#ice sheets

A series of maps showing "The Waning of the Ice Sheet in New York State" over time from Annual Report of the New York State Museum 65th:v.1 suppl. (1911).

Hello! For this post, I’m going to be talking about a glacial period, known as Pulse-1a. This was a period in geologic history that occurred around 21,000-14,600 years ago. This time period was originally discovered through coral records back in 1989 on the island of Barbados in the Caribbean. 

This time period is so unique because in a very short frame of time, sea levels rose about 120 meters, about 4 meters per century, which is a very rare occurrence. Since then, many scientists have debated how global sea level could have risen to such lengths.

The main theory is that this meltwater came from retreating ice sheets. During this time, temperatures were rising. Prior to, a portion of the world had been covered by ice sheets, a well known ice sheet being the Laurentide ice sheet, which covered Canada completely and parts of America. Global temperatures were rising, particularly around 14,700 years ago, known as the Bolling-Allerod period, a time of abrupt warming. Not too long before, a Heinrick event occurred, 16,000-17,000 years ago, which also caused global temperatures to spike. This caused ice sheets to melt, particularly the Laurentide ice sheet, which is what caused such a great rise in sea levels.

However, there are some problems with this theory. The Bolling-Allerod period was more at the end of 1A, so it may not account for the time before when global sea levels were still rising. The same goes for the Heinrich event. There are a few other prominent theories, with some scientists believing that the source could have come from the Southern Hemisphere, instead of just the Northern Hemisphere, from the Laurentide ice sheet. Some also believe that the meltwater may have come from the Antarctic region, using glacial isostatic adjustment. However, there is still not enough evidence to fully prove any theory. Due to constraints with geologic time frames, no method currently has been enough to fully support these theories.

Various methods have been used to find the source of the meltwater. Geologists have derived mathematical equations to find relative sea levels to find the source. Many have also used geophysical modeling to reconstruct past sea levels of 1a. However, more research is needed to conclude what the true source is. However, there is still not enough evidence to fully prove any theory. Due to how limited the scientific methods are, there's still research being done on this event today, and many geologists are still trying to figure out the source of meltwater. Only time will tell what the true cause is.

A view of the eroded type of iceberg

Changes in Antarctica’s glaciers and ice sheets: in pictures

Photograph: Anadolu/Getty Images

A view of the sloping iceberg

A view of the pool-type iceberg

Have you ever thought about how dinosaurs lived on a warm, swampy Earth and how we live on one that’s cold enough to keep pretty much the entirety of Greenland and Antarctica buried under kilometers-thick sheets of solid ice and wondered, hmm, how did we get from there to here? The short answer is that it took 50 million years of declining atmospheric carbon dioxide concentrations and dropping temperatures, not to mention building an ice sheet or two. For the longer story of the last 50 million years of climate change, including some of the reasons why, catch this episode of our podcast with Dr De La Rocha! You’ll hear about plate tectonics and continental drift, silicate weathering, carbonate sedimentation, and the spectacular effects the growth of Earth’s ice sheets have had on Earth’s climate. There are also lessons here for where anthropogenic global warming is going and whether or not its effects have permanently disrupted the climate system. Fun fact: the total amount of climate change between 50 million years ago and now dwarfs what we’re driving by burning fossil fuels, and yet, what we’re doing is more terrifying, in that it’s unfolding millions of times faster.

Bonus content: If you want to see sketches and plots of the data discussed in this episode, you can do so here!

!!Nerd alert!! 

If you're interested in the primary scientific literature on the subject, these four papers are a great place to start.

Dutkiewicz et al (2019) Sequestration and subduction of deep-sea carbonate in the global ocean since the Early Cretaceous. Geology 47:91-94.

Müller et al (2022) Evolution of Earth’s plate tectonic conveyor belt. Nature 605:629–639.

Rae et al (2021) Atmospheric CO2 over the last 66 million years from marine archives. Annual Review of Earth and Planetary Sciences 49:609-641.

Westerfeld et al (2020) An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science 369: 1383–1387.

Connect with Christina at her blog, on Twitter, and on Mastodon

You may wonder how melting ice sheets may make parts of the world cooler. Some might know of the albedo effect. The white of the ice reflects the energy of the sun back into space. As the ice melts there is less white to reflect the energy and more heat remains on Earth. If the heat is increasing, then how can parts of the world become cooler? This is because the chain reaction that leads to potential cooling isn’t linked to the amount of ice left, but the amount melted. When ice melts, fresh water is added to the ocean and this influences how the ocean functions.

Oceanic circulation is often called a conveyor belt by oceanographers. Ocean waters rise in the Northern Pacific Ocean and the Indian Ocean. Then they travel across the surface to eventually sink again in the North Atlantic Ocean. From there they travel across the bottom of the ocean back to the Pacific and the Indian Oceans. Figure 1 shows this circulation. It takes centuries for waters to travel along the entire conveyor belt, but if the circulation gets disturbed the effects may be noticeable quite a bit faster. This may be a good hundred years, but that is quite fast on a geological time scale.

Figure 1: This map shows a simplified image of the Great Ocean Conveyor Belt. Arrows indicate the direction in which the water travels. Blue indicates deep water currents and red indicates surface water currents (Source: IPCC 1996, based on Broecker (1987)).

So what happens if a lot of fresh water from icebergs gets added in the area where the waters sink? To answer this question we need to know the mechanisms behind the sinking of these waters or, as it is also called, the North Atlantic Deep Water formation. The warmer waters that arrive from the south are saltier without being heavy. This is due to the different properties water can have at different temperatures. As the waters travel north they become colder. The higher salt content causes the waters to become heavier and eventually sink. Much like water evaporates more when the air is warm and then falls down as rain when the air cools. When fresh water is added to these waters their salt concentration decreases and therefore the speed at which they sink decreases as well.

Scientists have measured a slowing down of North Atlantic Deep Water formation in certain places in recent years. They have also observed mass melting of icebergs in the distant past. These are called Heinrich events and can be observed in oceanic sediment by the large rocks that fell as the icebergs melted. Both computer models and natural records from prehistoric times show a chain of events caused by this melting and subsequent slowing and/or stopping of the North Atlantic Deep Water formation. Figure 2 shows a simplified map of climate anomalies from an event that happened 8,200 years ago. Lands near the North Atlantic cooled down, especially during winter, Africa and Asia saw decreases in rain, and the Americas suffered from increased winds.

Figure 2: Summary map of climate anomalies during the 8k event (from Alley & Argustsdottir (2005)).

But how can one part of this circulation cause effects as far away as India? Lets go back to the name ‘conveyor belt’. If you stick a screwdriver in the place where the conveyor belt at a supermarket cash registry disappears down, the whole belt will stop moving. The same applies to the oceanic conveyor belt to a degree. If no more water sinks in the North Atlantic Ocean the rest of the system will slow down and stop as well. The oceans have a massive influence on the climate of the world, especially on landmasses near these oceans. Warm waters from the equatorial region move north and release their heat (also shown in figure 1). During winters this means that temperatures do not get as low as they could. If no more warm waters move north this influx of warmth is also gone. North America and Europe suffer much colder winters in this scenario. Models and prehistoric records have shown the formation of sea ice in Europe in these conditions. In Africa and Asia monsoons are very important for the formation of rain. Monsoons form due to the balance of ocean temperatures and land temperatures. The slowing and/or stopping of the ocean conveyor belt will change this balance and the characteristics of the monsoons will change, leaving Africa and Asia drier than before.

However, we shouldn’t panic and start worrying about apocalyptic winters in Europe or massive crop failures in Africa and Asia just yet. The Earth’s climate is a system about balances. A balance can be upheaved, but as long as certain thresholds aren’t passed the balance will be restored. The slowing of North Atlantic Deep Water formation that has been measured in certain spots recently doesn’t mean that the whole system will be messed up. In other areas the waters are still sinking at regular speeds and scientists haven’t measured any of the other waters moving towards these sinking spots slowing down. Even if it came to it, the conveyor belt can restart and speed back up again, as it has in the past. This article isn’t written for the purpose of scaremongering. It is a look at a truly tiny part of the massive system of checks and balances that influence the climate on Earth. How complicated it is to predict how the human driven climate change will effect the Earth in the long term. There is a reason why we changed from talking about Global Warming to Climate Change. In the past Heinrich events and the climatic changes linked to it were most often observed at the end of ice ages. The Earth was warming, but would temporarily cool down again. However, the forcing behind the warming was too strong and the Earth would continue to warm. If the forcing had not been as strong, the Earth could have gone back to the way it had been before (the ocean conveyor belt never stayed still). This is why we talk about thresholds and tipping points. The Earth can regain her balance. It will take a long time, but we can help her along the way.

If you want to read more about the North Atlantic ‘conveyor belt’ and the associated climatic changes a good place to start is: Alley, R. B. (2007). Wally was right: Predictive ability of the North Atlantic" Conveyor belt" hypothesis for abrupt climate change. Annual Review of earth and Planetary sciences, 35(1), 241-272.

Reference:

Ice on Lake Superior | Feb. 2023