Future projections of Antarctic ice shelf melting

Climate change will increase ice shelf melt rates around Antarctica. That’s the not-very-surprising conclusion of my latest modelling study, done in collaboration with both Australian and German researchers, which was just published in Journal of Climate. Here’s the less intuitive result: much of the projected increase in melt rates is actually linked to a decrease in sea ice formation.

That’s a lot of different kinds of ice, so let’s back up a bit. Sea ice is just frozen seawater. But ice shelves (as well as ice sheets and icebergs) are originally formed of snow. Snow falls on the Antarctic continent, and over many years compacts into a system of interconnected glaciers that we call an ice sheet. These glaciers flow downhill towards the coast. If they hit the coast and keep going, floating on the ocean surface, the floating bits are called ice shelves. Sometimes the edges of ice shelves will break off and form icebergs, but they don’t really come into this story.

Climate models don’t typically include ice sheets, or ice shelves, or icebergs. This is one reason why projections of sea level rise are so uncertain. But some standalone ocean models do include ice shelves. At least, they include the little pockets of ocean beneath the ice shelves – we call them ice shelf cavities – and can simulate the melting and refreezing that happens on the ice shelf base.

We took one of these ocean/ice-shelf models and forced it with the atmospheric output of regular climate models, which periodically make projections of climate change from now until the end of the century. We completed four different simulations, consisting of two different greenhouse gas emissions scenarios (“Representative Concentration Pathways” or RCPs) and two different choices of climate model (“ACCESS 1.0”, or “MMM” for the multi-model mean). Each simulation required 896 processors on the supercomputer in Canberra. By comparison, your laptop or desktop computer probably has about 4 processors. These are pretty sizable models!

In every simulation, and in every region of Antarctica, ice shelf melting increased over the 21st century. The total increase ranged from 41% to 129% depending on the scenario. The largest increases occurred in the Amundsen Sea region, marked with red circles in the maps below, which happens to be the region exhibiting the most severe melting in recent observations. In the most extreme scenario, ice shelf melting in this region nearly quadrupled.

Percent change in ice shelf melting, caused by the ocean, during the four future projections. The values are shown for all of Antarctica (written on the centre of the continent) as well as split up into eight sectors (colour-coded, written inside the circles). Figure 3 of Naughten et al., 2018, © American Meteorological Society.

So what processes were causing this melting? This is where the sea ice comes in. When sea ice forms, it spits out most of the salt from the seawater (brine rejection), leaving the remaining water saltier than before. Salty water is denser than fresh water, so it sinks. This drives a lot of vertical mixing, and the heat from warmer, deeper water is lost to the atmosphere. The ocean surrounding Antarctica is unusual in that the deep water is generally warmer than the surface water. We call this warm, deep water Circumpolar Deep Water, and it’s currently the biggest threat to the Antarctic Ice Sheet. (I say “warm” – it’s only about 1°C, so you wouldn’t want to go swimming in it, but it’s plenty warm enough to melt ice.)

In our simulations, warming winters caused a decrease in sea ice formation. So there was less brine rejection, causing fresher surface waters, causing less vertical mixing, and the warmth of Circumpolar Deep Water was no longer lost to the atmosphere. As a result, ocean temperatures near the bottom of the Amundsen Sea increased. This better-preserved Circumpolar Deep Water found its way into ice shelf cavities, causing large increases in melting.

Slices through the Amundsen Sea – you’re looking at the ocean sideways, like a slice of birthday cake, so you can see the vertical structure. Temperature is shown on the top row (blue is cold, red is warm); salinity is shown on the bottom row (blue is fresh, red is salty). Conditions at the beginning of the simulation are shown in the left 2 panels, and conditions at the end of the simulation are shown in the right 2 panels. At the beginning of the simulation, notice how the warm, salty Circumpolar Deep Water rises onto the continental shelf from the north (right side of each panel), but it gets cooler and fresher as it travels south (towards the left) due to vertical mixing. At the end of the simulation, the surface water has freshened and the vertical mixing has weakened, so the warmth of the Circumpolar Deep Water is preserved. Figure 8 of Naughten et al., 2018, © American Meteorological Society.

This link between weakened sea ice formation and increased ice shelf melting has troubling implications for sea level rise. The next step is to simulate the sea level rise itself, which requires some model development. Ocean models like the one we used for this study have to assume that ice shelf geometry stays constant, so no matter how much ice shelf melting the model simulates, the ice shelves aren’t allowed to thin or collapse. Basically, this design assumes that any ocean-driven melting is exactly compensated by the flow of the upstream glacier such that ice shelf geometry remains constant.

Of course this is not a good assumption, because we’re observing ice shelves thinning all over the place, and a few have even collapsed. But removing this assumption would necessitate coupling with an ice sheet model, which presents major engineering challenges. We’re working on it – at least ten different research groups around the world – and over the next few years, fully coupled ice-sheet/ocean models should be ready to use for the most reliable sea level rise projections yet.

A modified version of this post appeared on the EGU Cryospheric Sciences Blog.

Advertisements

Modelling the Apocalypse

Let’s all put on our science-fiction hats and imagine that humans get wiped off the face of the Earth tomorrow. Perhaps a mysterious superbug kills us all overnight, or maybe we organize a mass migration to live on the moon. In a matter of a day, we’re gone without a trace.

If your first response to this scenario is “What would happen to the climate now that fossil fuel burning has stopped?” then you may be afflicted with Climate Science. (I find myself reacting like this all the time now. I can’t watch The Lord of the Rings without imagining how one would model the climate of Middle Earth.)

A handful of researchers, particularly in Canada, recently became so interested in this question that they started modelling it. Their motive was more than just morbid fascination – in fact, the global temperature change that occurs in such a scenario is a very useful metric. It represents the amount of warming that we’ve already guaranteed, and a lower bound for the amount of warming we can expect.

Initial results were hopeful. Damon Matthews and Andrew Weaver ran the experiment on the UVic ESCM and published the results. In their simulations, global average temperature stabilized almost immediately after CO2 emissions dropped to zero, and stayed approximately constant for centuries. The climate didn’t recover from the changes we inflicted, but at least it didn’t get any worse. The “zero-emissions commitment” was more or less nothing. See the dark blue line in the graph below:

However, this experiment didn’t take anthropogenic impacts other than CO2 into account. In particular, the impacts of sulfate aerosols and additional (non-CO2) greenhouse gases currently cancel out, so it was assumed that they would keep cancelling and could therefore be ignored.

But is this a safe assumption? Sulfate aerosols have a very short atmospheric lifetime – as soon as it rains, they wash right out. Non-CO2 greenhouse gases last much longer (although, in most cases, not as long as CO2). Consequently, you would expect a transition period in which the cooling influence of aerosols had disappeared but the warming influence of additional greenhouse gases was still present. The two forcings would no longer cancel, and the net effect would be one of warming.

Damon Matthews recently repeated his experiment, this time with Kirsten Zickfeld, and took aerosols and additional greenhouse gases into account. The long-term picture was still the same – global temperature remaining at present-day levels for centuries – but the short-term response was different. For about the first decade after human influences disappeared, the temperature rose very quickly (as aerosols were eliminated from the atmosphere) but then dropped back down (as additional greenhouse gases were eliminated). This transition period wouldn’t be fun, but at least it would be short. See the light blue line in the graph below:

We’re still making an implicit assumption, though. By looking at the graphs of constant global average temperature and saying “Look, the problem doesn’t get any worse!”, we’re assuming that regional temperatures are also constant for every area on the planet. In fact, half of the world could be warming rapidly and the other half could be cooling rapidly, a bad scenario indeed. From a single global metric, you can’t just tell.

A team of researchers led by Nathan Gillett recently modelled regional changes to a sudden cessation of CO2 emissions (other gases were ignored). They used a more complex climate model from Environment Canada, which is better for regional projections than the UVic ESCM.

The results were disturbing: even though the average global temperature stayed basically constant after CO2 emissions (following the A2 scenario) disappeared in 2100, regional temperatures continued to change. Most of the world cooled slightly, but Antarctica and the surrounding ocean warmed significantly. By the year 3000, the coasts of Antarctica were 9°C above preindustrial temperatures. This might easily be enough for the West Antarctic Ice Sheet to collapse.

Why didn’t this continued warming happen in the Arctic? Remember that the Arctic is an ocean surrounded by land, and temperatures over land change relatively quickly in response to a radiative forcing. Furthermore, the Arctic Ocean is small enough that it’s heavily influenced by temperatures on the land around it. In this simulation, the Arctic sea ice actually recovered.

On the other hand, Antarctica is land surrounded by a large ocean that mixes heat particularly well. As a result, it has an extraordinarily high heat capacity, and takes a very long time to fully respond to changes in temperature. So, even by the year 3000, it was still reacting to the radiative forcing of the 21st century. The warming ocean surrounded the land and caused it to warm as well.

As a result of the cooling Arctic and warming Antarctic, the Intertropical Convergence Zone (an important wind current) shifted southward in the simulation. As a result, precipitation over North Africa continued to decrease – a situation that was already bad by 2100. Counterintuitively, even though global warming had ceased, some of the impacts of warming continued to worsen.

These experiments, assuming an overnight apocalypse, are purely hypothetical. By definition, we’ll never be able to test their accuracy in the real world. However, as a lower bound for the expected impacts of our actions, the results are sobering.

A Little Bit of Hope

I went to a public lecture on climate change last night (because I just didn’t get enough of that last week at AGU, apparently), where four professors from different departments at my university spoke about their work. They were great speeches – it sort of reminded me of TED Talks – but I was actually most interested in the audience questions and comments afterward.

There was the token crazy guy who stood up and said “The sun is getting hotter every day and one day we’re all going to FRY! So what does that say about your global warming theory? Besides, if it was CO2 we could all just stop breathing!” Luckily, everybody laughed at his comments…

There were also some more reasonable-sounding people, repeating common myths like “It’s a natural cycle” and “Volcanoes emit more CO2 than humans“. The speakers did a good job of explaining why these claims were false, but I still wanted to pull out the Skeptical Science app and wave it in the air…

Overall, though, the audience seemed to be composed of concerned citizens who understood the causes and severity of climate change, and were eager to learn about impacts, particularly on extreme weather. It was nice to see an audience moving past this silly public debate into a more productive one about risk management.

The best moment, though, was on the bus home. There was a first-year student in the seat behind me – I assume he came to see the lecture as well, but maybe he just talks about climate change on the bus all the time. He was telling his friend about sea level rise, and he was saying all the right things – we can expect one or two metres by the end of the century, which doesn’t sound like a lot, but it’s enough to endanger many densely populated coastal cities, as well as kill vegetation due to seawater seeping in.

He even had the statistics right! I was so proud! I was thinking about turning around to join in the conversation, but by then I had been listening in for so long that it would have been embarrassing.

It’s nice to see evidence of a shift in public understanding, even if it’s only anecdotal. Maybe we’re doing something right after all.

Uncertainty

Part 5 in a series of 5 for NextGen Journal
Read Part 1, Part 2, Part 3, and Part 4

Scientists can never say that something is 100% certain, but they can come pretty close. After a while, a theory becomes so strong that the academic community accepts it and moves on to more interesting problems. Replicating an experiment for the thousandth time just isn’t a good use of scientific resources. For example, conducting a medical trial to confirm that smoking increases one’s risk of cancer is no longer very useful; we covered that decades ago. Instead, a medical trial to test the effectiveness of different strategies to help people quit smoking will lead to much greater scientific and societal benefit.

In the same manner, scientists have known since the 1970s that human emissions of greenhouse gases are exerting a warming force on the climate. More recently, the warming started to show up, in certain patterns that confirm it is caused by our activities. These facts are no longer controversial in the scientific community (the opinion pages of newspapers are another story, though). While they will always have a tiny bit of uncertainty, it’s time to move on to more interesting problems. So where are the real uncertainties? What are the new frontiers of climate science?

First of all, projections of climate change depend on what the world decides to do about climate change – a metric that is more uncertain than any of the physics underlying our understanding of the problem. If we collectively step up and reduce our emissions, both quickly and significantly, the world won’t warm too much. If we ignore the problem and do nothing, it will warm a great deal. At this point, our actions could go either way.

Additionally, even though we know the world is going to warm, we don’t know exactly how much, even given a particular emission scenario. We don’t know exactly how sensitive the climate system is, because it’s quite a complex beast. However, using climate models and historical data, we can get an idea. Here is a probability density function for climate sensitivity: the greater the area under the curve at a specific point on the x-axis, the greater the probability that the climate sensitivity is equal to that value of x (IPCC, 2007):

This curve shows us that climate sensitivity is most likely around 3 degrees Celsius for every doubling of atmospheric carbon dixoide, since that’s where the area peaks. There’s a small chance that it’s less than that, so the world might warm a little less. But there’s a greater chance that climate sensitivity is greater than 3 degrees so the world will warm more. So this graph tells us something kind of scary: if we’re wrong about climate sensitivity being about 3 degrees, we’re probably wrong in the direction we don’t want – that is, the problem being worse than we expect. This metric has a lot to do with positive feedbacks (“vicious cycles” of warming) in the climate system.

Another area of uncertainty is precipitation. Temperature is a lot easier to forecast than precipitation, both regionally and globally. With global warming, the extra thermal energy in the climate system will lead to more water in the air, so there will be more precipitation overall – but the extra energy also drives evaporation of surface water to increase. Some areas will experience flooding, and some will experience drought; many areas will experience some of each, depending on the time of year. In summary, we will have more of each extreme when it comes to precipitation, but the when and where is highly uncertain.

Scientists are also unsure about the rate and extent of future sea level rise. Warming causes the sea to rise for two different reasons:

  1. Water expands as it warms, which is easy to model;
  2. Glaciers and ice sheets melt and fall into the ocean, which is very difficult to model.

If we cause the Earth to warm indefinitely, all the ice in the world will turn into water, but we won’t get that far (hopefully). So how much ice will melt, and how fast will it go? This depends on feedbacks in the climate system, glacial dynamics, and many other phenomena that are quantitatively poorly understood.

These examples of uncertainty in climate science, just a few of many, don’t give us an excuse to do nothing about the problem. As Brian, a Master’s student from Canada, wrote, “You don’t have to have the seventh decimal place filled in to see that the number isn’t looking good.”. We know that there is a problem, and it might be somewhat better or somewhat worse than scientists are currently predicting, but it won’t go away. As we noted above, in many cases it’s more likely to be worse than it is to be better. Even a shallow understanding of the implications of “worse” should be enough for anyone to see the necessity of action.

A Conversation with Gavin Schmidt

Cross-posted from NextGenJournal

Dr. Gavin Schmidt is a climate modeller at NASA’s Goddard Institute for Space Studies, as well as the editor at RealClimate. I recently had the opportunity to interview Dr. Schmidt, one of the top scientists in his field, on what we can expect from the climate in the coming decades. Here is the entirety of the interview we completed for my article Climate Change and Young People.

Kate: In a business-as-usual scenario, what range of warming can we expect within the lifetimes of today’s young people – so to about 2070 or 2080?

Gavin: Well, we don’t have a perfect crystal ball for exactly what “business-as-usual” means, but the kind of projections that people have been looking at – which involve quite high increases in population and minimal changes in technology – you are talking about global temperature changes, by about 2070, of somewhere between two, three, five degrees Celsius, depending a little bit on the scenario, and a little bit on how sensitive the climate actually is.

That metric is a bit abstract to most people, so how will that amount of warming actually impact people’s lives?

That’s a very good question, because most people don’t live in the global mean temperature, or the global mean anything. Those kinds of numbers translate to larger changes, between four and six degrees of warming, over the land. As you go towards the poles it becomes larger as well, because of the amplifying feedbacks of ice albedo changes and reductions in snow cover.

Right now the range between a cold summer and a warm summer, in most mid-latitude places, is on the order of a couple of degrees. You’ll be looking at summers then – the normal summer then – will be warmer than the warmest summers that you have now, and significantly warmer than the coldest summers. The same will be true in winter time and other seasons.

How will that impact metrics such as agriculture, food prices, the economy…?

It’s easy enough to say that there are going to be some impacts – obviously agriculture depends on the climate that exists. People will adapt to that, they’ll plant earlier, but crops are very sensitive to peak summer temperatures. So you’ll see losses in the fatally sensitive crops. But then you’ll see movement north of crops that were grown further south. You have to deal with the other changes – in nutrient balances, water availability, soil quality. We’re not talking about just moving the subtropics further toward the poles.

Lots of other things are going to change as well. Pests travel much faster with climate than do other kinds of species: invasive species tend to increase faster, because they’re moving into an empty niche, than species that are already well established. There’s going to be changes to rainfall regimes, whether it snows or rains, how heavily it rains – a lot of those things will tax infrastructure.

You’ve got changes for people living on the coast related to sea level rise. That will lead to changes in the damaging effects of storm surges when any particular storm comes through. We’re also looking at more subtle changes to the storms themselves, which could even amplify that effect.

How much of this warming, and these impacts, are now inevitable? Do we have the ability to prevent most of it, and what would that take?

Some further changes are inevitable. The system has so much inertia, and it hasn’t even caught up with what we’ve put into the atmosphere so far. As it continues to catch up, even if we don’t do anything else to the atmosphere from now on, we’ll still see further warming and further changes to the climate. But we do have a choice as to whether we try and minimize these changes in the future, or we allow the maximum change to occur. And the maximum changes really are very large. It’s been said that if we allow that to happen, we’ll end up living on a different planet, and I think there’s some certain truth to that.

I hear you talking a lot about uncertainty, and that’s something a lot of people are paralyzed by: they don’t want us to take these actions because they think everything might be fine on its own. What’s your response to that attitude?

Any decision that you’re making now that has to do with the future is uncertain. We make decisions all the time: where to invest money, whether to buy a house – these things aren’t certain, and we still have to make decisions. The issue with climate is that no action is a decision in and of itself. That one is actually laden with far more uncertainty than if we actually try and produce energy more efficiently, try and use more renewables, adjust the way we live so that we have a more sustainable future. The uncertainty comes with what would happen if we don’t make a decision, and I find that to be the dominant uncertainty. But climate change is not unique in having to deal with decision making under uncertainty. All decisions are like that. It’s nothing special about climate change in that there’s uncertainty about what’s going to happen in the future. Any time we decide to do anything, there’s uncertainty about the future, yet we still manage to get out of bed in the morning.

Probably in response to this attitude, climate science has got a lot of bad press in the past couple years. What have your experiences been – what sort of reactions have there been to your research?

There are a lot of people, particularly in the US, who perceive the science itself – just describing what’s going on and why – as a threat to their interests. To my mind, knowing what’s going on in the planet and trying to understand why should just be information, it shouldn’t be a threat. But other people see it as a threat, and instead of dealing with either their perceptions or what the science actually means, they choose to attack the science and they choose to attack the scientists. Basically, you just have people adopting a “shoot the messenger” strategy, which plays well in the media. It doesn’t get us very far in terms of better understanding what’s going on. But it does add a sort of smokescreen to divert people’s attention from what the real issues are. That’s regrettable, but I don’t think it’s at all surprising.

And finally, are you at all optimistic about the future?

It depends on the day.

Climate Change and Young People

Cross-posted from NextGen Journal

What is the most important policy issue facing today’s young people? Climate change might not seem like an obvious contender, as it feels so distant. Indeed, the majority of impacts from global warming have yet to come. But the magnitude and extent of those impacts are being determined right now. Only today’s young people will still be around to witness the effects of today’s actions.

Many people see climate change as just another environmental issue that will only impact the polar bears and coral reefs. In fact, it’s far more wide-reaching than that. An increase of only a few degrees in average global temperature will affect human systems of all kinds: agriculture, public health, economics, and infrastructure, just to name a few.

Dr. Gavin Schmidt, a climate modeller at NASA’s Goddard Institute for Space Studies and one of the world’s top scientists studying global warming, says that significant changes in global temperature can be expected within the lifetimes of young people alive today – “somewhere between two, three, five degrees Celsius, depending a little bit on the scenario, and a little bit on how sensitive the climate actually is.” It might sound like a small change, until you look back at the history of the Earth’s climate and realize that the last ice age was only around 5 degrees Celsius cooler than today. Additionally, the rate of warming (which is the more important metric for the ability of species, including people, to adapt) is higher today than it has been at any time for at the least the past 55 million years. Human technology has far surpassed the natural forces in the climate system, to the point where significant future warming is inevitable. In fact, says Schmidt, the climate system “hasn’t even caught up with what we’ve put into the atmosphere so far. As it continues to catch up, even if we don’t do anything else to the atmosphere from now on, we’ll still see further warming and further changes to the climate.”

However, the future is still quite malleable. Two degrees of warming is bad, but five degrees is far worse, and the difference between the two ends of the spectrum will depend on what we decide to do about the problem. Since our emissions of greenhouse gases, especially carbon dioxide, are causing global warming, the solution is self-evident: cut our emissions, as quickly as we can reasonably do so. Implementing this solution is not so simple, as fossil fuels are currently highly integrated into the global economy. Luckily, free-market mechanisms exist which alter the price signals of fossil fuels to better reflect the damage they cause. A revenue-neutral carbon tax, which is offset by reductions in income taxes or paid back evenly to the public as a dividend, is one solution; a cap-and-trade program, which treats carbon emissions like a currency, is another. While virtually nothing has been done in North America to cut emissions, the rest of the developed world has made a pretty good start.

Here in North America, the outlook for action is somewhat bleak. In the United States, says Schmidt, many people “perceive the science itself – just describing what’s going on and why – as a threat to their interests…they choose to attack the science and they choose to attack the scientists.” The Republican Party has adopted this strategy of denial, to the point where top presidential candidates such as Michelle Bachmann and Rick Perry truly believe that climate change is a hoax scientists cooked up to get grant money. The Democrats largely accept the science, but after nearly a full term in office, President Barack Obama hasn’t made any progress on the cap-and-trade program he promised upon his election. In Canada, Prime Minister Stephen Harper has repeatedly said that he will follow whatever actions the United States takes, or does not take, on climate change policy.

It seems that action necessary to mitigate global warming won’t be taken unless citizens demand it. Otherwise, emissions will likely continue unabated until the problem is too severe to ignore any longer – and even then, the situation will get worse for decades while the climate system catches up. “No action,” says Schmidt, “is a decision in and of itself.”

What decision, then, will we make? Will we get our act together in time to keep the warming at a tolerable level? Or will we choose to let it spiral out of control? Will future societies look back on us with resentment, or with admiration? Remember, you and I are part of those future societies. But we are also part of today’s.

Thousands of years from now, it won’t matter what the US deficit was in 2011, or which nations went to war with each other, or how much we invested in higher education. These issues matter a great deal to people today, but they are very transient, like many aspects of human systems. Climate change, though, will alter the earth on a geological timescale. It will take the planet around one hundred thousand years to undo what we are doing. We are leaving behind a very unfortunate legacy to the entirety of future human civilization, and all life on Earth – a legacy that is being shaped as you read this; a legacy that we could largely avoid if we chose to.