earth’s oceans have already risen by around 0.2m since the late 1800s, with the rate of SLR accelerating in recent decades. In its 2013 fifth assessment report (AR5), the Intergovernmental Panel on Climate Change (IPCC) estimated that SLR was “unlikely” to exceed 1m this century, even if emissions were very high.
However, a number of studies published in the years since then suggest that the worst-case projections for SLR could be much higher – up to 2m or more this century.
Since AR5 in 2013, a large number of new studies on future SLR have been published. Many of these have shown substantially higher worst-case SLR estimates by the end of the 21st century than those published in the AR5 – largely due to a reassessment of the potential losses from Antarctic and Greenland ice sheets.
The figure below shows estimates of SLR by 2100, if emissions are very high, published between 1983 and 2018. The figure is based on research from Dr Andra Garner of Rowan University and colleagues.
Individual studies are shown in blue, with the dots representing the best-estimate and the bars representing the high- and low-end estimates (when available) from each study. The SLR projections found in the five IPCC assessment reports are shown in black, with the grey shaded region showing the IPCC range from each report until a new one is published.
Some early estimates in the 1980s and 1990s were quite high, but that was also a period when the science surrounding ice sheet melting was less well-understood.
While a number of studies over the last decade have best-estimates of future SLR in-line with IPCC projections, their worst-case estimates are nearly all higher than the upper end of the IPCC range. As Garner tells Carbon Brief:
“Since the publication of the IPCC report in 2013, we’ve seen the range of future SLR projections expand significantly, with some studies suggesting the possibility of up to 2.5m of global mean SLR by 2100.
“There are a number of factors driving the uncertainty in future SLR amounts and rates, but the behaviour of the Antarctic and Greenland ice sheets in a warming climate is, perhaps, one of the largest contributors to this uncertainty. In particular, as new studies have come out suggesting the possibility of larger contributions to sea level rise from the Antarctic ice sheet than previously thought, we’ve seen the upper bound of future SLR projections climb upwards.”
Antarctic ice sheets, in particular, have proven to be a large source of uncertainty in future SLR projections. One study, published in Nature in 2016, suggested that a previously unconsidered process known as “marine ice-cliff instability” (MICI) meant the glaciers in the Antarctic were more unstable than scientists had thought. The paper concluded that “Antarctica has the potential to contribute more than a metre of SLR by 2100 and more than 15m by 2500” if future emissions are very high.
However, an article published in the Atlantic reported that the authors of the study “have lowered some of their worst-case projections for the 21st century” after making improvements to their model. The results are likely to put Antarctica’s contribution to SLR in 2100 at “about a foot” (30cm), the article says, which is “much closer to projections made by other glaciologists”.
A recent paper – also in Nature – argued that while MICI was not necessary to reproduce the SLR seen in ice ages, even without MICI Antarctica will likely contribute around 0.15m to sea level by 2100 under RCP8.5, with a “likely” range of 13-31cm. These estimates are notably higher than the 0.05m best-estimate contribution from Antarctica published in the IPCC AR5.
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| Source: CarbonBrief |
Now, note, this is for the scenario where emissions are very high, the so-called "business as usual" (BAU) case, where emissions keep on growing indefinitely. Maybe BAU is unlikely, because the pressure to do something is increasing steadily. But so far, emissions are still rising.
So this has to be the case used for planning ahead for mitigation of the consequences of global heating. For example, if you are planning the sea defences of the Netherlands out over the next 30 or 40 or 50 years, it's no good hoping that emissions will stop growing. And equally, you can't assume that if it is BAU, that sea level rise will be at the low end of the ranges on the right hand of the chart above. Most likely, they will be in the middle. But they might not. They might be worse. The economic consequences of SLR being at the high end of ranges are orders of magnitude worse than if they are at the low end. So even though they might be equally probable, the outcome planners must worry about is the top end of the ranges. That means 2 metres by 2100.
What's more, the plausible range of SLR has risen over the last decade as the science about the melting of Greenland and Antarctica has improved. Whereas thirty years ago a one metre rise was at the top end of the forecasts, now it's in the middle. All the risks are that sea level rise will be worse than you thought. And if the SLR is at the top end of the forecasts (remember, these are probable ranges, so the top end is possible even if it is unlikely) then the sea level could rise 2 metres (6.5 feet) by 2100. If you were a politician planning for the future in the Netherlands, would you start raising the dikes now, and taking other measures to protect your country? Or would you believe the soothing lies told by denialists and do nothing? Yet, in Miami, where flooding is already a problem, one which will lead to the end of Miami well before 2100 if the worst case projections are realised, people still vote for the Republicans. In Australia, beach-side townships are being eroded by sea level rise, yet those same communities continue to vote for the denialist LNP coalition. You can't cure stupid.
The probability is that emissions will go on rising. Not enough is being done to stop that. Even if they stop rising, the new carbon dioxide emitted just adds to what's already in the air, since it takes thousands of years for carbon dioxide to be removed from the atmosphere. So for the level of CO₂ in the atmosphere to peak and also start falling, emissions each year have to actually fall to near zero. Levelling off is nowhere near enough. Even on the most optimistic scenarios, that won't happen until 2060 or later.
So, we can rely on luck and wishful thinking. Or we can responsibly and intelligently assess the risks and do something, both to cut emissions and to prepare for the worst. The largest polluters are still doing nothing—in China (35% of global emissions) they are still building coal power stations and funding the construction of coal power stations in developing countries, and in the USA (15% of global emissions), we have a full-on denialist administration which is doing its damnedest to stop the closure of coal power stations and slow the roll-out of EVs. Without these two countries doing much, much, more to slash emissions than they are doing, emissions will just keep on rising. When the market realises just how serious the risks of SLR are, expect property prices in low-lying places to plunge.
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