Understanding Sea Level Rise, p3: it’s about timescales, speed of change & carbon budget – world aims for 29-55m sea level rise

In our previous article of the series we’ve looked at an overview of global sea level rise forecasts for the year 2100 – and seen that these forecasts have a very large spread, and also seem to increase with time of publication (from problematic (decimetres) to catastrophic (multiple metres)).

Today we show ‘the full story of anthropogenic sea level rise’ – a story that requires us to zoom out to much larger timescales, to learn to respect the immense thermal inertia of oceans and ice sheets while simultaneously coming to terms with the full magnitude of the sea level rise that our CO2 emissions in the 20th and 21st century are causing: that is 29 to 55 metres in total, depending on the amount of fossil fuels we choose to continue to dig up and burn.

Best sea level rise graph ever - Nature Climate Change
Almost no one seems to be able to place sea level rise, as a consequence of climate change, in its proper scientific context. But if you only learn to properly interpret this one Nature graph you’d be a shiny exception to that rule..!

The above graph comes from ‘Consequences of twenty-first-century policy for multi-millenial climate and sea-level change.’ We think it’s one of the best climate studies of 2016. It was published in Nature Climate Change in February by a group of no less than 22 (paleo) climate scientists from 21 different universities and other climate research institutions.

Sea level changes in response to climatic changes, it clearly illustrates, are intrinsically slow – following the large thermal mass of oceans and ice sheets. But if you look at the millennium-scale you see that sea level changes to several degrees of warming are also very large – and what happened between the cold and mild state of Earth (Pleistocene-Holocene boundary) could repeat itself between the mild and hot state of Earth (Holocene-Anthropocene boundary – following human carbon emissions).

The big similarity is the possibility of tens of metres of sea level rise. The big uncertainty? Look at the bottom part of the graph: the speed of the change. After the Last Glacial Maximum (coldest part of last ice age, about 18,000 years ago) the sea levels seemed to rise steadily for a very long time. But newer research shows within this steady change there may have been very rapid hiccups too – in both directions – like the Younger Dryas, some 12,000 years ago. After the Holocene there is a clear possibility for even more rapid changes – as the change in climate forcers (CO2 rise now versus Milanković-cycles after Pleistocene) is also happening much faster – faster even than at any known time in geological history.

‘Consequences of twenty-first-century policy for multi-millenial climate and sea-level change’

There’s a lot of public confusion about sea level rise. We measure an annual growth of about 3 millimetres per year, adding up to decimetres over the course of this century – or possibly multiple metres, judging by forecasts that include ice sheet dynamics and other melting feedbacks to the equation. And then there’s the final sea level rise – the most likely new equilibrium state following the rise in atmospheric CO2 that is the result of our current emissions, an equilibrium that would be tens of metres higher – also an irreversible change to system Earth for tens of thousands of years.

Now try to take political responsibility for that – and chose one of the following 21st-century emissions scenarios: 1,280, 2,560, 3,840, 5,120 gigatonnes (petagrammes) of carbon.

Those concentrations the 22 authors of the Nature Climate Change study link to a climatic warming of 2 degrees (for 1,280Gt) to 7.5 degrees* Celsius warming (for 5,120Gt).

[*) We think that’s slightly optimistic still. The authors use ‘equilibrium climate sensitivity’ (ECS) of 3.5 degrees to calculate the warming for different carbon budgets. In the short run (multiple decades) that’s close enough to our climate sensitivity experts assessment (“close to 3 degrees, possibly a bit higher”) – but it ignores substantial thermal inertia ‘beyond ECS’ – warming we’ve tried to place in context in the concluding part of our global temperature trend series (under paleoclimate & ‘Earth System Sensitivity’).]

Under 2 degrees warming – they deduce from the paleorecord [further reading: Pliocene sea level rise, Eemian interglacial sea level rise, ‘MIS11’ interglacial sea level rise] and climate models – in the long run sea levels would rise by: 0.8m for thermal expansion (peaking sooner at 1.1m), 0.25m for small (mountain) glaciers, 4m from Greenland* melting and 24m from Antarctica (West and East Antarctic ice sheets combined) – to a total of 29 metres.

[*) That too might be a conservative estimate. Other studies suggest the temperature tipping point for complete melting of the Greenland ice sheet might be as low as 1.6 degrees Celsius global average warming.]

Under business as usual emissions followed by runaway warming (carbon feedbacks) to 7.5 degrees in the long run sea levels would rise by: 2.8m for thermal expansion (peaking at 3.4m), 0.25m for small (mountain) glaciers, 7m from Greenland melting and 45m from Antarctica – to a total of 55 metres sea level rise.

Certainty: all mountain glaciers disappear – most rapid sea level rise in first centuries

In this 5,120 Gt scenario even Antarctica would turn to a largely ice-free state – and Greenland would be fully ice free within 2,500 years. In both the best and the worst warming scenario all the world’s small mountain glaciers disappear.

Although this shows the full extent of the ‘final’ Holocene-Anthropocene sea level rise – a change that reaches its full extent after about 10,000 years, it is also important to note that under all scenarios the most rapid part of this sea level rise takes place in the first centuries.

How rapid exactly? That’s what we will try to investigate in the next part of the sea level rise series.

© Rolf Schuttenhelm | www.bitsofscience.org

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