Sea level in the IPCC 6th assessment report (AR6)

My top 3 impressions up-front:

• The sea level projections for the year 2100 have been adjusted upwards again.

• The IPCC has introduced a new high-end risk scenario, stating that a global rise “approaching 2 m by 2100 and 5 m by 2150 under a very high greenhouse gas emissions scenario cannot be ruled out due to deep uncertainty in ice sheet processes.”

• The IPCC gives more consideration to the large long-term sea-level rise beyond the year 2100.

And here is the key sea-level graphic from the Summary for Policy Makers:

This is a pretty clear illustration of how sea level starts to rise slowly; but in the long run, sea-level rise caused by fossil-fuel burning and deforestation in our generation could literally go off the chart and inundate many coastal cities and wipe entire island nations off the map. But first things first.

Observed Past Rise

Let’s dive a little deeper into the full report and start with the observed sea level change. Since 1901 sea level has risen by 20 cm, a rise unprecedented in at least 3,000 years (disclosure: I co-authored some of the research behind the latter conclusion).

Since 1900 the rise has greatly accelerated. During the most recent period analyzed, 2006-2018, it’s been rising at a rate of 3.7 mm/year – nearly three times as fast as during 1901-1971 (1.3 mm/year). The IPCC calls this a “robust acceleration (high confidence) of global mean sea level rise over the 20th century”, as did the SROCC in 2019.

The finding of sea-level acceleration is not new. The AR4 already concluded in 2007: “There is high confidence that the rate of sea level rise has increased between the mid-19^th and the mid-20^th centuries.” And the AR5 found in 2013 that “there is high confidence that the rate of sea level rise has increased during the last two centuries, and it is likely that global mean sea level has accelerated since the early 1900’s.” (Which has not stopped “climate skeptics” from repeatedly claiming a lack of acceleration.)

The reason for earlier hedged wording by the IPCC was the possibility of natural decadal variability affecting the trend estimates, but the AR6 now concludes “that the main driver of the observed global mean sea-level rise since at least 1970 is very likely anthropogenic forcing”. That is the result of so-called “attribution studies” – attempts to differentiate with the help of a combination of data, models, pattern detection and statistics between all possible human-caused and natural factors in the observed changes. However, on the level of basic physical reasoning, it is of course a no-brainer that warming will cause land-ice to melt (and melt faster as it gets hotter) and ocean waters to expand, so sea-level rise is the inevitable result.

And there is this:

New observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components contributing to sea level rise, including expansion due to ocean warming and melting of glaciers and ice sheets (high confidence).

IPCC AR6

That’s an important consistency check; the independent data add up to the overall observed rise.

The Future Until 2100

It is virtually certain that global mean sea level will continue to rise over the 21st century in response to continued warming of the climate system.

IPCC AR6

By how much? That depends on our emissions and is shown in the following figure. The take-away message is: for high emissions we’d likely get close to a meter, sticking to the Paris agreement would cut that down to half a meter.

And how does that compare to the recent previous reports? Here is the comparison the IPCC shows:

If you look at the 2100 projections for the last three reports (AR5, SROCC, AR6) you can see that the numbers have increased each time – and remember that the AR5 numbers had already increased by ~60% compared to the AR4. This illustrates the fact that IPCC has been too “cautious” in the past (which is not a virtue in risk assessment), having to correct itself upward again and again (all the while “climate skeptics” try to paint the IPCC as “alarmist”, for want of any better arguments to play down the climate crisis).

Related to that are notable changes in grappling with uncertainty and risk. The IPCC is now showing very likely (5-95 percentile) as well as likely (17-83 percentile) ranges. In the AR5, it had made the rather ad-hoc argument that “global mean sea level rise is likely (medium confidence) to be in the 5 to 95% range of projections from proces-based models”. So their likely range was actually the modelled very likely range.

The IPCC now splits the uncertainty into two types, hence the two different shadings in the uncertainty bars, in an attempt to also cover uncertainty in processes which we still cannot confidently model. They write:

Importantly, likely range projections do not include those ice-sheet-related processes whose quantification is highly uncertain or that are characterized by deep uncertainty. Higher amounts of global mean sea level rise before 2100 could be caused by earlier-than-projected disintegration of marine ice shelves, the abrupt, widespread onset of Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI) around Antarctica, and faster-than-projected changes in the surface mass balance and dynamical ice loss from Greenland. In a low-likelihood, high-impact storyline and a high CO2 emissions scenario, such processes could in combination contribute more than one additional meter of sea level rise by 2100.

Note that this uncertainty goes to one side: up. For estimating this uncertainty they use an expert survey as well as a smaller but more detailed structured expert judgement. I co-authored the survey (see also 7-minute video about it) with Ben Horton and others, as well as a predecessor survey published in 2014, and I am happy to see that the IPCC now includes this type of expert judgement to assess risks that can’t yet be modelled reliably, but cannot be just ignored either. In dealing with the climate crisis, it simply is not enough to consider what is likely to happen – it is even more important to understand what the risks are.

Think about it: If someone builds a nuclear facility near to your house, would you be satisfied with knowing that it is “likely” to work well (say, 83% certain)? Or would you like to know about a few percent chance that it could blow up like Chernobyl in your lifetime?

With the high-end risk scenarios, the IPCC is catching up with other assessments such as the US National Climate Assessment of 2017, which already showed a “high” scenario of 2 meters and an “extreme” scenario of 2.5 meters of rise by 2100.

The Long Term Future

One of the headline statements of the AR6 is:

Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level.

IPCC AR6

That’s because huge ice sheets take a long time to melt in a warmer climate, and the ocean waters take a long time to warm up as you go further down, away from the surface. So by what we are doing now in the next couple of decades we determine the rate and amount of sea-level rise for millennia to come, condemning many generations to continually changing coastlines and forcing them to abandon many coastal cities, large and small. That we cannot turn this back is the reason why the precautionary principle should be applied to the climate crisis.

Just look at the ranges expected by the year 2300, in the right-hand panel of the first image above. Even in the blue mitigation scenario, which limits warming to well below 2 °C, our descendants may well have to deal with 2-3 meters of sea-level rise, which would be catastrophic for the people living at the world’s coastlines. Not only would it be extremely hard and costly – if possible at all – to defend cities like New York during a storm surge with a so much higher sea level. We would see massive coastal erosion happening all around. And remember that “nuisance flooding” is already causing real problems after just 20 cm of sea-level rise, for example along the eastern seaboard of the US!

At least with this Paris scenario and a good portion of sheer luck, we may get away with less than a meter rise. But with further unmitigated increase in emissions, a desastrous 2 meter rise is about as likely as an utterly devastating 7 meter rise. What would our descendants think we were doing?