TheNZThrower
Active Member
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:
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:
Koonin first starts on p.149-150 by imploring the audience to question whether sea level rise is anthropogenic or problematic:
From p.155 onwards, he lays out his thesis on the dangers of GMSL rise: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?
Koonin's points are: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:
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:
- Ever since the early 90s, sea level has risen at an average rate of 3mm/yr
- This is higher than the average GMSL rate since 1880
- GMSL rates have varied a lot over the 20th century
- There has been a period from 1925-1940 which had a rate close to the 3mm/yr of the 21st century
- 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)
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:
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:
As do Church and White 2011: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.
Jevrejeva et al. also imply that the GMSL rise is anthropogenic: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.
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: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 contrary to the implications of Koonin, the rising GMSL rate is a cause for concern.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.
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