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Sea level rising

In this video, Dr. Nathan B. English talks through the impacts of rising sea levels.
<v ->In this step, we’ll look at the affect</v> of increased anthropogenic greenhouse gas emissions in the atmosphere on oceans. And again, this is an effect, we’ve talked quite a bit about the causes. And I also want to indicate that in this step we’ll be using the same scenarios and representative concentration pathways and terminology that we used in the previous step. So I’m showing you here the global surface temperature change relative to 1850-1900 that I showed you in the previous step in the upper left hand corner here, so panel A. And again we’re looking at the scenarios that examine the RCP 8.5, 7.0, 4.5, 2.6, and 1.9.
1.9 is this Paris Agreement targets, which we’re not very likely to hit. Stringent controls adopted rapidly on CO2 and greenhouse gas emissions. The intermediate scenario, and then the worst case scenario is 8.5. And again you see this reflection of the mean return on the global temperature increase by 2100 that the models have returned. Now I will say that in 1997, I believe, when the first IPCC report came out they did temperature projections and when you go back and look at the temperature projections from the 1990’s in 2020, those temperature projections were almost spot on in the intermediate scenario, and quite accurate.
Personally, I’m pretty confident that this is a good range, and the only place where I wonder if it may be wrong is when we start to consider feedback effects and those feedback effects would tend to bump this top worst case scenario even further up.
So I think we’ve got a lot of good confidence, IPCC has a good track record, it’s based on some solid quantitative science and over 100 years worth of climate science is considered and thought about when these reports come out. Let’s talk about the ocean.
These other three panels, B, C, and D, and again these are from the summary for policy makers, refer to ocean factors. And the first one here is sea ice area. This is the Arctic, this is north of Greenland and Canada and Russia, and you can see that already in the early 20th century we see these pretty significant continuous and persistent decreases each year in the areal extent of sea ice. And this doesn’t count depth of the sea ice, which is thinning rapidly as well. And when you think about polar bears, they depend on sea ice to survive, so the demise of sea ice will greatly impact and decrease polar bear populations.
Now you can see that we’re looking at a practically ice free Arctic Sea in the near future, by 2050 in the worst case scenario. And maybe not ice free, we might save it and bring it back with the most stringent controls. Sea ice is important because sea ice is bright white, it actually reflects a great deal of solar radiation rather than absorbing it. And now ocean is quite dark and when you don’t have sea ice you have dark ocean and that ocean absorbs that solar radiation and then re emits it as infrared. So keeping those polar ice caps is really important as far as providing a sort of shield from increased absorption of heat into our earth’s system.
Now the other thing about declining sea ice is it opens these areas up for resource exploration, including greater fossil fuel resource exploration. So there’s another feedback effect where decreasing sea ice leads to greater fossil fuel extraction leads to increased warming which means even more sea ice decreases. You can see how that is a positive feedback loop. Now over here we look at global ocean surface pH, and that is the power of hydrogen. And pH is a measure of acidity. Oceans are actually a little bit alkaline, they have lots of salts and other total dissolved solids in there that give them a pretty alkaline character.
And we can see that the addition of CO to the atmosphere has actually been absorbed by the ocean. The oceans are one of the greatest absorbers of CO2 in the atmosphere, but when they absorb CO2, they create carbonic acid which adds to the acidity of the ocean. You can see the ocean acidity is declining and has been declining slowly but steadily over the last 20 to 30 years. And if we keep going there’s a real chance we’ll actually begin to acidify our oceans quite significantly. And this has impacts on phytoplankton, on foraminifera, and corals.
Any organism that depends on a carbonate skeleton or a silicate skeleton to survive. So you don’t want to change the chemistry of the oceans too much because you’ll impact it’s ability to be biologically productive and also to absorb CO2. As the oceans become acidic they can absorb less and less CO2. And then finally lets talk about sea level change. This is global mean sea level change relative to 1900, so not to the 1850-1900 average, this is relative to 1900. And I will say right now that sea levels around the world are actually quite different to each other.
You can get differences to 10’s of meters between the global sea level in the Atlantic and the global sea level in the Pacific relative to each other. And in lots of places you get three to four meter tide differences every day aren’t unusual. These rises in sea level are important because 40% of the world’s population lives within about 100 kilometers of the coast. So large changes in sea level, especially in really flat coast lines, so passive continental margins, can go quite far inland and have a big impact on lots of people. So sea level is a really important and significant effect of increased levels of greenhouse gases in the atmosphere.
So what’s the tie, what’s the link between sea level change and greenhouse gases in the atmosphere? Its warming. Its the fact that if you warm the ocean, actually cold water takes up less volume than warm water, and the oceans absorb a huge amount of heat from global climate. In fact, they’re big heat sinks for us and they’re one of the reasons that we haven’t burned ourselves up into little crisps. So thermal expansion right now accounts for about 42% of the sea level rise we’ve seen. And that’s just a very small increase in the temperature of the surface oceans has led to about
a 40% of this 3.6 millimeters per year in sea level rise, is thermal expansion. And I’ll just tell you right now, the average level of sea level rise across the world is 3.6 millimeters per year.
That’s about 3 centimeters per decade.
Which doesn’t seem like much, but if you’ve got a house or if you’re running a port, 3 centimeters can make a big difference.
Or if you’ve got a fresh water aquifer that’s near the coast line, that can make a difference as well. Ice sheets, Greenland, and Antarctica make about 23% of the current sea level rise. So this is melt waters from heating over Greenland and Antarctica that are melting the ice sheets there and they contribute about 23% to this 3.6 millimeters per year. And then high elevation glaciers, so the Rocky Mountains, the Himalayas, Andes, and other mountain glaciers are melting rapidly. And we see these changes actually, most remarkable of all. You can look at photographs of European glaciers from the 1800’s and in some places you can’t even see those glaciers anymore.
They’ve receded and retreated so far back, they’ve melted so much that you can’t see them from where the photographs were taken in the 1800’s. So together I know these don’t add up to 100%, they change every now and then, and estimates differ across studies, but roughly the most expansion we see, the greatest contributor to sea level rise today, is actually just heating the oceans up and hot things expand. And that’s what’s happening to our oceans.
Now, this is an interesting story, they’ve got another storyline in here right, which is this low likelihood high impact storyline, including ice sheet instability. Turns out when you start melting ice sheets, you can actually lubricate the bottom of those ice sheets and then they begin to slide into the ocean. And because the ocean is at a lower elevation than these ice sheets they’re losing altitude and when you go down in altitude you actually warm the air. And because you warm the air you’re going to melt those ice sheets even quicker.
So there’s this low likelihood, we don’t think its likely to happen, but if the ice sheets start to slide off into the oceans, we’re looking at really rapid changes to the sea level over a very short period of time. As it is now, only - I say only, in the worst case scenario there’s only a meter of sea level rise by 2100. Best case scenario, because the oceans are real slow learners and they lag global climate quite a bit because they’re so huge, they just have all this sort of climate inertia, we’ve locked ourselves into a half meter of sea level change already no mater what we do.
So I strongly recommend you don’t buy property within a half meter of the current mean sea level. Because you’ll be needing to build a retaining wall quite soon. The scenarios here don’t differ too much, and that’s because of the slow thermal inertia of the oceans. Now, what I want to show you next is, because of that slow thermal inertia, the environmental scientists who’ve written and thought about this report, especially the sea level portion of it, have thought about well what’s it going to look like in 2300, what have we already locked ourselves into? So 200 years from now, these are the sea levels we’re looking at.
So our great grandchildren will live in a world where global sea levels are between a half and three meters higher than they are today. Just based on energy and greenhouse gases we’ve already added to the atmosphere in the last century and a half. So we’ve pretty much locked ourselves into a half meter of sea level rise unless something drastic changes. And in 300 years, the worst case scenario is that we get between two meters and seven meters of sea level rise. Think about New York City, that’s going to be bad news for New York City. Wall street will become a diving board.
And in really catastrophic scenario, 15 meters of sea level rise can’t be ruled out with high emissions, and that’s considering we lose some of the ice sheets. I think Greenland and the West Antarctic ice sheet. So its sort of a long term problem, but we have to think about it today because we are locking in changes today that won’t become apparent for another 300 years. That’s something we need to consider and think about. So I’ll leave you with that. Again, just 3.6 millimeters per year
and thermal expansion is 42% of it. The slow thermal inertia of the oceans means that sea level rise is slow now, but will continue for a long, long time, even after we make changes to our global carbon and greenhouse gas emissions.

One of the major effects caused by increasing atmospheric temperatures is rising ocean levels.

In this step, Dr. Nathan B. English revisits the scenarios he introduced in the Global Warming step to explore the effect of increased anthropogenic greenhouse gas emissions on the oceans.

Video guide

Since this video is a little longer than most videos in this course, here is a guide to the topics it covers.

Topic Timestamp
The effect on oceans 0:10
The future of sea ice 2:40
Global ocean acidity 4:40
Sea level change 6:00
Future sea levels 11:30

Now it’s your turn

Dr. English refers to the oceans as a ‘slow learner’, meaning that the impact of changes to greenhouse gas emissions may take longer to realise. What impact do you think this will have on the way policymakers address the problem of rising sea levels? Share your thoughts in the comments.


Figures SPM.1, SPM.2, SPM.5 and SPM.8 from IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson- Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.
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