By Judith Curry
Part II of the Climate Etc. series on sea level rise –the geological record provides context for the recent sea level rise.
Part I provided a context for examining observations and causes of global sea level rise. I greatly appreciate the comments on the blog posts, and the resources and comments I received via email.
Part II provides an overview of definitions of sea level and the causes of sea level variations and rise. An overview of the geological record of sea level rise is provided, with a focus on Holocene (the current interglacial). Historical and archaeological evidence (prior to the instrumental period) of sea level variations is also discussed.
Definition of sea ‘level’ and causes of sea level rise
A recent paper by Rovere et al. (2016) provides a helpful overview of the definitions of sea level, why it varies and how it is measured.
An understanding of sea-level change requires maintaining a clear distinction between global (or eustatic) sea-level and local relative sea-level. Sea level changes can be driven by either variations in the masses or volume of the oceans (‘eustatic’), or by changes of the sea surface relative to the land (‘relative’). Several techniques are used to observe changes in sea level, including satellite data, tide gauges and geological or archeological proxies.
Eustatic change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world’s oceans or net changes in the volume of the ocean basins. Determination and interpretation of sea level rise is complicated by the fact that both mean sea level and the solid earth surface move vertically with respect to each other. This movement in effect changes the shape of the ‘bathtub’. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. This is referred to as glacial isostatic adjustment (GIA).
Around the world, significant regional variations occur in the rate and direction of sea-level change. While some regions of the world’s oceans are today rising rapidly, in other regions sea level is falling. This is partly due to variations in the rate of warming and salinity changes and proximity to discharges of meltwater. Primarily, these variations reflect the influence of major ocean circulation systems that redistribute heat and mass through the oceans. Hence, at any location around or within the oceans, the observed sea level trend can differ significantly from the global average.
Melting of sea ice has no impact on sea level – water or ice that is already floating does not change the sea level by melting/freezing. Ice on a continent that melts and runs into the ocean increases sea level due to increasing the mass of ocean water. Antarctic ice shelves are in the ocean but are supported by the continent of Antarctica – melting these ice sheets will increase sea level.
The geological record
The geological record provides some insights and constraints into understanding how much, and how fast, sea level might rise in the coming centuries. It also provides critical context for understanding whether current sea levels and rates of sea level rise are unusual.
How is past sea level revealed in the geologic records? Proxy records of sea level are preserved in a variety of marine and terrestrial settings, such as sediments and organisms in deep ocean cores or once-submerged shorelines, and uplifted fossil reefs. Analysis of oxygen isotopes in tiny ocean organisms, and radiometric techniques are used to interpret and date the records. (JC note: I could use some good overview references here)
The IPCC AR5 summarizes our understanding of the geological record of sea level variation in the Last Interglacial (LIG) period:
There is very high confidence that maximum global mean sea level during the last interglacial period (~129 to 116 ka) was, for several thousand years, at least 5 m higher than present and high confidence that it did not exceed 10 m above present, implying substantial contributions from the Greenland and Antarctic ice sheets. This change in sea level occurred in the context of different orbital forcing and with high latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present.
For the time interval during the LIG in which GMSL was above present, there is high confidence that the maximum 1000- year average rate of GMSL rise associated with the sea level fluctuation exceeded 2 m kyr–1 but that it did not exceed 7 m kyr–1. Faster rates lasting less than a millennium cannot be ruled out by these data. Therefore, there is high confidence that there were intervals when rates of GMSL rise during the LIG exceeded the 20th century rate of 1.7 [1.5 to 1.9] mm yr–1.
During the past 20,000 years (the Holocene), since the end of the last glaciation, sea level has risen a total of about 120 m above modern shorelines, initially at a rate many times faster than observed anywhere today. The figure below shows sea level for the last 24,000 years. These data were compiled from a number of different regions and proxies. Sea level was lowest between 22,000 and 18,000 years ago, rising sharply between 15,000 and 8,000 years ago.
During deglaciation between 19,000 and 8,000 years ago, sea level rose at extremely high rates (Cronin, 2012). At the onset of the deglaciation, a ~ 500-year long, glacio-eustatic event may have contributed as much as 10 m to sea level with an average rate of about 20 mm/yr. During the rest of the early Holocene, the rate of sea level rise varied from a low of about 6.0–9.9 mm/yr to as high as 30–60 mm/yr during brief periods of accelerated sea level rise. For reference, these values are compared to modern values of sea level rise ranging from 1 to 3 mm/yr.
A zoom in on the past 8000 years is shown in the figure below. On average, sea level has been relatively stable and slow to rise over the past 6,000 years. However, some of the data (i.e. Malacca) indicate that sea levels approached or exceeded modern values ~ 6,000 – 4,000 years ago.
From the IPCC AR5:
Since the AR4, there has been significant progress in resolving the sea level history of the last 7000 years. RSL (relative sea level) records indicate that from ~7 to 3 ka, GMSL likely rose 2 to 3 m to near present-day levels. Based on local sea level records spanning the last 2000 years, there is medium confidence that fluctuations in GMSL during this interval have not exceeded ~ ±0.25 m on time scales of a few hundred years.
Rates of sea level rise
An important way that the geologic record provides perspective on projections of future sea level rise is through assessment of past rates of sea level rise. For reference, the recent rate of global sea level rise is 3 mm/yr. Rohling et al. (2013) provide a good overview on what we know about rates of sea level rise in the Holocene, past ice ages and interglacials:
Information about rates of SLR is most easily obtained from deglaciations, when ice ages terminated and sea level rose by up to 120–130 m at mean rates of about 1 m/cy [10 mm/yr] but with rapid steps bracketed by slower episodes. During one of these rapid steps (‘meltwater pulse 1a; mwp-1a’), SLR rates reached 4–5 m/cy [40 to 50 mm/yr] for several centuries. Rapid steps of 2 m/cy [20 mm/yr] also occurred during previous deglaciations. Note that past deglacial SLR rates characterise transitions from glacials with 2–3 times the present-day ice volume, to interglacials with ice volumes similar to the present.
Away from deglaciations, data for 75–30 ky ago, when sea level fluctuated between about 60 and 90 m below the present level, reveal rates of SLR when major Northern Hemisphere ice sheets were consistently present (with fluctuating volume). This is a significantly different state than deglaciations. All phases of considerable ice-volume reduction had SLR rates of 1–2 m/cy [10-20 mm/yr] (comparable with mean rates during deglaciations). This suggests that peak rates during deglaciations may reflect special conditions, but that rates of 1–2 m/cy [10-20 mm/yr] are not exceptional for natural fluctuations. Nevertheless, these rates concern times with much greater ice volume than today, and with intense global climate fluctuations.
The most valuable information on rates of SLR comes from periods when global ice volumes were similar to present. The last five glacial cycles contain two interglacials that were up to 2C warmer than the pre-industrial state, with sea level up to 10 m higher than today. LIg global temperature was about 1.6 +/-0.5 C higher than pre-industrial temperature and sea level peaked 6–9 m above the present level, which implies a 10–15% ice-volume reduction relative to present. Subsequent studies proposed 1000-year average LIg rates of 0.26 m/cy [2.6 mm/yr] and 0.56–0.92 m/cy [5.6 – 9.2 mm/yr], which is consistent with a 1000-year smoothed estimate of 0.7 +/- 0.4 m/cy [7 +/- 4 mm/yr]. Data from western Australia suggest a rapid rise within the LIg at 0.6 m/cy [6 mm/yr]. We infer that LIg SLR likely occurred at sustained rates of 1 m/cy [10 mm/yr] or less.
Rohling et al. concluded:
Hence, modern change is rapid by past interglacial standards but within the range of ‘normal’ processes.
Holocene Climatic Optimum
Of particular interest is the so-called ‘mid-Holocene highstand’ between about 6000 and 3000 years ago, with substantial regional variations. I don’t see any mention of this in the AR5. Here are a few local examples from the recent literature:
- Rio de la Plata, Argentina and Uruguay: the peak of the sea level high stand c. +4m [above present] between 6000 and 5500 cal yr BP Prieto et al. (2016)
- Southeast Australia: during the mid-Holocene (c. 2–8 kyr BP), when sea level was 1–2 m above today’s level Lee et al. (2016)
- Japan: The Holocene-high-stand inferred from oyster fossils is 2.7 m at ca. 3500 years ago, after which sea level gradually fell to present level. Yokoyama et al. 2016
- Western Australia. possibly corresponding to the mid-Holocene sea-level highstand of WA of at least 1-2 m above present mean sea level. May et al. (2016)
- Strait of Makassar. Radiometrically calibrated ages from emergent fossil microatolls on Pulau Panambungan indicate a relative sea-level highstand not exceeding 0.5 m above present at ca. 5600 cal. yr BP Mann et al. 2016 _
- Scotland: RSL [relative sea level] was <1 m above present for several thousand years during the mid and late Holocene before it fell to present. Long et al. (2016)
- Japan: Relative sea level during Holocene highstand reached 1.9 m [higher than today] during 6400–6500 BP Chiba et al. (2016)
- Denmark: The data show a period of RSL [relative sea level] highstand at c. 2.2 m above present MSL [mean sea level] between c. 5.0 and 4.0 ka BP Sander et al. (2016)
- China: sea level rising steadily to form a highstand of ~2-4 m [above present sea level] between 6 and 4 kyr BP Bradley et al. (2016)
- Africa: maximum 5 to 4 ka BP [5000 to 4000 years before present] (Ramsay, 1995) during a highstand about 3.5 m above the present sea level, Accordi and Carbone (2016)
- Persian Gulf: a highstand of > 1 m above current sea level shortly after 5290–4570 cal yr BP before falling back to current levels by 1440–1170 cal yr BP Lokier et al. 2015
- French Polynesia: we find that local rsl was at least ∼5 ± 0.4 m higher than present at ∼5.4 ka Rashid et al. 2014
The sea level high-stand was associated with the so-called Climatic Optimum or the Holocene Optimum, during 8000 to 4000 BC when average global temperatures reached their maximum level during the Holocene and were warmer than present day.
Last 2500 years
Since the AR5, an important new paper has been published on global sea levels over the past 2500 years (Kopp et al. 2016). They compiled a global database of regional sea level reconstructions from 24 localities, many with decimeter-scale vertical resolution and subcentennial temporal resolution. Also included are 66 tide-gauge records.
The key figure summarizing their results is shown below.
The paper concluded that global sea level (GSL) varied by ∼ ± 7-11 cm over the pre-Industrial Common Era (CE), with a notable decline over 1000–1400 CE coinciding with ∼0.2 °C of global cooling.
Despite the incomplete coverage and regional variability, sensitivity analyses of different data subsets indicate that key features of the GSL curve are not dependent on records from any one region. By contrast, the rise over 1400–1600 CE and fall over 1600–1800 CE are not robust to the removal of data from the western North Atlantic.
Kopp et al. found that 20th century rise was extremely likely faster than during any of the 27 previous centuries. Because their model is insensitive to the small linear trends in GSL over the Common Era, the relative heights of the 300-1000 CE and 20th century peaks are not comparable.
The Kopp et al. estimates differ markedly from previous reconstructions of Common Era GSL variability. For example, Grinsted et al. (2009) predicts GSL swings with ∼4 times larger amplitude (with much higher sea levels during the Medieval Warm Period).
Credibility of the Kopp et al. analysis is enhanced by semi-empircal prediction based on these rates that is close to model results using a budget approach.
An evaluation of the status of late Holocene sea level rise constructions is provided in a recent proposal by an international group of sea level experts (including Kopp), entitled: Towards a unified sea level record: assessing the performance of global mean sea level reconstructions from satellite altimetry, tide gauges, paleo‐proxies and geophysical models. Excerpts:
Furthermore, pre‐industrial reconstructions of GMSL based on sea level proxies are limited to one single study (Kopp et al., 2016). The approaches and datasets used in the different published estimates of past GMSL change differ considerably, and there has been no consistent assessment of the differences between the individual reconstructions.
Their proposal for improving our understanding of pre-industrial sea level:
Following the same approach as in WP1, a series of surrogate synthetic proxy records will also be created. Since the focus is on Late Holocene time scale, the synthetic sea level fields will be created using a millennial simulation with the Earth System model MPI‐ESM‐P AOGCM. The point‐wise information will correspond to the locations and temporal resolution of all available proxy records from paleo sea level studies (Kopp et al. 2016) and random noise will be added to each mimicking the limitations of actual proxy records. Gaussian process regression will be used to convert the non‐equidistant proxy records and their climate model surrogates into the required temporal resolution. Paleo‐sea level reconstruction techniques from Kopp et al. (2016) and Dangendorf et al. (under review) will be then applied to the surrogate time series and compared to the a priori known modelled GMSL curves.
Historical and archaeological records (pre-instrumental)
The Kopp et al. analysis does not settle the issue of whether sea levels during the Medieval and Roman Warm Periods were higher than current levels, or whether there were any large decadal-scale periods with large rates of sea level rise. Here we consider historical and archaeological information.
1) The most rapid phases [of sea level rise] were between 8000 and 5000 BC, and that the rise of general water level was effectively over by about 2000 BC, when it may have stood a metre or two higher than today.
2) The water level may have dropped by 2 metres or more between 2000 and 500 BC. What does seem certain is that there was a tendency for world sea level to rise progressively during the time of the Roman Empire, finally reaching a high stand around 400 AD comparable with, or slightly above, present.
3) The slow rise of world sea level, amounting in all probably to one metre or less, that seems to have been going on over the warmer centuries in Roman times, not only submerged the earlier harbour installations in the Mediterranean, but by 400 AD produced a notable incursion of the sea from the Wash into the English fenland, and maintained estuaries and inlets that were navigable by small craft on the continental shore of the North Sea from Flanders to Jutland.
4) The existence of pre-Norman conquest salterns – saltpans over which the tide washed and from which salt-saturated sand was taken – outside the later sea dykes on the Lincolnshire coast may point to a period of slightly lowered sea level between the late Roman and the medieval high water periods.
5) Our survey of the European scene during the warmer centuries of the Middle Ages would not be complete without mention of the things that suggest a higher stand of the sea level, which may have been rising globally during that warm time as glaciers melted .
Fig 60 [not shown] draws attention to the greater intrusions of the sea in Belgium, where Bruges was a major port, and in East Anglia where a shallow fjord with several branches led inland toward Norwich. [Bear in mind that the land here has been sinking due to isostatic forces since the ice age. If relative sea levels were as high then as now, it would mean absolute levels were higher than.]
In a previous post at Climate Etc., Historic variations in sea levels. Part I: Holocene to Romans, Tony Brown assessed some anecdotal, local evidence for higher sea levels during the Roman era.
JC note: TonyB et al, please let me know if you have any information/references on sea levels during the medieval warm period
The geological record for sea level rise provides important context for recent sea level rise. However, the uncertainties in the geological sea level record are substantial, associated with sparse sampling, uncertainties in the proxy methods and uncertainties in the analysis methods.
Is the 20th century sea level rise unusual? Sea level was apparently higher at the time of the Holocene Climate Optimum (~ 5 ka), at least in some regions. I have not seen an overall assessment of this, but there have recently been numerous publications providing local evidence for higher sea levels during this period.
Whether or not sea level was higher during the Medieval Warm Period than current levels remains uncertain, and there is substantial disagreement among different reconstructions on the sea level during the MWP, with the Grinsted et al finding substantially higher sea level values during the MWP (around 1150 AD).
Kopp et al. find the 20th century rate of sea level rise to be the highest in the last 27 centuries. However, since their data is barely resolved at 100 year time scales (with decimeter vertical resolution), I would not place high confidence in their conclusion. Eyeball examination of Grinsted et al.’s Figure 7 shows possibly higher rate of sea level rise between ~1000 and 1100 AD. Overall, I find Kopp et al.’s analysis to be more convincing (apart from overconfidence in the relative rate of 20th century sea level rise).
The pace of interesting and important paleo sea level rise research seems to have accelerated since publication of the AR5, I will be following this closely.
Forthcoming Part III: The observational (historical) record