In between our series about the global temperature trend and our (upcoming) series about climate & biodiversity, let’s do a short series about sea level rise, shall we?
Because sea level science is another subject that is so easy to misunderstand, again largely due to inert processes and complicated feedbacks – having to count from millimetres (per year, current rise) to possibly tens of metres (eventual sea level rise under official UN temperature targets), depending as always on time scale – and somewhere in between coming to grasps with different projections of speed and forecasts of acceleration, ranging from ‘consensus science’ (IPCC) to cutting edge peer-reviewed publications about ice sheet dynamics (Hansen, 2015/2016) – and from model studies to paleoclimate comparisons.
Anyway – let’s get started. Put your goggles on, we’re going to try to snorkel through some relevant science!
A couple of things to know about water density
What better way than to start with looking at sea level rise caused by thermal expansion. The sea level rise that does not look at Earth science, but at physics instead – an interesting characteristic of water (and many other substances), namely that is expands as it warms.
(In fact, you would indeed expect water to expand as it warms, and shrink as it cools. You would then also expect this contraction of water to continue as it cools further to solid state (freezing) and be heaviest as ice. Simple physics would dictate a sunken ice floor at the bottom of the Arctic Ocean, not one floating on top. But here water is odd – it also expands (and quite dramatically) when it turns to ice – in fact water is the densest and therefore heaviest around +4 degrees Celius, as illustrated in the graph on top. And that’s why ice skating exists.)
Sea level rise caused by thermal expansion is relatively well understood
First of all let’s be clear: sources of sea level rise add up. Climate change (warming) causes both small glaciers and large ice sheets to melt – a process that adds extra water to the world’s oceans, raising the sea level. The simultaneous process of ocean warming meanwhile leads to the mentioned expansion – therefore an additional sea level rise.
In its latest (2013) report the IPCC states predictions for thermal expansion sea level rise have smaller uncertainty than predictions for ice melting-induced sea level rise – expressing ‘high confidence’ in the first and ‘medium confidence’ in the second (we’ll get to that later).
Observations of past (20th century) sea level rise illustrate this – a relatively steady line for thermal expansion, compared to a more shaky line for additional sea level rise contributions from small glaciers and large ice sheets – see the below NASA graph:
Here you see how thermal expansion (depending on emissions scenario/amount of caused warming) contributes to 21st century sea level rise, ranging roughly between 15 to 30 centimetres from RCP2.6 to RCP8.5 – according to IPCC AR5:
Taking it one step further. From millimetres to decimetres, to metres – all without including ice loss(!)
Thermal expansion sea level rise is a slow process (because it takes very long for the oceans to warm to their full (±4km) depth), adding decimeters this century – yet several meters after 2100.
One of the things that makes communicating sea level rise so hard, is you never know what to aim for. Should you tell how fast the seas are rising now? How high they might be by the end of this century – or should you tell a totally different story, the full story?
We find that 2100 line a bit artificial. What if the people during the renaissance would have put a fix on the future in the year 1600 – that would have been a bit unpractical wouldn’t it?
The full story is that a large chunk of the CO2 we emit will remain in the atmosphere for a very long time and that all the extra heat this absorbs will stay in system Earth for even longer (as it slowly sinks to deeper ocean waters!) and that ice sheets like the one on Greenland and West Antarctica will likely keep melting until they are fully gone, once the process has reached a certain speed.
This is where we get to estimates of final sea level rise (for a given amount of emitted CO2 – or for any ‘preferred’ temperature rise limit, for instance the 1.5/2 degrees target) and suddenly you should realise we are in fact talking about something in the order of 25 metres – if indeed (very unlikely!) we manage to prevent further warming, warming beyond 2 degrees that is.
Now if you’d do a quick reversed calculation and remove added water from a disappeared Greenland and West-Antarctic ice sheet we’re still left with (±25 minus ±12 =) some 13 metres of unaccounted sea level rise, judging by paleoclimate comparisons.
Now of course uncertainty margins are high – and there are indeed other potential sources of sea level rise (East Antarctic ice sheet to name one, also precipitation changes) – but let’s at least understand that if the entire warming of ‘just’ 2 degrees would lead to several metres of sea level rise, ‘only’ coming from thermal expansion of the oceans – as illustrated by this somewhat older (2001) IPCC graph:
To be a bit more scientific about the matter before we end, let’s quote an interesting study by the Potsdam Institute for Climate Impact Research, published in 2013 in PNAS (called ‘The Multi-millenial sea level commitment of global warming’). This research group thinks of the final future sea level rise Greenland would contribute about 25 percent, Antarctica (combined) about 50 percent, smaller glaciers about 5 percent – and thermal expansion about 20 percent.
© Rolf Schuttenhelm | www.bitsofscience.org