The most interesting thing about Frederikse et al. is that not only do they publish a new sea level reconstruction based on tide gauge data, to reckon how much sea level has risen, they also attempt to reckon where that sea level rise came from.
Here’s their estimate of sea level since 1900:
And here’s what it says about the rate of sea level rise, according to my usual analysis: a lowess smooth in red, and PLF (Piece-wise Linear Fit) in blue:
Several things are clear. First, the rate of sea level rise has changed over the years, sometimes faster (the 1910s, 1930s, 1940s, 1990s, 2000s), sometimes apparently not even rising (the 1920s, 1960s), but the 2010s are “off the chart,” a colorful way of saying that sea level rise was significantly faster than previous decades.
The latest rate, according to these data, is about 5 mm/yr. Over a century, that’s half a meter (about 20 inches). That’s the global amount, lots of places will see more or less because of local conditions (particularly, vertical land movement). It seems to me to be extremely unlikely, downright implausible in fact, that the average over this century will be less than that.
Perhaps more important is that Frederikse et al. also attempted to quantify the causes of sea level rise. Mainly, those are put in three categories: steric (thermal expansion of sea water), melting of land ice (glaciers and ice sheets), and terrestrial water storage (TWS). The contribution of land ice melt is further subdivided into three categories: glaciers, Greenland, and Antarctica.
Here is their estimate of each factor’s contribution to sea level rise since 1900:
Glacier melt has raised sea level more since 1900 than any other factor, but has not shown much recent sign of acceleration, and appears to be slower now than in the first half of the century. Greenland melt was also a major factor in the early 20th century, and has shown signs of both deceleration and acceleration with high rates recently. Steric change caused sea level fall in the first decade of the 1900s, but has risen steadily since and is now moving fast. TWS has caused an overall drop in sea level since 1900, of about 10 mm. Antarctica has countered with about 10 mm net contribution to sea level rise, but took quite a while to get started.
We can also gain some insight by shifting all the curves, to show each factor’s contribution to sea level rise since 2000:
Steric is in the lead; Greenland may have contributed the most since 1900, but since 2000 the “big gun” has been thermal expansion of sea water. Glaciers are still at it, and so is Greenland, but two factors are now behaving differently than before. TWS is no longer lowering sea level, it’s raising it, and Antarctica was doing almost nothing until recently, but has now started making its contribution felt.
I applied the same analysis to the contributions, as I did to the sea level data itself. Fascinating results emerge, and the most striking is the rate of sea level rise/fall due to Antarctic ice melt:
This leads me to suspect that much of the reason behind the enhanced rate of sea level rise in the last decade is that after nearly a century of inaction, Antarctica has started to kick in.
In the long run (to the end of this century and beyond), we don’t expect TWS to be a major factor. Thermal expansion will continue, as fast or faster than now but not tremendously so; it takes a long time for heat to penetrate deep into the ocean. Glaciers will keep melting, but probably not much faster than now if at all. The fact is that even if we melt all the world’s alpine glaciers and the extra heat penetrates far into the ocean, we’ll still only get about 1 meter of sea level rise, and it will take a long time — longer than this century — for that to happen.
Then there are the big boys: Greenland and Antarctica. If all of Greenland melts there’s 7 meters of sea level rise, and if Antarctica goes, over 50.
Greenland and Antarctica aren’t going to melt this century. But they might dramatically increase the rate at which they discharge ice into the ocean. According to Richard Alley, a leading glaciologist, this tends to happen where the ice and the ocean meet, and the two phenomena which bring it about are the disintegration of floating ice sheets, and calving-cliff retreat.
When an ice sheet disintegrates it tends to happen quickly; we’ve seen the picture of the Larsen B ice shelf falling apart in a matter of days, after having survived many thousands of years. When it did, it no longer acted as a “buttress” to hold back land-based ice from reaching the sea. The flow rate just about doubled — in the twinkling of an eye, geologically speaking.
Computing how it works is straightforward, in that we know all the physics. It’s also impossible, in that there are too many variables and processes to keep track of them all, even with a supercomputer. So, models are developed to simulate processes, and they’re getting better — we’ve actually learned a lot — but according to Richard Alley, we still have a lot to learn before we’re ready to make reliable predictions.
When the rates at which ice is discharged double or triple, if that happens in enough places — and especially in east Antarctica — the rate of sea level rise could become terrifyingly high. Imagine 30 mm/yr — six times the present rate — more than an inch per year. That’s 3 meters per century, and 3 meters is about 10 feet so goodbye, Miami. Goodbye, New Orleans. Goodbye, a lot of places.
And, according to Richard Alley … this is what keeps him up at night.
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“melt all the world’s alpine glaciers and the extra heat penetrates far into the ocean”
I have waded through glacier meltwater. It is not going to warm up the ocean.
Hi, Keith. Glacial meltwater, or melting mountain ice and seasonal snowpack, makes up the headwaters of the Ganges, Mekong, Amazon, Brahmaputra, Indus and other major rivers.
That meltwater isn’t still chilled after flowing down all the miles to the sea.
Glaciers terminating at sea level are a different story – which is why ice loss from Greenland is worrisome.
Perner, K., Moros, M., Otterå, O.H. et al. An oceanic perspective on Greenland’s recent freshwater discharge since 1850. Sci Rep 9, 17680 (2019). https://doi.org/10.1038/s41598-019-53723-z
Keith, you must have misread the ‘and’. It’s about adding two factors together: glaciers + tms = only 1 meter. Not a statement that glacier melt heats up the ocean.
That said, I’ve actually dove in sub zero C seawater and watched fresh water runoff immediately turn to slush as it ran into the seawater.
This warmed the seawater a bit what with the phase change.
Really important and well-written post, Grant. Thanks for all the work you do to clarify things like this for us.
You present an interesting breakdown, tamino…thanks.
The impact of Antarctica may be sooner than we all thought:
The paper by Fredrikse is very informative, but it does only include data up til 2018, and (as you can see at page 394), only observations from GRACE. However, the data from GRACE-follow-up – allthough they are for a very short period yet, do not support a continued accelerated contribution through ocean mass (which includes Greenland, Antarctica, Glaciers and terrestial WS). Any estimation of ocean mass contribution before 2002 is with large uncertainty, and after 2015, the trend has no longer been increasing.
This probably relates to a less intense melting on Greenland, which peaked in 2012, as can be seen in the report from DMI , see fig. 2 http://polarportal.dk/fileadmin/user_upload/PolarPortal/season_report/polarportal_saesonrapport_2020_EN.pdf
This is also quite evident in NASA’s sea level pages, where you can find the last observations on ocean mass here; https://sealevel.nasa.gov/understanding-sea-level/key-indicators/ocean-mass
Hence, it remains to be seen whether ocean mass will be a driver of accelerated sea level rise in the next decades.
Tor Ole writes: “Hence, it remains to be seen whether ocean mass will be a driver of accelerated sea level rise in the next decades.”
But what do you think? Do you think ocean mass will be a driver of accelerated sea level rise in the next decades? We know the global average surface temperature is increasing; we know the Greenland ice sheet is losing mass, and we know the Antarctic ice sheet is losing mass. Are you predicting the amount of precipitation mass retained on the land surface will increase by the amount of ice mass loss from the two ice sheets?
Should we depend on that happening? Should we let it “remain to be seen?” What would you do right now, if you were in charge?
Martin, you write; “But what do you think? Do you think ocean mass will be a driver of accelerated sea level rise in the next decades? We know the global average surface temperature is increasing…”
Yes we now that global average temperatures are increasing – approximately by 0,2 C per decade, and have kept this pace from approximately 1980 (or 1975). This may suggest that sea level will continue to rise by 3,5-4,5 mm/year, but hardly that the increase must accelerate? Ocean heat content and thermosteric sea level rise appear rather unchanged the last 20-30 years.
As regards the arctic temperature rise, the last decades have shown a much larger increase than the global average, but as I showed you regarding Greenland mass loss, there may be indications of a slower increase the last 10-15 years, for instance in Greenland temperatures, as reported in a recent publication; https://rmets.onlinelibrary.wiley.com/doi/10.1002/joc.6771
(from the abstract:) ” We provide an updated analysis of instrumental Greenland monthly temperature data to 2019, focusing mainly on coastal stations but also analysing ice-sheet records from Swiss Camp and Summit. Significant summer (winter) coastal warming of ~1.7 (4.4)°C occurred from 1991–2019, but since 2001 overall temperature trends are generally flat and insignificant due to a cooling pattern over the last 6–7 years”.Flat tempertures since 2001 means that Greenland in this period has warmed slower than the global average.
I think we can all assume that the RCP 8.5 now is quite unlikely, and hopefully emissions will be lowered gradually- perhaps corresponding to somewhere between the RCP 4.5 and RCP 2.6 scenario (I admit 4,5 probably most likely). https://www.i4ce.org/where-do-the-five-new-ipcc-scenarios-come-from-climate/ .
In that case, I do not think that there is any clear indications of further acceleration in sea level rise the next 2-4 decades, allthough the contribution from Antarctica may be the least predictable factor.
“Wait and see” is perfectly valid in science when definitive data are not in. “Wait and see” is perfectly INvalid in risk analysis.Which domain to you purport to inhabit?
Tor Ole writes: “Yes we now that global average temperatures are increasing – approximately by 0,2 C per decade, and have kept this pace from approximately 1980 (or 1975). This may suggest that sea level will continue to rise by 3,5-4,5 mm/year, but hardly that the increase must accelerate? Ocean heat content and thermosteric sea level rise appear rather unchanged the last 20-30 years.”
I want to ask you why a constant increase in global average temperature “may suggest that sea level will continue to rise by 3,5-4,5 mm/year,” but first a couple of points:
Tamino just published his analysis showing that multiple data sets show an increase in the rate of global average temp increase post 2000, so I don’t think you can say it is increasing at a constant rate:
Nor has ocean heat content been “rather unchanged.” See here:
But ok, let’s use your assumption about global average temp: It is increasing at a constant rate. And thermosteric sea level rise may be stable as well, but upper ocean heat content is clearly increasing. Why do you say this suggests that sea level will continue to rise by 3,5-4,5 mm/year? I think it does not suggest that.
For Greenland, the average surface temp over the ice is still increasing, if only at your assumed constant rate. Then the temp over the Greenland ice sheet is increasing, and that air is moving. Ice melts faster as the temperature of the air/water around it increases, when the air/water is moving. I think that suggests the melt rate of the Greenland ice sheet will increase even if the increase in global average temp remains constant.
For Antarctica, we are more interested in the increasing temperature of the ocean around the edges where the great glaciers terminate, not so much the air temperature over the entire ice sheet. It appears that the ice shelves are getting ready to collapse, exposing the glaciers to relatively warm (increasingly so) ocean currents. I think that suggests we should expect some major sudden ice losses from those glaciers.
Tor Ole wrote: “I admit 4,5 probably most likely.”
So your answer to my question: “What would you do right now, if you were in charge?” is that you would not try to hold the global average temp increase to 1.5C. You would take the more relaxed approach of RCP 4.5.