How has sea level changed over history?
Scientists think that in ancient times the global climate and sea level went through many cyclic changes. Why do they believe this and what is the pattern of the sea level according to written history? This post addresses the gross features of the global mean sea level (GMSL), and not minor variations and trends.
Ice and marine sediment cores
Scientists study the chemicals in ice cores from Greenland and rock deposits from the ocean floor because they can indicate what the climate was like when the ice or rocks were formed.
The belief that in ancient times the global climate and sea level went through cyclic changes comes from measurements in oxygen isotope (δ18O) measurements. Benthic δ18O records are used as a proxy for global ice volume. As this results in a cyclic pattern of changes in sea level, I will refer to it as the Cyclic Sea Level (CSL) model.
Cyclic Sea Level (CSL) model
Scientists infer paleo sea levels indirectly from oxygen isotope (δ18O) measurements. Since temperature changes and ice sheet volumes both impact oxygen isotope values, the isotope signals resulting from each of these effects must be disentangled. Then the ice sheet volumes are related to the GMSL. There is a graph below of δ18O (oxygen isotope) measurements from deep sea sediments (a), which shows fluctuations over the past 4 million years of geologic time (Yokoyama and Purcell, 2021). From this graph scientists deduce that there have been many cold periods (ice ages), each separated by a warmer interglacial period. The sea level since the last interglacial is reconstructed based on direct methods, such as corals (b). This indicates that the sea level has increased by more than 120 meters since the end of the last ice age. More detail is shown of the last 35,000 years (c). They claim that this shows “Rapid rises and falls of sea level”, but the changes are over a period of about 10,000 years! This is about 400 generations, which is not rapid from a human perspective. All these graphs use the geologic time scale, which makes uniformitarian assumptions about earth history and rejects written biblical history.
A CSL model that shows the marine isotope stage (MIS) is shown below. It has multiple ice ages.
All these graphs show numerous “ice ages” (of low sea levels). But how can this pattern be explained? Scientists often invoke Milankovitch cycles – where regular changes in the Earth’s tilt and orbit combine to affect which areas on Earth get more or less solar radiation. But this explanation is not convincing. And lower temperatures have been proposed as the cause of an ice age, but these do not contain sufficient moisture for enhanced snowfall. Another possible cause is elevation of land, but this is unlikely for ice sheets over appreciable areas.
To develop an ice age the oceans need to be warm at mid and high latitude and the land masses need to be cold (Batten et al., 2017). This cannot be produced by slow and gradual evolutionary scenarios, however the global flood provides a simple mechanism for an ice age. The oceanic heating comes from hot subterranean water (Gen. 7:11) and from volcanic activity. The volcanic activity would have also added dust and aerosols to the atmosphere, reducing land temperatures. For example, the little ice age may have been triggered by a volcano (Mitchell, 2013).
Based on studies like those described above and a belief in the geologic time scale, scientists suggest that there have been at least five major ice ages and many minor ice ages:
– The Huronian ice age (2,100 to 2,400 million years ago)
– The Cryogenian ice age (630-720 million years ago), when it is suggested that glacial ice sheets reached the equator.
– The Andean-Saharan ice age (420-460 million years ago)
– The late Paleozoic ice age (260-360 million years ago)
– The Quaternary ice age (cycles of glaciation from 2.6 million years ago to 10,000 years ago). The Antarctic ice sheet has existed continuously across this period. Ice sheets expanded during the glacial periods and contracted during the interglacial periods. Initially the fluctuation period was about 41,000 years, but later it slowed to about 100,000 years. They suggest that there have been numerous “ice ages” during the Quaternary ice age. Over the past 740,000 years there have been eight glacial cycles. And 17 glacial cycles have been suggested for the entire period. During the glacial periods because of the volume of ice on land, sea level was about 120 meters (394 ft) lower than present.
It is assumed that the most recent glaciation period, often known simply as the “Ice Age,” occurred between about 120,000 and 11,500 years ago. Since then, earth has been in an interglacial period (using geological time).
Unfortunately the current concern for global climate change is largely driven by these assumptions about paleoclimate. Scientists use indirect (proxy) measurements to make statements about how variables like global mean sea level have changed in the past and then they compare this to the present situation. Of course any changes in the present appear to be very rapid compared to changes over the hypothetical geologic time scale. This method is not robust, but it is speculative. If a risk assessment was done, it’s likely that their predictions are unreliable.
Mono Sea Level (MSL) model
I’m skeptical of ancient sea levels derived indirectly from proxies such as oxygen isotopes. There is no way that these relationships can be verified. Just like there is no way that the geologic time scale can be verified. That’s why I prefer direct measures of historical sea levels like shoreline features.
A simpler model (see schematic graph) that is consistent with recorded history in the Bible is as follows:
– It begins with the widespread formation of massive sedimentary rock layers during the flood, with no land above sea level.
– Then tectonic uplift of sedimentary rock layers forms mountain ranges and continental drift forms continents.
– The sea level is higher than at present, because there is no ice. The IPCC says that with the current geomorphology this would be 66m above the current sea level.
– Then the sea level decreases to about 120m below the current sea level because of a single ice age. The evidence for multiple ice ages can be more readily understood as resulting from advance and retreat phases of a single ice age after the flood (Batten et al., 2017). And ancient fluctuations in δ18O (oxygen isotope) in ice cores probably reflect individual storms, not annual seasons.
– Then the sea level increases to the current sea level because of ice melting. This is consistent with orally-transmitted Australian Aboriginal stories describing coastal flooding and times when sea levels were lower than today.
Because this model has only one cycle, I will call it the Mono Sea Level (MSL) model. I believe that historic shoreline features can be explained by such a model.
According to the IPCC, “Paleo-evidence shows that the GMSL has been about 70 m higher and 130 m lower than present within the past 55 million years (of geologic time) and was likely 5 to 10 m higher during the Last Interglacial” (IPCC, 2021). And “The Earth was largely ice free during the early Eocene Climatic Optimum (EECO) and complete loss of current land ice reservoirs would raise the GMSL by 66 m” (IPCC, 2021). Thus the starting GMSL in the MSL model is about 66m higher than at present.
As the ice layers melted at the end of the Ice Age, the sea level rose above present-day levels through eustatic flooding of coastal margins. This is referred to as the Flandrian transgression (Silby, 2004). The sea later retreated to present-day levels through upward isostatic adjustment of the land after the massive ice layers had gone, and downward adjustment of the sea floor because of extra water.
Ancient sea levels
Bezore (2019) studied the coastline of southern Victoria under the current conditions and those of previous times. Submarine coastal landscapes were used to indicate past sea level. It was noted that regional sea level in Victoria is considered to closely follow GMSL trends.
The study extended to a depth of 80m below current sea level. The findings were interpreted in terms of a CSL model determined from proxy data by Grant et al (2014), shown below.
Five drowned sea stacks were discovered that formed when the sea level was about 60m lower. This was unexpected under the CSL model because, “Previously, these features were believed to be so highly erosional that they would not be able to withstand multiple sea level cycles and would likely form and erode within a single sea level stillstand”. But they are more readily explained under the MSL model because there are no multiple sea level cycles to erode them.
Submerged sea cliffs were observed at about 30 m and 50-60 m below present sea level. Likewise, it was assumed that they had endured multiple sea level cycles. And it was assumed that “cliffs require on the order of a thousand years or more to form”, which is an unusual statement as surely sea cliffs could form in soft rock in a shorter period. For example, at the current rate of shoreline retreat (10cm/year), Demons Bluff will erode a shore platform 50 m wide in 500 years.
Shore line platforms were observed about 50 m (ranging 38-57 m) below present sea level. These were below four submerged cliffs. Likewise, it was assumed that the platforms had endured multiple sea level cycles.
Bezore concludes that “the sea cliffs, shore platforms, and sea stacks found submerged offshore of Victoria were formed when sea level was 50 – 60 m below present, corresponding with sea level during the beginning of MIS 3 (60k years ago)”.
Although the findings of this investigation were interpreted in terms a CSL model, they could be interpreted in terms of a MSL model.
When a glacier (or ice sheet) grows and moves across the landscape, it pushes rocks and sediments. When the glacier melts, it leaves piles of these rocks behind. The rock piles are called moraines. These moraines provide evidence that glaciers once covered large parts of the world.
Layers of varves (alternating layers of fine and course particles) can be deposited downstream of glaciers. However, ancient rock layers that have been interpreted as varves are probably evidence of submarine landslides.
Occam’s razor aids scientists who are developing theoretical models. It is the principle that “Entities should not be multiplied unnecessarily”. Or, “in explaining a thing no more assumptions should be made than are necessary”. Or, “when you have two competing theories that make exactly the same predictions, the simpler one is the better”. The razor shaves away unlikely or extraneous explanations for a phenomenon. For example, if two computer programs do the same job, the shorter one, in which less code can go wrong, is probably preferable.
Occam’s razor makes no absolute assertions. It does not claim that the simplest answer is always correct. It merely suggests that, among all possible answers to a question, the best bet is generally the one that requires the fewest assumptions.
Observations of previous sea levels can be interpreted in terms of either a CSL model or a MSL model. Occam’s razor suggests that a MSL is preferable. Therefore, someone should interpret the relevant data in terms of a MSL model to see if this can be verified.
It is unnecessary to accept the current complicated evolutionary ideas of about 30 separate ice ages over the past 2.5 million years, with the most recent ones lasting 100,000 years and the earlier ones lasting 40,000 years.
Studies of past sea levels and climates are always interpreted using a CSL model with multiple ice ages. However this model is not robust as it relies on inferences from indirect (proxy) information and geologic time. And it rejects written biblical history. A mono sea level (MSL) model (with one ice age) is presented that is based on written biblical history and is supported by Occam’s razor. It would be interested to see how the experimental data can be interpreted in terms of a MSL model.
Batten D et al., 2017, “What about Ice Ages”, The creation answers book, Creation Book Publishers, p.199-210.
Bezore R, 2019, “The morphology and evolution of rock coasts over eustatic cycles in temperate, wave dominated environments”, PhD Thesis, University of Melbourne.
Grant K et al., 2014, “Sea-level variability over five glacial cycles”, Nat. Commun. 5, 5076.
IPCC, 2021, “Climate change 2021. The physical science basis”, Working Group 1, p.TS-44, 2-77.
Mitchell E, 2013, “Was the Medieval ‘Little Ice Age’ triggered by an Indonesian volcano?’, Answers In Genesis.
Sibley A, 2004, “Post-glacial flooding of coastal margins within the biblical timeframe of Peleg”, J Creation, 18(3), 96-103.
Yokoyama Y and Purcell A, 2021, “On the geophysical processes impacting palaeo-sea-level observations”, Geosci. Lett. 8, 13.
Written, October 2021