Understanding Sea Level Rise 6: SLR benefits of keeping warming below 1.5 degrees (IPCC & Pattyn)

If you do one about agriculture, you gotta do one for your sea level rise series too, we heard you say.

Sea level rise projections IPCC SR15 - comparing 1.5 & 2 degrees
Well, here it is. A brief overview of relevant statements derived from IPCC’s special report on 1.5 degrees (IPCC SR15) supplemented with an interesting more recent study about ice sheet tipping points.

At first glance, if you read the sea level rise paragraph (3.3.9) of chapter 3, IPCC SR15’s chapter about climate change impacts, you may think it does not offer a very strong additional argument for raising the global climate policy ambition from a 2 degrees to a 1.5 degrees target. That is the most likely difference between these two mitigation pathways is ‘just’ 10 centimetres of sea level rise by the year 2100.

Now of course if you’re already close to an adaptation threshold, 10 centimetres of extra sea level rise can make a big difference. But in itself – well – it’s 10 centimetres.

A more notable thing to take home from IPCC SR15 sea level paragraph are the raised upper limits for 21st century sea level projections, also at these relatively strong mitigation scenarios. Limiting warming to 1.5 degrees can still lead to 80 centimeters of global average sea level rise, while a 2 degrees scenario can even lead to a full metre of sea level rise by the year 2100 – as we read on page 207:

“There is medium confidence that GMSL rise will be about 0.1 m (within a 0.00–0.20 m range based on 17–84% confidence-interval projections) less by the end of the 21st century in a 1.5°C compared to a 2°C warmer world. Projections for 1.5°C and 2°C global warming cover the ranges 0.2–0.8 m and 0.3–1.00 m relative to 1986–2005, respectively ( medium confidence ).”

An ice sheet tipping point can make a bigger difference

However if you keep reading the chapter until you reach the special paragraph about tipping points associated with abrupt climate change, there’s more to learn about the possible sea level rise differences between the two mitigation scenarios.

Greenland ice sheet tipping point - Nature Climate Change graph
Various interacting mechanisms that favour ice loss and ice growth are temperature dependent, and combined lead to the possibility of melting tipping points for major ice sheets, like the Greenland ice sheet graphed above. Taken from 2018 publication in Nature Climate Change.

This SR15 paragraph includes interesting statements about possible tipping points of major ice sheets. Then we are talking about the very long term, and a much larger difference between the risks of a 1.5 and 2 degrees scenario. Below we quote from paragraph, page 271, about sea level rise beyond the year 2100:

“Policy decisions related to anthropogenic climate change will have a profound impact on sea level, not only for the remainder of this century but for many millennia to come (Clark et al., 2016). On these long time scales, 50 m of sea level rise (SLR) is possible (Clark et al., 2016). While it is virtually certain that sea level will continue to rise well beyond 2100, the amount of rise depends on future cumulative emissions (Church et al., 2013) as well as their profile over time (Bouttes et al., 2013; Mengel et al., 2018). Marzeion et al. (2018) found that 28–44% of present-day glacier volume is unsustainable in the present-day climate and that it would eventually melt over the course of a few centuries, even if there were no further climate change. Some components of SLR, such as thermal expansion, are only considered reversible on centennial time scales (Bouttes et al., 2013; Zickfeld et al., 2013), while the contribution from ice sheets may not be reversible under any plausible future scenario (see below).”

Now if we get to the assessments of the actual melting tipping points for Greenland and Antarctica, we find these may be very low – in fact within the range between 1.5 and 2 degrees global average temperature rise. For Greenland the lowering ice height feedback may be crucial, whereas for Antarctic tipping points marine melting feedbacks are thought to be dominant:

“For Greenland, where melting from the ice sheet’s surface is important, a well-documented instability exists where the surface of a thinning ice sheet encounters progressively warmer air temperatures that further promote melting and thinning. A useful indicator associated with this instability is the threshold at which annual mass loss from the ice sheet by surface melt exceeds mass gain by snowfall. Previous estimates put this threshold at about 1.9°C to 5.1°C above pre- industrial temperatures (Gregory and Huybrechts, 2006). More recent analyses, however, suggest that this threshold sits between 0.8°C and 3.2°C, with a best estimate at 1.6°C (Robinson et al., 2012). The continued decline of the ice sheet after this threshold has been passed is highly dependent on the future climate and varies between about 80% loss after 10,000 years to complete loss after as little as 2000 years (contributing about 6 m to SLR).”

Also for the West-Antarctic ice sheet and large portions of the East-Antarctic ice sheet possible tipping points for complete melting are assessed. The precise position of associated thresholds is highly uncertain and dependent on future emissions. According to IPCC SR15 under the most favourable emissions scenario (concentration pathway RCP2.6) passing of Antarctic tipping points may still be prevented:

“The Antarctic ice sheet, in contrast, loses the mass gained by snowfall as outflow and subsequent melt to the ocean, either directly from theunderside of floating ice shelves or indirectly by the melting of calved icebergs. The long-term existence of this ice sheet will also be affected by a potential instability (the marine ice sheet instability, MISI), which links outflow (or mass loss) from the ice sheet to water depth at the grounding line (i.e., the point at which grounded ice starts to float and becomes an ice shelf) so that retreat into deeper water (the bedrock underlying much of Antarctica slopes downwards towards the centre of the ice sheet) leads to further increases in outflow and promotes yet further retreat (Schoof, 2007). More recently, a variant on this mechanism was postulated in which an ice cliff forms at the grounding line and retreats rapidly though fracture and iceberg calving (DeConto and Pollard, 2016). There is a growing body of evidence (Golledge etal., 2015; DeConto and Pollard, 2016) that large-scale retreat may be avoided in emissions scenarios such as Representative Concentration Pathway (RCP) 2.6 but that higher-emissions RCP scenarios could lead to the loss of the West Antarctic ice sheet and sectors in East Antarctica, although the duration (centuries or millennia) and amount of mass loss during such a collapse is highly dependent on model details and no consensus exists yet.”

Nonetheless also for Antarctica it is supposed that tipping points for complete melting of major ice sheet sections can lie between 1.5 and 2 degrees global average temperature rise:

“The long-term committed future of Antarctica and the GMSL contribution at 2100 are complex and require further detailed process-based modelling; however, a threshold in this contribution may be located close to 1.5°C to 2°C of global warming.”

Summarising the above there is a real chance that tipping points for melting of both Greenland and Antarctica could lie between 1.5 and 2 degrees. It is difficult though to attach specific values for global average sea level rise to these tipping points:

“In summary, there is medium confidence that a threshold in the long-term GMSL contribution of both the Greenland and Antarctic ice sheets lies around 1.5°C to 2°C of global warming relative to pre-industrial; however, the GMSL associated with these two levels of global warming cannot be differentiated on the basis of the existing literature.”

Well, ‘existing literature’ of course keeps expanding. Shortly after IPCC SR15 another interesting ice sheet tipping point study was published in Nature Climate Change, by an international research group led by Frank Pattyn of the Université libre de Bruxelles. And it sort of echoes these same values in their assessment of possible melting thresholds for Greenland and Antarctica:

“[…] large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5–2.0 °C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance–elevation feedback, whereas for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening.”

So, yes, also when it comes to sea level rise – limiting global average warming to 1.5 degrees instead of 2 degrees, can make a big difference (in the very long run).

© Rolf Schuttenhelm | www.bitsofscience.org

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