I just finished the third week of learning with this online course by the University of Exeter, which I found on futurelearn.com. This post is supposed to help me summarise what I have learnt, check my understanding, and make comments or ask questions. I hope that it will also be interesting or helpful to you! 🙂 (My comments and questions are in bold).
What was covered?
To put it in a nutshell, this past week, I have learnt a lot about how climate change affects the world’s ice sheets, permafrost regions and oceans and about what consequences the resulting changes might have.
Retreat & disappearance of glaciers
I learned that the world’s glaciers have shown a rapid response to global warming over the past two decades, with increased ice loss, rising flow speed, frontal retreat and thinning of the ice. In Greenland, half of the total mass loss is due to surface melt, the remainder is due to blocks of ice breaking off and forming icebergs where the glacier meets the sea (calving).
The amount of ice lost as iceberg calving is increasing as the ice sheets are spreading out faster. Glaciers spread out under their own weight. When the ice is softer because of warming, it spreads out more easily and more ice reaches the edges of the glacier, where it calves off and is lost to the ocean.
Moreover, the ice is also flowing faster because lakes of meltwater on the surface of the ice sheet can drain abruptly, because of which meltwater reaches the bed, where it has a lubricating effect on ice sheet flow. The results of this are a short-lived acceleration and more ice calving off at the margins. The lakes, which emerge because meltwater collects in topographic depressions on the surface, can drain so abruptly because they have a warming effect on the underlying ice as their surface is darker and thus has a lower albedo than the surrounding ice. The meltwater also transports heat to the interior of the ice sheet, which causes it to soften and flow even faster.
I learned that buttressing is a process which opposes spreading of the ice sheet and which is being compromised by rising temperatures. When an ice sheet spreads to the coast under its own weight, it either runs aground on a high spot in the bed, is topographically constrained, or flows over the ocean forming a floating ice shelf. When water and air temperatures increase, floating ice shelves start to melt and may be lost completely eventually. When this happens, a friction component is removed, which causes the glacier to flow faster and to lose more mass at its edges. As I had already learnt in the previous weeks, the loss of floating ice to the ocean does not contribute to sea level rise. However, more land ice being lost because of the glacier flowing faster does.
I did not fully understand why a floating ice tongue is such an important friction component for a glacier, as ice seems to float and move around so easily on water. Is that little bit of friction already enough to reduce the amount of land ice calving off at the edges of the glacier?
Normally, in the summer, half of the surface of the Greenland ice sheet naturally melts. At high elevations, most of the resulting meltwater quickly refreezes in place. Near the coast, some of it is retained by the ice sheet, the rest is lost to the ocean. Thus, it is important to note that calving, just like melting, is a natural ablation mechanism, which is not automatically evidence of climate change. Furthermore, it is still necessary to learn more about ice sheets to determine how big the influence of climate change is, as they are also influenced by other processes like changes in ocean currents or shifting of the North Atlantic pressure systems.
However, the points mentioned above show that factors like air temperature and water temperature can significantly change how much ice calves off, which is why it does make sense to see a connection between rising global temperatures and the rapid increase in ice loss from glaciers.
The online course gave multiple examples of significant and unusual calving events and of glacial retreat in the last years. One was that of the July 2012 calving event at the Helheim glacier in South East Greenland, during which 1.5 cubic kilometres of ice were lost to the ocean. 0.25 cubic kilometres of this ice had been above sea-level, which means that it contributed to sea level rise after it broke off. The force of the iceberg impacting the fjord bottom as it rotated produced an earthquake that was measured by seisometers around the world. That summer, Greenland experienced the largest melt extent in the satellite era. We were also shown pictures of the retreat of the Muir Glacier in Alaska and told that glaciers in that part of the world are not only retreating, but disappearing, which is extremely worrying when we think about the albedo feedback and sea level rise.
Determining the health of a glacier
To determine the health of a glacier, you have to look at its mass balance, which means that you have to measure the normal inputs against the normal outputs. Glaciers gain mass in their highest altitudes through snowfall, avalanches and wind-blown snow and lose mass because of calving, melting, and avalanches. The area of the glacier that gains more mass than it loses is called the accumulation zone, while the area that loses more mass than it gains is called the ablation zone. If accumulation is higher than ablation, the glacier has positive mass balance, if it is the other way around, it has a negative mass balance.
The most obvious way to measure the input and output of a glacier is to take field measurements using a stick. You can just drive a stick into the ice, see how thick it is, come back a couple of months later, and see what has changed. However, this process would have to be repeated multiple times all across the glacier to get a good amount of information, so it is much more accurate and less time consuming to use observations of the Earth from satellites or aircrafts, a method called remote sensing. This way, thickening and thinning of the ice, the retreat of the edge of the glacier, and the weight of the ice sheet can be measured. With this information, the sensitivity of the glacier to climate change can be determined.
One observation from this that was talked about in the online course is that the equilibrium line altitude is rising as more of the glacier is in net ablation. This means that as temperatures are rising, a glacier has to be located ever higher up in order to experience temperatures at which the size of its ice sheet does not decline.
Permafrost is soil in the high latitudes which remains below freezing all year round. Fifteen percent of the Northern Hemisphere is underlain by permafrost to some extent and it is so globally important because it stores around 1.6 trillion tonnes of carbon. This carbon could be released if the ground thaws, which would effectively be irreversible on relevant time scales because it has the potential to unleash a significant positive feedback mechanism (rising temperatures -> will thaw permafrost regions most (warming greatest in the Arctic) -> rapid decomposition of carbon stores -> carbon dioxide & methane released into atmosphere -> thickened blanket -> further warming -> more permafrost thaw…).
It was pointed out in the course that the Arctic has already warmed by one degree Celsius and that it has been observed that there are now thicker active layers (thawed soils above permafrost) and more lakes from melt water in permafrost regions.
Furthermore, if permafrost regions thaw, they turn into wetlands where large thermokarst lakes form that tend to be depleted in oxygen. When that is the case, decomposition in those lakes is anaerobic, which means that methane instead of carbon dioxide is released. Methane has a warming potential rougly 25 times greater than carbon dioxide.
Moreover, there are potentially 400 billion tonnes of methane hydrates (pockets of methane gas trapped within frozen water lattices that are formed at low temperatures and high pressure) deep within the permafrost regions. If only a small fraction of this is released, it could have global implications in terms of amplifying warming.
Currently, methane hydrates are only exposed along coastlines due to erosion from the sea and only eight percent of global atmospheric methane comes from permafrost regions. However, we still should not neglect the danger of it as it could trigger a vicious cycle and a runaway warming process and as some energy companies are apparently seriously looking at methane hydrates as potential fuels.
Reasons for ocean acidification
At the beginning of learning about this new topic, I was reminded of how important the oceans are not only for Earth’s biodiversity, but also for our health. They contain 99% of the living space for animals on Earth, are home to 250,000 species that we know about (there are probably a lot more), and its phytoplankton provide the oxygen for at least one in three breaths we take. Additionally, the oceans are an important source of food for us and they take up about a fourth of man-made carbon dioxide emissions.
Even though the oceans have been more acidic in the past, the rate of change was generally slow enough for organisms to adapt. The rare occasions when ocean acidification happened rapidly brought about mass extinctions of marine calcifying organisms. The chemical reactions that are leading to ocean acidification today are caused by carbon dioxide dissolving into seawater.
When carbon dioxide is taken up by the ocean , it reacts with water to form carbonic acid, which is unstable and quickly dissociates (CO2 + H2O <=> H2CO3 <=> HCO3– + H+, so carbon dioxide + water <=> carbonic acid <=> bicarbonate ion + hydrogen ion). The hydrogen ions have an acidifying effect on the water and as the carbon dioxide levels in the atmosphere increase, so does the concentration of hydrogen ions in the oceans.
The ocean has a natural buffering system for this, called the carbonate buffer. Carbonate ions, which enter the seawater through natural weathering of rocks or from the shells of dead marine animals, soak up the hydrogen ions as they are released. This has kept the pH of the oceans stable for millions of years (CO32- + H+ <=> HCO3– , so carbonate ion + hydrogen ion <=> bicarbonate ion). The bicarbonate ions have an alkalising effect on the water.
At this point, i had a question about this carbonate buffer. If the bicarbonate ions have an alkalising effect on the seawater, and if they occur together with the hydrogen ions when carbonic acid dissociates, why does this process cause ocean acidification at all? Or is the acidifying effect of the hydrogen ions stronger than the alkalising effect of the bicarbonate ions? Could that be the reason why you need extra carbonate ions to form more bicarbonate ions?
It is only explained that the processes which put carbonate ions into seawater are quite slow, which is why they cannot keep up with the rate of carbon dioxide being released into the atmosphere and taken up by the oceans. As a result, the oceans are acidifying. The effect of bicarbonate ions is not explained.
Furthermore, I learned that the pH of the ocean has fallen by 0.1 of a pH unit since the industrial revolution, which is equivalent to a 30% increase in the hydrogen concentration. This kind of change is a real shock to marine animals and it is only expected to become much worse, with predictions stating that there will be a 120% increase in the hydrogen concentration by the end of the century. The pH of seawater is now 8.1 and is expected to fall to 7.6 by 2100 if we continue to release carbon at the rates that are currently predicted.
Effects of ocean acidification on marine invertebrates
Marine invertebrates make up the biggest proportion of ocean biodiversity with 76%. They are very important bottom of the food chain animals, which means that any effects of climate change on them will escalate up the food chain to the fish that we rely on as food. A large number of marine invertebrates have calcium carbonate skeletons. Once formed, these structures are quite vulnerable to dissolution unless they are surrounded by seawater saturated with calcium carbonate. As the seawater becomes under saturated with carbonate ions because they are soaking up hydrogen ions, there is a tendency for the calcium carbonate skeletons of marine invertebrates to start dissolving and there is much less carbonate available for the forming of new structures.
Moreover, as all organisms produce carbon dioxide when they respire, they all have to get rid of it in order to prevent becoming acidic. The buffering system in us organisms is much the same as that of seawater: we can use bicarbonate inside our cells to buffer any acidosis from increased carbon dioxide. However, some animals are much better at controlling their internal carbon dioxide levels than others and there are some marine animals that are quite susceptible to having internal acidosis from ocean acidification.
Furthermore, ocean acidification can also impact the reproduction of marine invertebrates, which is a very sensitive part of any animal’s life cycle. Marine invertebrates reproduce by releasing eggs & sperm directly into the seawater, where fertilisation takes place. Eggs and sperm are very small and less able to control the conditions inside them, which is why they can easily be affected by more acidic conditions in the ocean. There is also a problem for sea urchin larvae, who have to grow tiny calcium carbonate shells in order to swim and feed properly. Their shells do not form properly when exposed to conditions expected for the end of the century. The result will be that much less sea urchin larvae will make it to adult phases, which will also affect the populations of other marine animals.
When it comes to the implications ocean acidification has for marine invertebrates, there are still a lot of questions that are being asked. For example, it would be interesting to know which species will be most affected and which might cope and how. Additionally, scientists are not sure how ocean acidification might interact with other stresses like environmental pollution and chemical contamination.
Next, I learned that most of the heat trapped by the thickening blanket of greenhouses gases (over 90% of excess heat in recent decades) is going into the oceans, which is warming them from the top down. Most of it is being absorbed by the top 700 metres. The result of this is thermal expansion and sea level rise.
Moreover, the heat stress on coral reef environments may lead to bleaching, where corals lose their colour because their photosynthetic symbiont is expelled and starve. I was given an example of an extreme bleaching event which happened after the 2016/17 El Niño event. 90% of corals in the northern section of the Great Barrier Reef bleached. The IPCC warns that two degrees Celsius of global warming (which we are currently on track for) will result in the loss of 99% of warm water corals by 2100. When a reef is tipped into a declining state, algae may take over, which can have a range of negative environmental impacts because of the amount of oxygen they use and of toxins they produce.
Ocean pollution was briefly mentioned, too, even though it is not strictly related to climate change. It is probably the human effect on the environment that most people know and are passionate about because it is the one that is most visible. Examples for ocean pollution are oil spills like the 2010 oil spill in the Gulf of Mexico and plastic pollution, which both have devestating effects on the marine environment.
Oil as well as plastic can kill marine animals by being ingested by them, by trapping them, or by suffocating them. In one study, 90% of sea birds apparently had plastic in their stomachs and research from the University of Exeter shows that plastic micro-beads, which are found in everyday cosmetics, are eaten by tiny marine animals and enter the food chain. As a result, a plate of six oysters can have around 50 plastic micro-beads in it. I learned that there is no marine ecosystem where we have looked so far that does not contain this micro-plastic, not even in the Arctic: 1 litre of Arctic sea ice has been found to contain 12,000 pieces of micro-plastic!
Lastly, I learned a little more about this topic that has been continuously coming up throughout the online course: sea-level rise. It was discussed how both sea-level rise and ocean acidification could have severe economic impacts on communities, especially on poorer communities on the coast. Sea-level rise is already impacting people’s ability to have homes and to grow food where they live near the coast, as houses and fields are being flooded.
The IPCC predicts that sea-levels will rise anywhere between 26 to 98cm until 2100, with there probably being a lot of regional variations. The most notable short-term impact of sea-level rise is likely to be salinity intrusion, where salty water from the ocean is able to infiltrate the groundwater further inland. This groundwater is used for agriculture and is a very important source for drinking water in many parts of the world. Thus, there are plans underway in some areas to combine hard mitigation solutions like dykes, embankments and dams with soft adaption solutions like education, relocation and salt-tolerant crops.
Another economic impact of sea-level rise will be that on the tourism industry, as the shorelines are already starting to retreat. The income from this industry is crucial for a lot of developing countries that have a coast. Thus, sea-level rise might hit them very hard economically, in multiple ways.
At the same time, ocean acidification and ocean warming might have negative effects on fisheries as fish populations might drastically decline and many people who have the choice might stop eating fish as marine animals ingest more and more plastic.
This week has again shown me how important it is that we reduce our use of fossil fuels dramatically and as fast as possible. It is maybe easy for us to underestimate the role the Earth’s ice sheets and oceans play in our well-being, as they are not our typical environments, but it is a fact that they have an immense impact on the answer to the question whether we are or are not able to survive on the planet. The lifestyle of excess and unnecessary consumption of one group of people on the planet is having a devastating effect not only on ecosystems but also on other peoples’ livelihoods.
We have some very good answers to the question “What can we do?”, some regarding technology, some social and economic changes, some addressing personal lifestyle. It is largely up to the wealthier part of the population, mostly people in developed countries, to decide whether they really want to live the way they are living if it is costing the planet’s ecosystems and millions of fellow humans. Each of us should make a decision and act on it, because each of us can actually make a difference. Because remember: You may be a small part of this world, but you are not insignificant!