This is the third lecture in this series on the cryosphere. If you haven't watched the first two, stop right now and go back to the first one. This time, we are discussing global impacts of Arctic changes.
Specifically, we'll look at how changes in sea ice extent and permafrost thaw impact the global climate, and how changes in land ice impact sea level rise, which is also a global phenomenon. The ice-albedo climate feedback loop is one of Earth's most effective air conditioning systems. The ice is white, which means it reflects sunlight back into the atmosphere and into space, keeping the surface of Earth cool.
We say that ice has a high albedo, which is a measure of its capacity to reflect solar radiation. When there is a lot of ice, A lot of energy is reflected back. When temperatures rise a bit, this air conditioning system is able to maintain the system cool anyways.
This is called a negative feedback. The ice maintains cool conditions despite a potential small increase in temperature. We say negative, not because it's bad, but because it slows down the change.
In this case, it slows down the rise in temperature, so it's going counter. to the rising temperature. But eventually sea ice starts shrinking under warmer temperatures, which means that less and less solar energy is reflecting back. Instead, solar radiation is absorbed by the dark blue oceans that have low albedo. This accelerates warming even more because water also has a high heat capacity, i.e. it can relate a it can retain a lot of heat.
This is a positive feedback. The warmer it gets, the more sea ice we lose, the more heat is absorbed by the dark waters, the more heat we have, the warmer it is, and that goes on and on like in a domino effect. For some time, scientists really feared a runaway process where if a certain threshold or tipping point is reached in terms of low sea ice extent, there might be a rapid domino effect leading to very rapid sea ice loss.
The idea was that one day the ocean would absorb so much heat that summer sea ice would no longer form, locking the Arctic into a no sea ice world. But fear not! Based on computer simulations, it is now argued that the Arctic sea ice does not have a tipping point. This means that even if you completely remove sea ice in summer, the ocean loses so much heat in winter that ice regrows rapidly and some of it would survive the following summer.
Glaciers are different from sea ice and we know that they have a no return turning point. The permafrost carbon climate feedback loop could amplify the ongoing climate warming. Soils in the Arctic are frozen, which means that decomposition occurs very slowly. Usually, in a temperate or warm ecosystem, plants grow, then they die, then their biomass becomes litter that gets quickly decomposed by fungi, insects, and microbes.
And whatever tiny fraction of carbon that's left accumulates into the soils over long timescales. In the Arctic, The story is different. The conditions are cold enough that the decomposition of that dead litter, so those those dead plants, does not happen much. Instead, the plants grow very slowly because it's cold, but they also almost don't decompose.
So the soils accumulate thick layers of carbon-rich plant detritus that only have partly been degraded. These layers then freeze into the permafrost. and they remain mostly intact for thousands of years or more. It is estimated that permafrost soils contain more than twice the amount of carbon that is stored in the Amazon forest. But, as the permafrost is thawing under warmer conditions, these layers of plant residues become available for decomposition once again.
In addition, since a lot of these areas become flooded, The active microbes are the ones generating methane because they can live under anaerobic conditions, so in a flooded kind of situation. And methane is a greenhouse gas that has a short lifespan in the atmosphere but that is a lot more potent or effective at sequestering heat than carbon dioxide. Methane is a very potent greenhouse gas. A molecule of methane is 25 times stronger than carbon dioxide.
Methane is formed in millions of lakes around the Arctic where permafrost is found. and each year these lakes are emitting already tremendous amounts of methane. But when we look at how much carbon is in permafrost, still frozen, and the potential for that permafrost to thaw in the future, we estimate that more than 10 times the amount of methane that's right now in the atmosphere will come out of these lakes. So, the fear here is that as climate warms, the permafrost thaws, which floods some areas with these carbon-rich soils, which then turn on those bacteria and microbes that can generate methane, and then that methane is being emitted to the atmosphere, which increases the greenhouse gas concentration, which increases that initial warming.
In other words, another positive feedback loop. That bottom picture with the tilted trees, and we call them scientifically drunken trees, so those drunken trees are showing an area that is actively thawing. The drunk trees, while they're tilted because they are on the margin of an area that is currently collapsing, so the ice in that higher soil is thawing, and so now the soil is kind of subsiding down and collapsing. So those trees are usually going to fall into the swampy area that is more of an orange color on that picture.
This is a peatland. It's a type of wetland environment. It's very wet, and this is where the methane is potentially formed and emitted to the atmosphere.
An interesting aspect of what's happening in the permafrost region is that nature has multiple ways to counter, or fight, if you wish, this carbon loss to the atmosphere. First, warming conditions mean that plants grow faster. This is one way to absorb CO2 out of the atmosphere and store it into plant biomass and eventually into the soil. Another observation is that when the permafrost thaws, this makes available a lot of water that benefits plant growth. These two phenomena greatly explain what is referred to as Arctic greening.
You can see on the map here regions in green where net photosynthesis has increased over the past decades. So think about this, if the Arctic is getting warmer in the summertime this means that plants can grow for a longer time so they're happier they take CO2 out of the atmosphere when they do photosynthesis, and they grow taller, fatter, and they march northward, which means that overall there's an increase in that carbon on land, and then less carbon in the atmosphere. So this is a kind of natural countermeasure. And it adds up.
Look at the scale of this greening effect. So in a way, this CO2 concentration or this CO2 absorption by the plants fights... against those methane emissions that we discussed earlier for the decomposing soil.
And to this day, scientists still disagree on which will win the battle. Will there be an overall net increase in CO2 being stored, or will there be a net increase in methane being emitted to the atmosphere? Lastly, as you might have heard before, glacier ice loss leads to sea level rise.
Sea level has been rising at a rate of about an eighth of an inch per year over the past few decades. For example, from 1993 to 2004, sea level increased by 2.6 inches based on satellite data. Higher sea levels mean that deadly and destructive storm surges push farther inland than they once did.
It also means more flooding near the coastal areas of course, and potential seawater intrusion into some aquifers. Glacial ice melting is not the only explanation for sea level rise. The other one is thermal expansion which is caused by the warming of the ocean.
So water expands as it warms, those molecules of H2O. The issue with land ice loss is that there is potential for runaway process to happen. On this schematic, and this is a part of the West Antarctic Ice Sheet, you see that warm, deep ocean water can flow under the ice shelf, chewing away at the grounding line. Melting can be as much as 20 to 50 meters of ice thickness each year.
As the glacier's base recedes, the brakes holding the continental ice ease up and the glaciers feeding the ice shelf accelerate. So the glacier is feeding, kind of moving, imagine a conveyor belt of ice going from the top of the glaciers towards the ocean, and so that conveyor belt accelerates and that further thins and recedes the whole ice sheet. These days, based on repeat satellite measurements, We know that the rate of ice flow in this part of the world is estimated at over 1.5 km per year.
We know that, or we think that one day, the ice shelf could split up and repeatedly kind of peel off, and these ice cliffs that are peeling off from it could topple. That's an example of a runaway process. Quick recap here.
At the beginning of the lecture, we asked the question, how are the changes in the cryosphere impacting the world? We saw through two case studies that feedback loops that happen in the Arctic can further accelerate the ongoing climate warming. One has to do with sea ice loss and lower albedo, and the other one has to do with permafrost thaw and soil carbon. We also discussed How ice melting contributes to sea level rise, and that is something we need to watch because of potential runaway melt events.