Debunked: Steve Koonin's downplaying of sea level rise

[TheNZThrower]

In his book Unsettled, theoretical physicist Steve Koonin has quite a few claims to make about sea level rise, and how the broader scientific community represents it. I will be focusing on his claims about acceleration in sea level rise for this post.

Koonin first starts on p.149-150 by imploring the audience to question whether sea level rise is anthropogenic or problematic:

people are very concerned about rising sea levels. Should they be? Are sea levels changing in response to our warming globe? And—even if our nation's monuments are safe for another thousand generations—are our human futures in danger as a result?

From p.155 onwards, he lays out his thesis on the dangers of GMSL rise:

So for the past three decades, sea level has been going up by about 3mm (0.12 inches) each year—higher than the overall average rate (1.8 mm or 0.07 inches per year) since 1880. To judge how the rate of sea level rise might have increased under growing human influences, IPCC's AR5 presents a figure (reproduced as Figure 8.5) that displays year-by-year eighteenyear trends (rates of rise) and their uncertainties from three different tide gauge analyses, as well as the eighteen-year trend in the satellite data then available. (Each year's plot point is the trend over the eighteen years that follow; the single point for satellite data plotted in 1994 therefore reflects the average over the interval from 1994 to 2011.)

While the rate in more recent decades is indeed higher than the average rate over the twentieth century, it has to be viewed in context with the substantial variability over past decades, which is evident from even a glance at Figure 8.5. As the IPCC put it:

It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/year between 1901 and 2010 ... and 3.2 [2.8 to 3.6] mm/year between 1993 and 2010. It is likely that similarly high rates occurred between 1920 and 1950.

In fact, the rate of rise between 1925 and 1940—a period almost as long as the eighteen-year satellite record then available—was almost the same as that recent satellite value, about 3 mm (0.12 inches) per year.

Since the rate varies so much, it's hard to know for recent years what's human-caused and what's natural. And while the IPCC's 2019 Special Report on the Oceans and Cryosphere in a Changing Climate (or SROCC) expresses high confidence that the satellite data from 1993 to 2015 shows an acceleration (that is, the rate of rise is increasing), the implications are murky both because of the shortness of the record and because there was acceleration well before human influences were significant. The AR5 had this to say:

It has been clear for sometime that there was a significant increase in the rate of sea level rise in the four oldest records from Northern Europe starting in the early to mid-19th century (Ekman, 1988; Woodworth, 1990, 1999; Mitchum et al., 2010). Estimates of the change in the rate have been computed, either by comparing trends over 100-year intervals for the Stockholm site (Ekman, 1988; Woodworth, 1990), or by fitting a quadratic term to all the long records starting before 1850 (Woodworth, 1990, 1999). The results are consistent and indicate a significant acceleration that started in the early to mid-19th century (Woodworth, 1990, 1999), although some have argued it may have started in the late 1700s (Jevrejeva et al., 2008).

Koonin's points are:

1. Ever since the early 90s, sea level has risen at an average rate of 3mm/yr
2. This is higher than the average GMSL rate since 1880
3. GMSL rates have varied a lot over the 20th century
4. There has been a period from 1925-1940 which had a rate close to the 3mm/yr of the 21st century
5. There has been acceleration in GMSL since the 19th century, and even possibly the 18th century prior to significant anthropogenic forcing (per Jevrejeva et al. 2008)

In addition, his word choice appears to cast doubt on anthropogenic GMSL rise. Especially with his emphasis on the variability of GMSL rise rates in the 20th century and of prior periods in the 20th century where GMSL rise is similar to the 21st century.

The impression that Koonin is trying to create about the science behind GMSL rates is that there is nothing to worry about the current rate of GMSL rise simply because there has been prior periods in the 20th century where the rate of rise has been similar, and that acceleration in GMSL rise has occurred prior to significant anthropogenic influences. Therefore it is difficult to parse out anthropogenic influences from natural variability in GMSL rise trends, including that of acceleration.

Koonin's source for Fig. 8.5 is this graph from IPCC AR5 citing 3 studies on GMSL acceleration trends:

Source: https://archive.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter03_FINAL.pdf

Seems to show high variability in the GMSL rise rates, right? Sure. But it doesn't negate that there is a trend of accelerating GMSL rise. Jevrejeva et al. note that there is acceleration despite the variability:

We present a reconstruction of global sea level (GSL) since 1700 calculated from tide gauge records and analyse the evolution of global sea level acceleration during the past 300 years. We provide observational evidence that sea level acceleration up to the present has been about 0.01 mm/yr2 and appears to have started at the end of the 18th century. Sea level rose by 6 cm during the 19th century and 19 cm in the 20th century. Superimposed on the long-term acceleration are quasi-periodic fluctuations with a period of about 60 years. If the conditions that established the acceleration continue, then sea level will rise 34 cm over the 21st century. Long time constants in oceanic heat content and increased ice sheet melting imply that the latest Intergovernmental Panel on Climate Change (IPCC) estimates of sea level are probably too low.

As do Church and White 2011:

We estimate the rise in global average sea level from satellite altimeter data for 1993–2009 and from coastal and island sea-level measurements from 1880 to 2009. For 1993–2009 and after correcting for glacial isostatic adjustment, the estimated rate of rise is 3.2 ± 0.4 mm year−1 from the satellite data and 2.8 ± 0.8 mm year−1 from the in situ data. The global average sea-level rise from 1880 to 2009 is about 210 mm. The linear trend from 1900 to 2009 is 1.7 ± 0.2 mm year−1 and since 1961 is 1.9 ± 0.4 mm year−1. There is considerable variability in the rate of rise during the twentieth century but there has been a statistically significant acceleration since 1880 and 1900 of 0.009 ± 0.003 mm year−2 and 0.009 ± 0.004 mm year−2, respectively. Since the start of the altimeter record in 1993, global average sea level rose at a rate near the upper end of the sea level projections of the Intergovernmental Panel on Climate Change's Third and Fourth Assessment Reports. However, the reconstruction indicates there was little net change in sea level from 1990 to 1993, most likely as a result of the volcanic eruption of Mount Pinatubo in 1991.

Jevrejeva et al. also imply that the GMSL rise is anthropogenic:

The calculated acceleration of 0.01 mm/yr2 using the 300 year long GSL accounts for 6 cm sea level rise in the 19th century, about 19 cm during the 20th century and will contribute 34 cm sea level rise during the 21st century. This estimate assumes that the conditions that produce the present day evolution of sea level will continue into the future–though the acceleration will depend on the actual rate of temperature increase in the 21st Century.

So it appears that Koonin's emphasis on GMSL variability and on the GMSL rise rate of specific points in the 20th century distract from the fact that there is an acceleration in the rate of rise per his own sources. This leads us to the point that the IPCC AR5 also notes that extreme high sea level events have also increased as a consequence of GMSL rise:

It is likely that the magnitude of extreme high sea level events has increased since 1970. A rise in mean sea level can explain most of the increase in extreme sea levels: changes in extreme high sea levels are reduced to less than 5 mm yr–1 at 94% of tide gauges once the rise in mean sea level is accounted for.

So contrary to the implications of Koonin, the rising GMSL rate is a cause for concern.

 

[Ann K]

Forgive me for not addressing the statistics themselves, but it seems to me that information from three measured points would be insufficient to make sweeping generalizations about the sea level as a whole. Would we not be better advised to take computations and predictions from such contributory causes as the overall amount of ice melt over land surfaces and the amount of surface temperature change?

(Isostatic land surface rebound as the weight of ice decreases also means that a small increase in tidal measurement is under-representative of the actual amount of sea level rise, if I understand that correctly, but the extremely long time frame of that change means that it's immaterial for this discussion.)

 

[Mendel]

What are "natural" causes of sea level rise?

 

[Henk001]

As Tamino in his 'Open Mind' blogpost "So What" pointed out:

And now to the really important part, which is not the math but the physics. Whether sea level showed 20th-century acceleration or not, it's the century coming up which is of concern. And during this century, we expect acceleration of sea level rise because of physics. Not only will there likely be nonlinear response to thermal expansion of the oceans, when the ice sheets become major contributors to sea level rise, they will dominate the equation. Their impact could be tremendous, it could be sudden, and it could be horrible.

The relatively modest acceleration in sea level so far is not a cause for great concern, but neither is it cause for comfort. The fact is that statistics simply doesn't enable us to foresee the future beyond a very brief window of time. Even given the observed acceleration, the forecasts we should attend to are not from statistics but from physics.

https://tamino.wordpress.com/2011/03/31/so-what/
Sea level rise is mainly a response to temperature. If sea temperature rises the ocean water will expand, and together with air temperature rise land ice will melt, or ablate at glacier fronts. Both effects respond potentially non-linear to the temperature rise. Up until now thermal expansion has been the main contributor to sea level rise and appears to have followed the ocean temperature (quasi-)linearly. So when the sea temperature rise showed an acceleration (as in 1920-1940), so did sea level rise. The last two decades the Greenland ice sheet mass loss has a slowly increasing contribution and the (West-)Antarctic ice melt more recently started to contribute to an as yet lesser extent.
But the past sea level rise or its acceleration is in itself not a predictor of future developments, so I think Koonin's argumentation is a distraction.

 

[FatPhil]

Sea level rise is mainly a response to temperature. If sea temperature rises the ocean water will expand, and together with air temperature rise land ice will melt, or ablate at glacier fronts. Both effects respond potentially non-linear to the temperature rise. Up until now thermal expansion has been the main contributor to sea level rise and appears to have followed the ocean temperature (quasi-)linearly. So when the sea temperature rise showed an acceleration (as in 1920-1940), so did sea level rise. The last two decades the Greenland ice sheet mass loss has a slowly increasing contribution and the (West-)Antarctic ice melt more recently started to contribute to an as yet lesser extent.
But the past sea level rise or its acceleration is in itself not a predictor of future developments, so I think Koonin's argumentation is a distraction.

Thermal expansion has not been the main contributor, it's about 40%:

via: https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level

 

[Henk001]

Thermal expansion has not been the main contributor, it's about 40%:

via: https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level

Apparently, I stand corrected.

 

[Eburacum]

What are "natural" causes of sea level rise?

As noted above sea level rises and falls are caused by temperature changes, and to a much lesser extent, changes in the level of the ground due to geological processes. Natural sea-level changes therefore are almost exclusively caused by changes in the planet's overall temperature.

1/ The temperature of the planet changes naturally over time due to the luminosity of the Sun. The Sun is a slightly variable star, but it is much more stable than most stars in the night sky. Over a timescale of millions and billions of years, the Sun is increasing in luminosity, because the Sun becomes more dense as it converts hydrogen into helium, and this makes the Sun brighter.

2/ The Earth's temperature also changes due to the eccentricity of the planet's orbit, and the tilt of the planet with respect to that eccentricity. When the Earth's orbit is particularly eccentric, the amount of sunlight absorbed by the planet can change significantly, and this effect is amplified (or reduced) by the angle of the Earth's tilt during the year and the precession of the equinoxes. These changes in orbital parameters can be predicted accurately, resulting in the so-called Milankovich cycles, which cause long-term fluctuations in the Earth's orbit and ultimately have a very large role in the cyclic nature of Ice Ages.

3/ The landforms and continents on the Earth change slowly over time, and these changes can also affect the temperature of the Earth. for instance the Earth currently has a nearly circular sea at the North Pole, which can trap ice, and a nearly circular continent at the Antarctic which is almost completely covered in ice. Millions of years ago neither of these conditions existed, and the ocean currents of the world moved much more freely near the poles; this reduced ice formation at both locations, and so the Earth was just a little bit warmer, since ice reflects sunlight quickly back into space and cools the planet.

4/ Over a very long timescale, the level of carbon dioxide in our atmosphere has been slowly reducing due to natural geological processes. Carbon is slowly absorbed by sediments and buried underground at a rate which is slightly faster than it is released by volcanoes. However sometimes volcanic activity increases (for unknown reasons), and the carbon dioxide level goes up again; this increase can however be offset by increased particulates in the atmosphere, which change the thermal qualities of the planet as well.

In short, it is very complicated. Human changes to the atmosphere are all superimposed on top of these natural changes, so quite often it is difficult to see the effects clearly; but overall human changes to the atmosphere are in the direction of warming, so we should not think that human effects are trivial or can be ignored.

 

[Mendel]

Natural sea-level changes therefore are almost exclusively caused by changes in the planet's overall temperature.

So it basically comes down to whether climate change is man-made; and if we agree that human activity has a large part of it, then that's also true for sea level change as a consequence.

 

[Ann K]

Before the Industrial Revolution, the point in history at which we started using lots of fossil fuels and which is generally accepted as the beginning of our current man-made warming trend, there were the natural cycles that caused ice ages to come and go. Archaeologists frequently find evidence of human civilization in now-submerged areas along the current coastlines. Fishing boats on Dogger Bank in the North Sea sometimes pull up things like worked flint tools from what we call "Doggerland", a large land mass that connected Britain and continental Europe that was submerged many thousands of years ago as the glaciers melted.