AR6 and Sea Level Rise, Part 1

By Andy May

This is the first of a three-part series on the IPCC’s discussion of sea level rise in their latest report, AR6 (IPCC, 2021). The report claims that the rate of sea level rise is accelerating. It is fair to ask why they think this, what evidence do they offer?

We find the following in the AR6 Summary for Policymakers:

“Global mean sea level increased by 0.20 [0.15 to 0.25] m between 1901 and 2018. The average rate of sea level rise was 1.3 [0.6 to 2.1] mm yr–1 between 1901 and 1971, increasing to 1.9 [0.8 to 2.9] mm yr–1 between 1971 and 2006, and further increasing to 3.7 [3.2 to 4.2] mm yr–1 between 2006 and 2018 (high confidence). Human influence was very likely the main driver of these increases since at least 1971.” [Bold added]

AR6 Summary for Policymakers, page SPM-6 (IPCC, 2021)

And the following in AR6, Chapter 9:

“Global mean sea level (GMSL) rose faster in the 20th century than in any prior century over the last three millennia (high confidence), with a 0.20 [0.15–0.25] m rise over the period 1901 to 2018 (high confidence). GMSL rise has accelerated since the late 1960s, with an average rate of 2.3 [1.6–3.1] mm yr-1 over the period 1971–2018 increasing to 3.7 [3.2–4.2] mm yr-1 over the period 2006–2018 (high confidence). New observation-based estimates published since SROCC [Special Report on the Ocean and Cryosphere in a Changing Climate, 2019] lead to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components. While ocean thermal expansion (38%) and mass loss from glaciers (41%) dominate the total change from 1901 to 2018, ice sheet mass loss has increased and accounts for about 35% of the sea level increase during the period 2006–2018 (high confidence).”

On page 9-8 (Chapter 9, page 8) of the AR6 report:

And, farther in Chapter 9:

“At the basin scale, sea levels rose fastest in the Western Pacific and slowest in the Eastern Pacific over the period 1993–2018 (medium confidence). …The anthropogenic signal in regional sea level change will emerge in most regions by 2100 (medium confidence).” [bold added]

AR6 Chapter 9, page 8.

It is a little distressing that in the Summary for Policy Makers (SPM) they conclude that human influence was “very likely” the main driver of the acceleration in sea level and in Chapter 9 they admit they do not expect to observe an anthropogenic signal in regional sea level change before 2100.

Global Mean Sea Level (GMSL) might have increased more in the 20th century than at any time in the past 3,000 years, but how would anyone know? There were no tide gauges or satellites 3,000 years ago. Neither tide gauges nor satellites are accurate at the millimeter level, and certainly historical records and geological proxies from three thousand years ago are not. The geological proxies of past sea level are explained by Willis Eschenbach here. Warning, if you have a sensitive stomach, don’t look at Willis’s plots!

Further, why should this mean anything? Glaciers advanced to their lowest Holocene elevations during the Little Ice Age from 1600 to 1850, swallowing entire villages in the process (Behringer, 2010, pp. 89-90). The Little Ice Age was the coldest period in the entire Holocene—roughly 11,700 years ago to the present day. As Little Ice Age glaciers melt, one would expect sea level to rise a little, but just how significant is this? More importantly can we be confident the rate of GMSL rise is accelerating?

We also find it strange that they conclude the rate of GMSL rise is increasing based on comparing linear least squares fits to selected portions of the sea level record. Figure 1 plots the entire NOAA GMSL record by quarter since April 1880. The x axis is the number of quarters (three-month periods), and mean sea level is given on the y axis through 2020. This record is built from the Church and White (Church & White, 2011) GMSL data to 2010 and University of Hawaii Fast Delivery data after that.

Figure 1. NOAA Mean Sea Level from 1880 to 2020 by quarter. Data collected by Philip Townsend. Most of this record was prepared by (Church & White, 2011). The record after 2010 is from the (University of Hawaii Sea Level Center, 2021). The period from 1971 to 2018, mentioned in the AR6 quotes above is shown in orange and a linear least squares fit to the period is displayed.

AR6 selects numerous specific intervals in the quotes above to justify their claim that the rate of sea level rise is accelerating. This claim is visually dubious since the data look a bit wavy, but linear. The period of apparent acceleration from 180 to 300 (1925-1952), looks suspiciously like 420 to 510 (1985-2007). The entire record, from April 1880 to the end of 2020 has a slope of 1.65 mm/year with an R2 of 0.97. We could cherry pick periods all day and not resolve anything significant regarding acceleration or the lack of it. The least squares statistics for the AR6 cherry-picked periods mentioned in the quotes above are given in Table 1 and compared to four I cherry-picked.

While AR6 claims acceleration is occurring with high confidence, the previous report states:

“The trend in GMSL observed since 1993, however, is not significantly larger than the estimate of 18-year trends in previous decades (e.g., 1920–1950).”

AR5: (IPCC, 2013, p. 290)

One wonders why AR6 has a different view only seven years later.

Table 1. Cherry-picked sea-level rise rates.

AR6 would have us believe that because a least squares linear fit to the rise in sea level is larger from 2006 to 2018 than from 1971 to 2018 it is accelerating. Yet from 2012 to 2020 the rate is nearly as low as from 1971 to 2018. The largest rate of rise in Table 1 is only 15 inches or 38 cm per century, hardly alarming when global tides, in the open ocean, average more than twice that; and coastal tides are often ten times that value daily. Climate changes on a temporal scale of centuries, as we can see comparing the Little Ice Age to the Medieval Warm Period, so an instrumental record from 1880 to 2020 is unlikely to capture the full range of sea level rates. Estimates of sea level rise gathered from historical and geological records show that sea level has risen much faster in the past, as shown in Figure 2 created by Robert Rohde.

Figure 2. A sea level change graph for the Holocene and the Last Glacial Maximum created by Robert Rohde. The rate of sea level rise was much higher than today from 14,000 years ago until about 7,000 years ago. The variations since 4,000 years ago are too small to measure.

Sea level is estimated using tide gauges mounted on coasts around the world. The very best of these gauges are only accurate to ±5 mm for a monthly average (NOAA, 2020). Satellite measurements of sea level are problematic unless meteorological conditions are perfect, and they are trying to measure the altitude of a moving surface. Satellite estimates of sea level change show that sea level rises and falls in complicated patterns around the world and the global sea level budget does not close. This suggests that we do not understand all the components of sea level change, and this makes the attribution of sea level change to human activities difficult. Global and regional sea level budgeting is made complicated by the fact that regional (basin wide) sea level changes are much larger than global sea level changes (AR6, page 1290). Further, satellite estimates of sea level are not accurate to one-half of a millimeter (Frederikse, et al., 2020).

Is the difference between an estimated global average rate of 3.8 mm/year and 1.8 mm/year statistically significant, considering the data used? Particularly when these measurements are made over a few decades? It seems unlikely, but let’s look at the data more closely.

The AR6 statements suggest that the rate of sea level rise is increasing due to human influence. This is presumably due to greenhouse gas (GHG) emissions causing surface warming, which then melts glaciers sitting on land. This raises two questions:

  1. Is the increase in the rate of sea level rise statistically significant?
  2. If so, could warming due to human GHG emissions have caused it?

From AR6:

“Heating of the climate system has caused global mean sea level rise through ice loss on land and thermal expansion from ocean warming. Thermal expansion explained 50% of sea level rise during 1971– 2018, while ice loss from glaciers contributed 22%, ice sheets 20%, and changes in land water storage 8%. The rate of ice sheet loss increased by a factor of four between 1992–1999 and 2010–2019. Together, ice sheet and glacier mass loss were the dominant contributors to global mean sea level rise during 2006-2018 (high confidence).”

AR6 page: SPM-14

Thus, ocean warming since the Little Ice Age, provides about half of sea level rise. Melting ice provides most of the rest.

From AR6:

“By 2100, GMSL is projected to rise by 0.28–0.55 m (likely range) under SSP1-1.9 and 0.63–1.02 m (likely range) under SSP5-8.5 relative to the 1995–2014 average (medium confidence). Under the higher CO2 emissions scenarios, there is deep uncertainty in sea level projections for 2100 and beyond associated with the ice-sheet responses to warming. In a low-likelihood, high-impact storyline and a high CO2 emissions scenario, ice-sheet processes characterized by deep uncertainty could drive GMSL rise up to about 5 m by 2150. Given the long-term commitment, uncertainty in the timing of reaching different GMSL rise levels is an important consideration for adaptation “

AR6 page TS-44

Some IPCC climate models predict up to 5 meters of sea level rise by 2150, when the current rate of sea level rise is less than 40 cm or 1.3 feet per century? Considering that the IPCC models have not predicted climate accurately after 30 years of trying (McKitrick & Christy, 2018), pardon my skepticism.


“It is virtually certain that global mean sea level will continue to rise through 2100 …

Beyond 2100, GMSL will continue to rise for centuries due to continuing deep ocean heat uptake and mass loss of the Greenland and Antarctic Ice Sheets and will remain elevated for thousands of years (high confidence).”

AR6 Chapter 9, page 9-9.

The first statement is likely true, we are still warming as we come out of the Little Ice Age and I would doubt a change in direction of glacier retreat before 2100, the second statement is pure speculation, projecting beyond 2100 is reckless.

In summary, the AR6 statements about acceleration of sea level rise are based on simple cherry-picked and crude linear least squares fits to sea level data for the past 140 years. They also incorporate data and trends of ocean warming and land-based glacier melting. The problem is the rate of rise of sea level is so small today and so linear that their attempts to predict large rates of sea level rise are statistically inept and almost comical. In the next post we examine the complexity of measuring GMSL, and later in this series we will provide a more statistically significant projection of sea level rise that is much less than the wild predictions from AR6.

The bibliography can be downloaded here.

Published by Andy May

Petrophysicist, details available here:

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