Professor Alan Mix was one of many scientists contributing to the latest report released by the Intergovernmental Panel on Climate Change, which includes over 230 authors from 65 nations. The IPCC report plays a role in addressing the ongoing issue of global warming by highlighting what is currently happening, such as extreme weather and rising sea levels, and what is expected in the future. Mix is a distinguished professor of Earth, ocean and atmospheric sciences and was the lead author on the “Ocean, Cryosphere and Sea Level Change” chapter. He joins us to share details on his findings.

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This transcript was created by a computer and edited by a volunteer.

Dave Miller: From the Gert Boyle studio at OPB, this is Think Out Loud. I’m Dave Miller. Earlier this month, the UN’s Intergovernmental panel on climate change, or IPCC, released the first half of its latest comprehensive report on the state of our warming planet. These reports are the culmination of the work of thousands of scientists from around the world. One of those scientists, one of the lead authors of the chapter on oceans is from Oregon, Alan Mix, is a distinguished professor of Earth, Ocean and Atmospheric Sciences at Oregon State University. He joins us now to talk about this latest report. Alan Mix, welcome to Think Out Loud.

Alan Mix: Thanks Dave. Thanks for having me.

Miller: You’re one of the lead authors of the Ocean Cryosphere (I had to look that up), meaning frozen water and sea level chapter. Let’s take a look at those one by one, starting with ocean heat. There are different ways to look at increases in ocean heat including surface temperature, total heat content and marine heatwaves. What do you see as the most significant findings about ocean heat in this latest report?

Mix: I think it’s probably not widely appreciated that most of the excess heat caused by global warming is going into the ocean. In fact, about 90% of it or so. We’ve seen about 1.1 degrees Centigrade or a couple degrees Fahrenheit warming in the atmosphere. But that’s just a small piece. That’s just a couple of a percent of the problem. Most of that heat is going into the ocean. That means it’s sort of like storing it in the bank. That means it’s there, and it’s going to make it harder on the other end if we get climate change under control to bring the temperature back down, because all that heat in the ocean is serving as a sort of buffer.

Miller: And it lasts longer because actually, it’s a thicker substance. It holds onto heat for longer.

Mix: Yeah, that’s right. Of course, the ocean is huge. It’s deep and that heat is penetrating into the subsurface of the ocean. It takes a long time for the ocean to circulate. So that means it’s a very, very slow process where that heat, as it builds up, will be able to come out over hundreds or perhaps even thousands of years. It just makes it really hard to bring things back down.

Miller: It’s interesting that you brought up thousands of years because most of the years that I saw in this chapter, and looking through other chapters in the report, talk about confidence levels for ‘x’ to happen by 2050 or by the year 2100. I saw a few references to the year 3000 or 4000, but that scale is also something that oceanographers and climate scientists are thinking about right now?

Mix: Yeah, that’s right. My job is actually working on the past, the geologic history of how the ocean and atmosphere changed. That’s important because we’re now in the process of leaving all historical precedent, everything we have history on. We’re approaching warming, for example, that happened during the last interglacial period 120,000 years ago. So, the way we figure out how these slow processes work, like the ocean, is to figure out how they worked in the past. That’s going to tell us how sensitive they are to change and how fast they can move heat around. Things like that.

Miller: The new report notes it’s virtually certain that upper ocean stratification will continue to increase throughout the 21st century. What’s the significance of that?

Mix: Well, that’s important for several reasons. First, as the surface of the ocean heats. That means the warmer water is less dense. So that stratifies, it makes it harder for the water to mix from the surface down into the subsurface. That also makes it hard, for example, for oxygen to penetrate from the atmosphere down into the subsurface. So we might expect to see a rise in low oxygen or what we call hypoxic systems and that produces things like dead zones that kill off bottom fish.

Miller: And that stratification is likely to increase in the coming century.

Mix: Yeah, that’s what we think. There’s a variety of ways that happens and it depends on how this warming affects the biology, the critters that grow in the surface ocean under sunlight. So it’s a pretty complex system on how all those things intertwined. But for sure this stratification isn’t helping.

Miller: There have been some recent reports about what’s known as the AMOC, the Atlantic Meridional Overturning Circulation. Before we get to the projections for what could happen, what is the AMOC?

Mix: The MOC there stands for Meridional Oceans Overturning Circulation. What happens is the ocean cools in the high latitudes. It gets more dense, the opposite of this warming and stratification, sinks and then circulates around through the interior of the ocean. That’s important because that circulation, that is a really big deal. A big thing. It moves heat around on the planet. When you move heat around the planet, that changes the climate in different places. So the question is then, are those really big things like that going to change?

Miller: Am I right? This is connected to the Gulf Stream and this is connected to the relatively moderate temperatures that Western Europe enjoys?

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Mix: Yeah, that’s right. The upper ocean moves northwards out of the warm parts of the world to replace the water that sinks in the high latitudes. You can think of that like a heat pump.

Miller: The report says it’s very likely to decline over the 21st century. What could that mean?

Mix: There’s a lot of debate about that right now, but it could mean that you might see some very odd regional manifestations of climate. Just because the globe is warming doesn’t mean every place on the planet is going to warm. If you change these heat transports, some places could get colder. So we could have some surprises. There’s some wild cards in the system.

Miller: This gets to some tortured language, maybe understandably because we’re dealing with tricky probabilities, but the report says that there is only medium confidence that decline will not involve an abrupt collapse before the year 2100. I guess I’m still trying to figure out what exactly an abrupt collapse would look like.

Mix: That’s medium confidence because there are a variety of models to simulate these things and some of them produce shutdowns of this ocean circulation and some of them don’t. We don’t really have a lot of observational evidence right now for it shutting down. There is some monitoring going on and it shows that it varies from year to year or decade to decade, but there isn’t evidence right now of a shutdown. So we’re relying on models to simulate that.

We do know from the paleo records, this record of what happened in the past, that there have been some pretty massive changes in the subsurface circulation. So we know these things are possible. That’s why we can’t exclude them. But right now it is really hard to predict things like total shutdown. We’re also learning that it’s not just AMOC. There are other MOCs out there. For example, there’s a Meridional Overturning Circulation that happens from the southern ocean that’s deeper than the MOC. You can think about this as a big machine with a bunch of gears that are intertwined and we don’t quite know how all the gears work. But we know that all the pieces of this circulation interact with each other. One of the things in the new report is some focus on the southern ocean, the area around Antarctica, and how that interaction with the surface interacts with the deep circulation and how the heat moves into the ocean. That too is intertwined in this AMOC stuff. So, it’s a really complicated puzzle to figure out. And so that’s why you’re hearing honest words like medium confidence.

Miller: Let’s turn to the cryosphere part of this, the ice part. Can you give us a broad sense for what the report says about sea ice or ice sheets or glaciers?

Mix: Let’s start with the glaciers and the ice sheets. The ice sheets in particular are a slow part of the system. We know now that Greenland is certainly in melt condition. It’s losing mass. In fact, I just read in the news that it rained at the summit of Greenland, I think yesterday, which is unprecedented. It’s not supposed to do that. So we know that that’s happening and we know it’s going to continue. And we know that, just from the warming we’ve already experienced, a lot of this loss of ice [is] already baked into the system. It takes a long time for these big ice sheets to respond. But they have to, and they will over a period. Again, this is one of these things that’s gonna go on for hundreds or perhaps thousands of years.

Miller: So that’s ice sheets and glaciers. What about sea ice itself?

Mix: Yep, sea ice. In fact, I was just up in the arctic a couple of years ago. We took a ship around the north end of Greenland and were the first ship into some of the fjords in north Greenland. It was pretty much open water up there, smooth sailing. And that’s surprising.

So certainly there’s evidence that we’re losing sea ice. We’re losing it pretty quickly. It doesn’t mean that it won’t freeze in the winter. But we’re expecting that sometime within the next century we’re going to see a condition that, towards the end of the summer, there may be very little ice up in the arctic. That’s a problem because the ice coating the surface ocean up there is white, so it reflects sunlight. As soon as it goes away, then you’re seeing more sunlight absorb and even more warming.

Miller: It’s one of those feedback loops that then feeds on itself. A lot of what we’ve been talking about already connects to the next big issue in your chapter, which is sea level rise. The chapter notes that, like a lot of things, this is not going to be uniform around the globe. Some places will see more than others, but in general, how big an increase in sea level are we looking at over the next century?

Mix: Yeah, that’s right. It’s variable from place to place. But if you look at the global average, we know now that sea level has gone up about, give or take, 8 inches over the last century or so. We also know, and this was debated until recently, but we’re really quite certain the evidence is good, that it’s accelerating. It’s accelerating in part because this melting of the glaciers is kicking into gear. So far, most of the sea level rise has been just from this absorption of heat by the ocean that we talked about at the beginning. That makes the water expand and it makes the sea level go up. But now we’re seeing a situation where that warming is continuing. But now the melting of the glaciers and ice sheets are adding to it. So that’s accelerating.

We know we’ve done about 8 inches of sea level rise so far. We know it’s accelerating. So the question is, what’s going to happen in the future? That’s kind of a choose your own adventure story here, because it really depends on what we do with CO2 emissions. So, under the best case scenario that’s played out in the report, we might see by the year 2100 maybe about a foot of sea level rise. Reality is that’s a pretty optimistic scenario. [It] is not clear that we’ll be able to achieve that one. A middle range scenario might see a couple feet of sea level rise and a bad scenario, in which we really don’t do much of anything, might see three feet of sea level rise. But it’s important to remember that that’s not the end of the sea level rise. It’s gonna keep going from that. So although we’re in policy around more near-term effects, it’s not going to stop in the year 2100. It is going to keep going.

Miller: What do you most want the general public to take away from this report?

Mix: There’s certainly some doom and gloom in the report, but there’s optimism too. We can’t go back to the world we had. It’s too late for that. But we can act now and minimize future damage and get it under control. So I think there’s a huge opportunity ahead and it really involves reducing our CO2 emissions. There’s no way around it. We have to do that and then we can choose our future adventure that’s within bounds, that’s reasonable. I think there’s huge opportunities for the economy, for jobs and so on, in making that happen. That’s often lost in the doom and gloom stories, that there is a way out of this and we should be able to do that. There’s lots of technologies developing now and it is possible. It is possible. We can’t go back, but we can limit the damage.

Miller: Alan Mix, thanks very much.

Mix: Thank you.

Dave Miller: That’s Alan Mix, a professor of Earth, Ocean and Atmospheric Sciences at Oregon State University.

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