Climate Change

It’s getting hot in here: IPCC and the ocean

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 [MassonDelmotte, 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.

As we are reminded on a near-daily basis, we are living through “unprecedented times.” By now I’m sick of hearing this phrase, but a new report from the Intergovernmental Panel on Climate Change (IPCC) has given it a whole new context. In their report, the IPCC reiterated that the rate of recent climate change is “unprecedented,”  and that it’s “unequivocal” that human influences – mostly in the form of carbon dioxide emissions – are the cause. For anyone paying attention, this isn’t a shock, but what does it mean for the oceans, and what does it mean for us?

Climate Change and Oceans

The IPCC report says that climate change will impact the oceans in four major ways: water temperature will continue to increase, ocean pH will drop (ocean acidification), oxygen levels in the ocean will fall, and sea level will rise as ice melts and warm water expands. All of these changes will be long-lasting, possibly for thousands of years. That isn’t long on a planetary scale, but on a human scale this is astronomical; the entirety of recorded human history lasts about 5000 years. If we looked forward 5000 years from now, we might just begin to see the ocean recovering from our actions… maybe.

Photo of a bleached coral reef, Scott Reef, in April 2016. When coral is stressed, which can occur when water gets too warm, it evicts the algae in its tissue that gives it its pigment. Photo by James Gilmour from the Australian Institute of Marine Science.

1) Warming Waters

The ocean is an amazing sink for both carbon and heat. As a result, the ocean has absorbed a lot of the radiation that has been trapped by greenhouse gases; when scientists look at how much the planet has warmed, ocean warming accounts for  91% of the change. As climate change ramps up, marine heat waves are expected to become more frequent, meaning sea creatures will have to contend with temperatures they are not used to and haven’t had to time to adapt to. It’s like how people in Portland had to contend with 114°F temperatures in homes without air-conditioning. Short-term heat waves are hard on marine life, but smaller, long-lasting changes can also be difficult to adjust to (find oceanbites articles on how heat impacts the ocean here).

2) Deoxygenation

Water temperature can also impact the amount of oxygen in the ocean. Colder water holds more oxygen, which fish and other sea creatures need to breath. As it heats up, the molecules in the water move around more quickly (because they have more stored energy), which in turn bounces oxygen molecules out.

A warming ocean may also lead to greater stratification, which is the vertical separation of water with different densities. Colder water is more dense, meaning it sinks deeper into the ocean. Warmer water, by contrast, is less dense and will stay high up. That’s why if you’ve ever gone swimming in a lake and you stop to tread water, sometimes your feet will get cold while your arms are warm at the surface. When water densities are too different, the water doesn’t mix well. Oxygen can’t get from the air down to deeper water – it gets trapped at the surface. If oxygen is not replenished and drops too low, it’s bad news for many sea creatures (particularly the ones that can’t swim away). (You can read more about ocean deoxygenation here).

3) Ocean Acidification

This photo shows how shelled animals struggle with ocean acidification. The shell on this marine snail should be hard and protective, but instead it is brittle and clear. Photo from NOAA’s National Ocean Services Photostream.

Ocean acidification is the other direct effect of increased CO2 in the atmosphere. While there is a lot of chemistry that goes into the process (you can find it here at oceanbites) the crux of the matter is that as the ocean absorbs CO2 from the air, the pH of the water drops, making the water more acidic. This is particularly hard on plants and animals that form some sort of carbonate shell, i.e. clams, coral, snails, crabs etc. Ocean acidification can both weaken existing shells and make it more difficult to form new ones. While both rising ocean temperatures and ocean acidification can be a lot for marine life to adjust to, having to contend with both stressors at the same time can make the process even harder.

4) Sea Level Rise

In the last century, sea level rose only about 0.m, but sea level is rising faster and faster. From 1971 to 2006, sea level rose at a rate of 1.3mm (0.05 in) per year. Today, sea level rises by almost three times that, 3.7mm (0.15 in) per year. Those measurements may seem small at face value, but they add up. If we limit global warming to 1.5°C (meaning we stop emitting carbon today), sea level will rise 2-3m (6.5-9.8ft) – enough to cause frequent flooding and erosion in many coastal areas. If we continue to burn carbon with reckless abandon, sea level could rise 19-22m (62-72ft).

The Situation is Dire… But Our Future is Still Ours to Decide

No matter how you slice it, things are serious. Under every emissions scenario (very low to very high emissions) the world will be 1.5°C warmer in the next 20 years than it was in 1850. That means that no matter what we do now, some warming is inevitable. If we take climate change seriously, it may feel a bit like the first several weeks of COVID lockdowns – where we were making lots of sacrifices, but case numbers remained high because the lockdown benefits hadn’t yet taken effect. In times like that, it’s easy to feel like nothing matters.

Miami is one of the cities most threatened by sea level rise in the world. Currently, high tide frequently leads to flooding, which has led several parts of the city to raise their streets by 2 feet. Photo from B137 on Wikimedia Commons.

But if we take climate change seriously, we will see the results later on. In the lowest emissions scenario, global temperatures in 2021 are still only about 1.4°C (2.52 °F) warmer than 1850; in the very high emissions scenario, we could be looking at a world 4.4°C (7.92 º F) warmer. This is a difference worth fighting for, as are smaller differences. For every 0.5°C increase in global average temperature, the number of events with extremely hot temperatures, extremely heavy rainfall, and agricultural drought increase, as do the number of areas that experience them. For every 0.5°C warming we prevent, more people are fed, more people get to stay in their homes, and more marine species have a place where they can survive.

We are at the unique point in time where what we do literally impacts the entire course of history. It’s time to step up and demand systemic change. If you’re wondering where to start, call your representatives, talk to your mayor, talk to your friends, find the people in your community who are already fighting for healthy air and for their grandchildren’s futures. The IPCC report delivered an unequivocal warning for “unprecedented” times. Let’s work to ensure we don’t have to continue describing our lives this way.

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