Reconciling recent ice mass balance estimates for Antarctica

This post is a slight modification and extention of a comment made at the cliscep post The Latest Antarctic Ice Sheet Alarmist Con

As a (slightly manic) beancounter I like to reconcile data sets. The differing estimates behind the claims of accelerating ice mass loss in Antarctica do not reconcile, nor with sea level rise data.
The problem of ice loss needs to be looked at in terms of the net of ice losses (e.g. glacier retreat) and ice gains (snow accumulation). Any estimate then needs to be related to other estimates. The Guardian article referred in the  cliscep post states

Separate research published in January found that ice loss from the entire Antarctic continent had increased six-fold since the 1980s, with the biggest losses in the west. The new study indicates West Antarctica has caused 5mm of sea level rise since 1992, consistent with the January study’s findings.

The paper is Rignot et al 2018 “Four decades of Antarctic Ice Sheet mass balance from 1979–2017“. The abstract states

The total mass loss increased from 40 ±
9 Gt/y in 1979–1990 to 50 ± 14 Gt/y in 1989–2000, 166 ± 18 Gt/y
in 1999–2009, and 252 ± 26 Gt/y in 2009–2017. In 2009–2017,
the mass loss was dominated by the Amundsen/Bellingshausen
Sea sectors, in West Antarctica (159 ± 8 Gt/y), Wilkes Land, in
East Antarctica (51 ± 13 Gt/y), and West and Northeast Peninsula
(42 ± 5 Gt/y). The contribution to sea-level rise from Antarctica
averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ±
2.0 mm since 1979, including 6.9 ± 0.6 mm from West Antarctica,
4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the
Peninsula (i.e., East Antarctica is a major participant in the mass
loss).

Jaime @ 19 May 19 at 7:56 am points to a New Scientist article in January claiming that Antartica ice loss has trebled. The underlying article is from Nature – The IMBIE Team – Mass balance of the Antarctic Ice Sheet from 1992 to 2017. The abstract states

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimeters (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5 ± 46 billion tonnes per year) being the least certain.

The key problem is in the contribution to sea level rise. The Rignot study from 1979-2017 gives 3.6 mm a decade from 1989-2017, about 4.1 mm and from 1999-2017 about 5.6 mm. The IMBIE team estimates over the period 1992-2017 7.9 mm sea level rise, or 3 mm per decade. The Rignot study estimate is over 50% greater than the IMBIE team. Even worse, neither the satellite data for sea level rise from 1992, nor the longer record of tide gauges, show an acceleration in sea level rise.

For instance from NOAA, the satellite data shows a fairly steady 2.9mm a year. rise in sea levels from 1992.

Using the same data, the University of Colorado estimates the average sea level rise to be 3.1 mm a year.

Note that in both the much greater variability in the Jason 2 data, and the slowdown in rise after 2016 when Jason 3 started operating.

The tide gauges show a lesser rate of rise. A calculation from 155 of the best tide gauges around the world found the mean and median rate of sea level rise to be 1.48 mm/yr. 

Yet, if Rignot is correct in recent years Antarctic ice loss must now account for around 22-25% of the sea level rise (satellite record) or almost 50% (tide gauges) of the measured sea level rise. Both show no accleration. What factors have a diminishing contribution to sea level rise over the last 25 years? It cannot be less thermal expansion, as heat uptake is meant to have increased post 2000, more than offsetting the slowdown in surface temperature rise when emissions accelerated. 

Kevin Marshall

Postscript

This is not the first time I have covered rather extreme claims in one of Prof Eric Rignot’s estimates of accleration in ice melt. Six years ago I looked at Rignot et al 2011 – Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise

I compared the 12 monthly rise in sea surface temperatures with the corressponding chart of ice mass balance loss for Greenland and Antarctica. The peaks and troughs corressponded nicely, with about 18 months between ice loss and sea level rise. This is quite remarkable considering that from Rignot et 2011 in the 1990s ice loss would have had very little influence on sea level rise. It is almost as though the modelling has taken the sea level data, multiplied by 360, flipped it, moved it back a few months then tilted to result show acceleration. 

Yet the acceleration of 14.5 ± 2 Gt/yr2 for Antarctica results in decadal increases not too dismillar from those in the abstract of Rignot et al 2018. This would validate the earlier results except for another paper. Shepherd et al Nov 2012 – Reconciled Estimate of Ice-Sheet Mass Balance had a long list of authors including Rignot and three of the four co-authors of the Rignot et al 2011. It set the standard for the time, and was  the key article on the subject in IPCC AR5 WG1. Shepherd et al Nov 2012 has the following Table 1.

For Antartica as  experienced no significant acceleration in ice mass loss in the period 1992-2011. 

Velicogna 2009 and Chen et al 2009 on Acceleration in Antarctic Ice Melt

This blog post started out as some musings on the different way of measuring the changes in the mass of Antarctic land ice, as a follow up to a couple of comments to Jo Nova’s posting “Antarctica gaining Ice Mass — and is not extraordinary compared to 800 years of data.” The problem with this is that it looks at just part of the total ice mass balance. These lead me to look at the major papers that looked to Total Mass Balance. There are two from 2009, using early data from the GRACE satellite gravity mission Velicogna and Chen et al. In comparing the various estimates, I discovered three anomalies that should have been detected as part of the peer review process.

Error in Velicogna Summary

The abstract notes

In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009. In Antarctica the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009, i.e., an acceleration of −26 ± 14 Gt/yr2 in 2002–2009.

When I tried to replicate this for Greenland, the figures worked out. Starting with 122 Gt/yr a year ice loss in 1992 and adding 30 to each year gives the “137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009“. But for Antarctica, adding 26 to each year cannot give “the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009“. However, if the statement is rephrased with the Greenland timescales as “the mass loss increased from 104 Gt/yr in 2002–2003 to 246 Gt/yr in 2007–2009” then the numbers work out.


The spread sheet is easy to construct. For Velicogna Antarctica, start with -90 in 2002 and subtract 26 from the preceding year. The average uses the “=AVERAGE()” function in Excel.

So why did this dating error occur? There is no apparent reason in the Velicogna paper to use two different averages over such a short time frame. I might suggest that there is another reason. The two papers were published weeks apart (Velicogna 13th Oct and Chen 22nd Nov) and used the same data for Antarctica over similar periods (Velicogna Apr 02 – Feb 09 and Chen Apr 02 – Jan 09). The impact of both would be enhanced if they had comparative statistics. For instance Zwally & Giovinetto 2011 state

Table 2 includes two GRACE-based mass loss estimates of 104 Gt/year (Velicogna 2009) and 144 Gt/year (Chen et al. 2009) for the period 2002–2006 and two estimates of 246 Gt/year (Velicogna 2009) and of 220 Gt/year (Chen et al. 2009) for the period 2006–2009.

Correcting Velicogna, it becomes

Table 2 includes two GRACE-based mass loss estimates of 142 Gt/year (Velicogna 2009) and 144 Gt/year (Chen et al. 2009) for the period 2002–2006 and two estimates of 233 Gt/year (Velicogna 2009) and of 220 Gt/year (Chen et al. 2009) for the period 2006–2009.

That is, the two papers become far more consistent if the averages are corrected. It would appear that Velicogna changed the dates without doing the maths.

Form of the acceleration

Velicogna states in the abstract

We find that during this time period the mass loss of the ice sheets is not a constant, but accelerating with time, i.e., that the GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time.

This quadratic trend is backed up by graphs on the NASA website (Antarctica) and NOAA websites (Greenland)


For ice melt Velicogna is stating that, not only would the trend be for each year to be greater than the previous year, but for the incremental increase to be greater than the last.

But, if ∂M is the change in ice mass, from the following functions were used in my spread sheet to replicate both Velicogna’s and Chen’s results.

For Velicogna 2009, Antarctica

∂M = -90 – 26(Year-2002)

For Velicogna 2009, Greenland

∂M = -122 + 30(Year-2002)

For Chen et al. 2009, Antarctica

∂M = -126 + 17(Year-2002)

These are all linear functions. I do not have access to Chen’s paper, but Velicogna’s abstract does not conform to her model.

Discontinuous functions in Chen et al. 2009

The abstract for Chen states

… our data suggest that East Antarctica is losing mass, mostly in coastal regions, at a rate of −57±52 Gt yr−1, apparently caused by increased ice loss since the year 2006.

Chen detection of increased ice loss is similar to Velicogna’s. But unlike Velicogna, Chen suggests that there is a discontinuous function. In other words, Chen’s graph would look like this.


Although it is possible to extrapolate from a discontinuous function, it would be highly misleading to do so. It suggests there is no underlying empirical relationship to be observed, in direct contradiction to Velicogna. Further, over a short period it is impossible to say whether this is the shift in the underlying rate of change in Antarctic melt, or if this new direction be quickly reversed. Fortunately, the two studies were published over three years ago, so there are alternative studies to compare the projection against. This will be the topic of the next post.

J. L. Chen, C. R. Wilson, D. Blankenship & B. D. Tapley Nature Geoscience 2, 859 – 862 (2009) Published online: 22 November 2009 doi:10.1038/ngeo694

Velicogna, I. (2009), Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophys. Res. Lett., 36, L19503, doi:10.1029/2009GL040222

H. Jay Zwally, Mario B. Giovinetto (2011) Surveys in Geophysics September 2011, Volume 32, Issue 4-5, pp 351-376, Overview and Assessment of Antarctic Ice-Sheet Mass Balance Estimates: 1992–2009 10.1007/s10712-011-9123-5

Climate Change Impacts in AR5 – It is better than we thought

BishopHill has a screen shot of Climate change impacts for the new IPCC report. He notes the similarity to the AR4.

The differences are noticeable and demonstrate a subtle dilution of AR4.

First, the disappearing Himalayan Glaciers have disappeared – to be replaced by disappearing glaciers in Latin America.

Second, the potential disappearing Amazon forest has disappeared, to be replaced by tree species extinction.

Third, no mention of potential catastrophic losses of Antarctic sea ice or land ice. So that means that sea level rises are less of a problem.

Fourth, there is no mention of coastal storm damage. Roger Pielke Jnr has won the argument on hurricanes?

Fifth, species extinction is much more localised.

Sixth, global loss of wetlands reduced, to more seasonal coastal flooding in Asia. So that means that sea level rises are less of a problem.

Check out for yourselves.

2007 AR4

Which was developed from the Stern Review.