IPC(Sea): Part 1

What the latest UN climate report says about sea level rise

Mar 05, 2022

“Our actions today will shape how people adapt and how nature responds to increasing climate risks.”
Hoesung Lee, IPCC Chairman

Earlier this week, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) published its most recent findings on the state of Earth’s climate. These reports take around seven years to compile and complete; the last one was issued in 2014, which resulted in the landmark Paris Climate Accords. Over 900 scientists from 67 countries volunteered their time to contribute to the report, which focused on the measurable impacts of global warming. It contains 18 chapters about the different manifestations of climate change; six “cross-chapters,” which provide extra analysis; a Synthesis Report aka a general synopsis, and several “fact sheets” aka summaries of the chapters.

Each of these chapters are over 40 pages long(!) and are full of dense sentences & repetitive conclusions, which makes them a joy to read. My aim is to go through every sentence of this report that discusses the ocean and coastal areas. I will then provide context for certain phenomena that the report mentions (e.g. sea level rise, ocean acidification, ocean circulation, geoengineering).

This week I read the IPCC’s Synthesis report, which seemed to conclude that sea level rise was the most urgent problem facing human civilizations. Here are the basics:

What is sea level rise?

Sea level rise (SLR) happens for two reasons: melting of land-based ice and expansion of water as it increases in temperature. The reason loss of sea ice does not affect sea levels is because the sea ice is already part of the ocean water. Think of a nice glass of ice tea, filled to the brim with ice cubes. As it sits out in the summer sun, the cubes will melt and water down the tea. But does the drink begin to spill over the sides? No. While the melting ice changes the makeup of the iced tea, it does not cause it to increase in volume.

If suddenly the bucket of ice sitting next to your tea tipped over, dropping additional cubes into your drink, only then would it overflow. In this metaphor, that bucket of ice is glaciers and polar ice caps. Both of these store ice on land (aka in some location outside of the tea) — if they were to melt, the liquid they fall into would obviously increase in volume.

The ocean is not just affected by land, however. Ocean-atmospheric interactions are key to the ocean’s temperature. When the atmosphere is warm, the cooler ocean waters will absorb some of that heat, trying to achieve thermal equilibrium — when two temperature extremes come into contact, one will cool the other and itself become warmed. However, the ocean’s natural cooling of the air is far lesser in magnitude than the air’s warming of the ocean. So the net effect is that sea water heats up. And as with any object, sea water expands as it warms. Remember in elementary school, when we learned about states of matter? Melting a solid turns it into liquid, and “melting” (aka evaporating) a liquid turns it into gas. In each successive state, the particles that make up the object get bigger and take up more space:

So, as the planet’s average surface temperature increases, more land-based ice will melt, causing sea levels to rise. And because of ocean-atmospheric interactions, the ocean itself will warm and expand, also causing sea levels to rise. The graphic below, while five years old, illustrates quite nicely the magnitude of each SLR factor under the 2014 report’s worst-case scenario:

you can see that thermal expansion & the two biggest land-based ice sheets will add the most to future SLR

Who is affected by SLR?

Generally, people living on coasts, islands, and in low-lying areas will experience excessive flooding, the most obvious and measurable effect of SLR. However, some of the earth’s crust is actually sinking back into the mantel because of a thing called isostasy. Isostasy is a measurement of the elasticity of the earth, aka its ability to “bounce back” after being compressed (cue Big Sean). Imagine a clean mattress with absolutely nothing on it:

If we place bowling balls on that mattress, parts of it will sink (get compressed) under the weight. Other parts of the mattress will rise as the mattress’s fluff is redistributed around the balls:

Now if we remove those bowling balls, the mattress will slowly return to its natural uniform state. The compressed parts will rise up, and the elevated parts will sink down. The same thing happens if an ice sheet covers a large part of the earth’s surface, like in the Last Glacial Maximum (aka the peak of the last ice age) roughly 23,000 years ago. As illustrated by the diagram I so helpfully retrieved from my marine geology class slides, when the ice sheet melts, the land will no longer be compressed, and it will rise up. Parts of the land that were NOT covered by the ice sheet will fall:

if this graphic is too confusing, just think of the mattress!

This phenomena is called glacial rebound, and it affects the rate of SLR in different parts of the earth. Even though global sea levels are rising at 3.6 mm/year, relative sea levels are changing at either a faster or slower rate depending on where the land was during the Last Glacial Maximum. Take North America, for example: everywhere in Canada and southward into the US, until northern New Jersey, was compressed by the glacier.

So, the land New England, the Midwest, and the Upper Great Plains are all rebounding, and that of the Gulf Coast and Mid-Atlantic states are sinking. This means that sea levels will rise faster than 3.6mm/year in areas that are sinking, and it will rise slower than 3.6mm/year (and may even be falling!) in areas that are rebounding. If you live in the US and are curious about SLR in your area, check out these report cards from the Virginia Institute of Marine Science at my alma mater, William & Mary.

I don’t live on the coast or in an area experiencing glacial rebound — should I be worried about SLR?

Yes, but not because your town will be inundated by flooding. As mentioned above sea levels rise as they warm, and larger amounts of warmer water make storms worse. In the Pacific, this can lead to stronger El Niños (another topic for another newsletter), which not only affect weather but also Eastern Pacific fisheries, specifically those of Chile, Peru, and Ecuador. Furthermore, inland & upland areas (e.g. the upper Midwest, Scandinavia) will be ideal locations to live, so large amounts of people from around the world (specifically the coasts & islands) are likely to relocate to these places. If such regions are not prepared, there may not be enough resources for all — or worse, there will be a lack of care and empathy for these climate refugees, making their transition even harder.

In summary, Sea Level Rise is one of the most obvious existential threats to humanity. So, how will it change as the climate changes?

Climate Change: SLR edition

Per the IPCC’s Synthesis report, climate change is getting worse and the window to stop irreversible change is almost completely closed. “Slow-onset processes” like SLR are affected by greenhouse gas emissions, and negative impacts from tropical cyclones have increased due to sea level rise. So, greenhouse gas emissions are worsening storms and flooding.

There are three time periods defined by the IPCC: near, mid, and long term. In the near-term, a span of time from the present until 2040, coastal cities and infrastructure will be continuously flooded until they are submerged and possibly lost because of rising sea levels. Displacement of people will undoubtably increase as climate-caused extremes increase in frequency and severity. In the mid- to long-term (2041-2060 and 2081-the century’s end) cities and settlements close to the land-sea barrier will see a rapid rise in climate change-related risks to their communities. One billion people will be at risk from “coastal-specific climate hazards,” meaning flooding and SLR by mid-century, including in small islands. According to The World Bank, over a quarter of the world’s population (2.2 billion people) currently lives in an area that would be inundated by a once-in-a-hundred-year flood. This number increases to a third of the world’s population if global average sea levels rise by 0.15m relative to current levels; it increases to well over two thirds with a SLR of 0.75 m — without population change and additional adaptation of course. In the second half of this century, climate change will also progressively reduce marine animal biomass (the number of animals in the ocean) because of habitat extinction, fewer nutrients in the water, and higher competition for habitable areas of the ocean.

The report states that “unavoidable sea level rise will bring cascading and compounding impacts” to coastal ecosystems, meaning that flooding and other natural disasters will happen more frequently & severely. This will result in losses of coastal ecosystems and ecosystem services and will damage coastal infrastructure. As the report states plainly,

“Sea level rise poses an existential threat for some Small Islands and some low-lying coasts.”

However, as mentioned above, SLR is a threat for all communities, a fact which the climate report did not mention in its summary. It is likely that the individual chapters and fact sheets have more depth, so I am likely to find a more detailed analysis when I get around to reading them.

So what’s happening and what should we do?

Specifically, the report details that sea level rise (SLR) poses a unique challenge to adaptation. This is because sea levels rise slowly, and any changes, while small in scale, will increase the “frequency and magnitude of extreme events,” for example tropical cyclones, which are projected to escalate in the coming decades. SLR’s measurable effect on extreme weather events and flooding would occur sooner under if land-based ice sheets and glaciers continue to melt. Around 815 million people lived in low elevation coastal zones in 2020; this number could increase by 100-200 million by 2050 because of SLR, ice melt, and glacial/isostatic rebound. There will be increases in rapid flooding, specifically in the Indian Ocean and tropical Pacific where if sea levels rise by 15-40cm the frequency of flooding could double. Projected rise levels are 18-24cm in the “business-as-usual” scenario, which would increase the magnitude of hurricane-induced flooding in the Caribbean.

So in order to stop all this, we should act literally *checks watch* this second. Some helpful adaptive responses to SLR include protecting civilizations by building walls and levees, accommodating flooding by building houses on stilts, and planned relocation of vulnerable communities. Such responses, however would be more effective if planned in advance and were guided by inclusive community engagement. An example of this is the Green New Deal, which would help restructure our society around sustainability & conservation. As the IPCC says, unless “urgent and timely action” is taken, the feedback loop illustrated below on the left will remain unchanged:

Effectively, there is little we laypeople can do to stop or even reverse climate change at this point. Pressuring politicians, joining activist groups, and making green personal choices (carpooling, using reusable plastics, etc.) will help create little societies where people do not exploit their resources. However, as emphasized in the report there needs to be a global collective effort to change the way we live, and until that happens the global average temperatures will continue to rise as will sea levels.

Any-who, next week we will focus on ocean acidification and discuss how that impacts plankton, corals, and humans alike... I may even read another full IPCC chapter! Stay tuned :)

In Circulation

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