Posts Tagged ‘science’

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.


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To continue my tradition of trying out all the Commonwealth countries, since my last post I have moved to the UK and begun a postdoc at the British Antarctic Survey in Cambridge. The UK is far nicer than Australians will lead you to believe – there are indeed sunny days, and gorgeous coastline, and great wildlife. None of these things are quite at Australian levels, but there are other things that at least partially make up for it. Like central heating, and the absence of huntsman spiders.

My PhD is now completely wrapped up, and I can officially use the title Dr., so I get very excited about filling in forms. For my postdoc I’m continuing to study interactions between Antarctic ice shelves and the ocean, but using a different ocean model (MITgcm), and focusing on a specific region (the Filchner-Ronne Ice Shelf in the Weddell Sea). This project includes some ice-sheet/ocean coupling, which I’m enormously, ridiculously excited about.

A postdoc is far more relaxing than a PhD, and far less existential. I know I’m only a few months in, but many of my colleagues hold a similar opinion. At last, there is no monolithic Thesis that everything is building up to, no pressure for all your research threads to converge into a coherent narrative before your scholarship runs out, no need to justify your continued existence (“how long have you been here, again?”) There is just a period of time for which your postdoc is funded, and you do as much science as you can during that time. You have more confidence in your own abilities, since you’ve done vaguely similar things before, and everyone else seems to take you more seriously too.

Much has been written on the mental health risks of doing a PhD, both in the scientific literature and in the media. I won’t pretend to be an alarming example of this, because many students have a much, much harder time than I did. But I did operate under elevated stress during the last year and a half of my PhD, and I noticed the effect this had on my life. Regular exercise was very effective in keeping my spirits up, but it didn’t really help the insomnia.

Here’s the pleasantly surprising bit: these effects appear to go away when you finish your PhD. I don’t know what else I expected – that I would be scarred for life? All I know is that I’ve slept well nearly every night since the day I submitted my thesis. And when I look at my giant list of things to do with my model, I don’t feel overwhelmed. I just feel excited.

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Pending examiner approval…soon I will be Dr Kaitlin!

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I’ve just given an interview at the Forecast podcast, hosted by Nature’s climate change editor, Michael White. Head over to the Forecast website to check it out.

What I love about Forecast is that it interviews climate scientists as fully-rounded human beings, rather than fact-generating robots. The humanisation of scientists is so important for science communication. If the audience feels like they can relate to a scientist, they’re more likely to trust them and take an interest in what they say. Michael does an exemplary job of this, because his interviews aren’t just limited to reporting the results of recent studies. He also asks his guests how that research was done, how they became a scientist in the first place, and what life is like as an academic. These are the sorts of conversations scientists actually have with each other, and it’s refreshing and fascinating to see them captured in online media.

I approached Forecast with an idea for a podcast I’d had rattling around in my brain for months. I am a person who stutters. I don’t think I’ve ever mentioned it on this blog before, but there you have it. When I’m talking my throat likes to close up and block the words from coming out. This is frustrating.

In academia these days there’s quite a lot of discussion about equity and diversity. Usually this is around gender, sometimes ethnicity, sometimes sexual orientation. But I’ve yet to hear any discussion about disabilities. Here I am with this very obvious disability (at least, it’s obvious every time I open my mouth) and I’m sure my colleagues have questions about it, but everyone is too polite to ask.

Those who do say anything – usually my closest friends, and only after I bring it up – generally say something along the lines of “you’re so brave to be a scientist anyway and not let this stop you”. I was really taken aback the first time I heard this, because I’d never even thought about it that way. My disability has never factored into my career choices for a second. There are very few jobs that don’t require some amount of speaking, and I like science and am good at science, so why would I do anything else?

But this got me thinking. About 1% of the population stutters (mostly men, I’m the unlucky exception) and surely there are some aspiring scientists in there. Maybe they’re not all as stubborn as I am. Maybe it would help them to hear from someone who has made it work.

So, there you have it. Hopefully this podcast helps somebody. You also get to hear about ice-sheet/ocean interactions, model development, my new postdoc, and all the different Commonwealth countries I have lived in.

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At the library

The DOI system is great and all, and I love being able to access nearly the entire scientific literature without having to leave my desk. But there’s something wonderful about looking up a textbook with no full-text access online, and instead walking down to the uni library with a Dewey decimal number scrawled on a sticky note. Pacing through the shelves of books, finding the one I want, taking it down, and smelling it.

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For many years politicians said, “We’re not even sure climate change is real, so why should we waste money studying it?”

And seemingly overnight, the message has become, “Now that we know climate change is real, we can stop studying it.”

Don’t believe me? This is what Larry Marshall, the chief executive of Australia’s federal science agency CSIRO, wrote in an email to staff earlier this month:

Our climate models are among the best in the world and our measurements honed those models to prove global climate change. That question has been answered, and the new question is what do we do about it, and how can we find solutions for the climate we will be living with?

And then he cut 110 of the 140 climate research jobs in CSIRO’s Oceans and Atmosphere division.

Larry’s statement on its own is perfectly reasonable, but as justification for cutting basic research it is nonsensical. As Andy Pitman, the director of my research centre, responded, “What he fails to realise is that to answer these new questions, you need the same climate scientists!”

Luckily climate scientists are a tough bunch, and we don’t take shit like this lying down.

Why CSIRO matters

There’s not very many people in the Southern Hemisphere, and certainly not very many people studying the climate. Australia is really the only Southern Hemisphere country with the critical mass of population and wealth to pull off a significant climate research programme. And historically, Australia has done this very well. I can attest that the climate modelling community is larger and more active in Australia than it is in Canada, despite Canada’s population being about 50% higher.

Some of this research is done in universities, mainly funded by Australian Research Council grants. I’m a part of this system and I love it – we have the freedom to steer our research in whatever direction we think is best, rather than having someone from the government tell us what we can and can’t study. But there isn’t very much stability. Projects are typically funded on a 3-5 year timeline, and with a success rate around 20% for most grant applications, you can never be sure how long a given project will continue. You wouldn’t want to be the official developers of a climate model in an environment like that. You certainly wouldn’t want to be in charge of a massive network of observations. And that is why we need governmental research organisations like CSIRO.

Right now CSIRO hosts the ACCESS model (Australian Community Climate and Earth-System Simulator) which is used by dozens of my friends and colleagues. ACCESS has a good reputation – I particularly like it because it simulates the Southern Ocean more realistically than just about any CMIP5 model. I think this is because it’s pretty much the only contribution to CMIP5 from the Southern Hemisphere. Even if a model is ostensibly “global”, the regions its developers focus on tend to be close to home.

The official ACCESS developers at CSIRO distribute new releases of the code, collect and resolve bug reports, organise submissions to model intercomparison projects like CMIP5, and continually work to improve model efficiency and performance. Some of this work is done in collaboration with the Bureau of Meteorology, the National Computational Infrastructure, the UK Met Office (which provides the atmosphere component of the model), and researchers at Australian universities. But CSIRO is the backbone of ACCESS, and it’s unclear what will happen to the model without its core development team. The Bureau and the universities simply will not be able to pick up the slack.

[It’s] completely understandable that someone who’s spent 20 years, for example, studying climate change, measuring climate change or modelling climate change, it’s perfectly understandable that they don’t want to stop doing that and we must respect that, and we must find a place for them in the rest of the innovation system, perhaps in an university.
Larry Marshall

Changing directions

In his letter to staff, Larry Marshall wrote that he wants CSIRO to focus on (among other things) “our management of the oceans, climate adaptation, climate interventions (geo-engineering), [and] emergency response to extreme events”. It’s clear that he wants climate research to move away from the question “Are we really, really, really sure that climate change is real?” and more towards “How will climate change impact us and what can we do about it?” Again, I completely agree. But here’s the thing, Larry: We’re already doing that. The research program that you want to eliminate is already evolving into the research program you want to replace it with.

Climate science is evolving in this manner all over the world, not just at CSIRO. Try publishing a paper that concludes “Yes, humans are changing the climate” and see how far you get. Your reviewers will almost certainly respond with “Is that all?” and hit reject. Papers like that were a big deal in the 80s and the 90s. But by now we’ve moved on to more interesting questions.

For example, I’m using ocean models to study how the Southern Ocean might melt the Antarctic Ice Sheet from the bottom up. I’m not doing this to “prove” climate change – how could I even do that in this context? – but to get a better understanding of how much and how fast the sea level will rise over the next few centuries. If we’re going to have to move Miami, Shanghai, New York, and countless other major coastal cities, it would be good to have a few decades’ notice. And we won’t have that kind of notice unless we understand what Antarctica’s doing, through a network of observations (to see what it’s doing right now) and modelling studies (to predict what it might do next).

Without measuring and modelling, our attempts at adaptation and mitigation will be shots in the dark. “The one thing that makes adaptation really difficult is uncertainty,” writes journalist Michael Slezak. “If you don’t know what the climate will be like in the future – whether it will be wetter or dryer, whether cyclones will be more or less frequent – then you can’t prepare. You cannot adapt for a future that you don’t understand.”

Someone’s going to have to convince me that measuring and modelling is far more important than mitigation – and at this point you know, none of my leadership believe that.
Larry Marshall

Scientists fight back!

My colleagues at CSIRO are having a difficult month. They know that most of them will lose their jobs, or their supervisors will lose their jobs, and that they may have to leave Australia to find another job. But they won’t know who’s staying and who’s going until early March – a full month after the announcement was first made. Due to Larry’s lack of consultation on this decision, the CSIRO staff are reportedly considering industrial action.

The really maddening part of this whole situation is that climate science was specifically targeted. Of the 350 job losses across CSIRO – which studies all kinds of things, not just climate science – 110 were to the climate unit of the Oceans and Atmosphere department, and a similar number to Land and Water. It wouldn’t be so insulting if CSIRO was trimming down a little bit everywhere to cope with its budget cuts. Instead, they’re directly targeting – and essentially eliminating – climate science. “”No one is saying climate change is not important, but surely mitigation, health, education, sustainable industries, and prosperity of the nation are no less important,” Larry wrote in response to mounting criticism. So are we going to cut all of those research programs too?

Climate scientists are rather battle-hardened after so many years of personal attacks by climate change deniers, and everyone jumped into action following CSIRO’s announcement. At the annual meeting of the Australian Meteorological and Oceanographic Society the following week, the attendees organised a bit of a demonstration. It’s not often that the Sydney Morning Herald publishes a photo of “angry scientists”:

(I know just about everyone in this photo. The guy front and centre shares a cubicle with me. Hi Stefan!)

Then scientists from outside Australia started to get involved. The normally-staid World Climate Research Program released an official statement condemning CSIRO’s decision. “Australia will find itself isolated from the community of nations and researchers devoting serious attention to climate change,” they warned.

A few days later an open letter, signed by 2800 climate scientists from almost 60 countries, was sent to the CSIRO and the Australian government. “Without CSIRO’s involvement in both climate measurement and modelling, a significant portion of the Southern Hemisphere oceans and atmosphere will go unmonitored,” the letter warned. You can tell it was written by scientists because it’s complete with references and footnotes, and the signatories are carefully organised both alphabetically and by country.

We’re not sure if our efforts will make any difference. We’ll find out next month whether or not these cuts really will go ahead. Media coverage of this issue has slowed, and there have been no more announcements out of CSIRO. But I suppose we’ve done everything we can.

I feel like the early climate scientists in the ’70s fighting against the oil lobby…I guess I had the realisation that the climate lobby is perhaps more powerful than the energy lobby was back in the ’70s – and the politics of climate I think there’s a lot of emotion in this debate. In fact it almost sounds more like religion than science to me.
Larry Marshall

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Around 55 million years ago, an abrupt global warming event triggered a highly corrosive deep-water current to flow through the North Atlantic Ocean. This process, suggested by new climate model simulations, resolves a long-standing mystery regarding ocean acidification in the deep past.

The rise of CO2 that led to this dramatic acidification occurred during the Paleocene-Eocene Thermal Maximum (PETM), a period when global temperatures rose by around 5°C over several thousand years and one of the largest-ever mass extinctions in the deep ocean occurred.

The PETM, 55 million years ago, is the most recent analogue to present-day climate change that researchers can find. Similarly to the warming we are experiencing today, the PETM warming was a result of increases in atmospheric CO2. The source of this CO2 is unclear, but the most likely explanations include methane released from the seafloor and/or burning peat.

During the PETM, like today, emissions of CO2 were partially absorbed by the ocean. By studying sediment records of the resulting ocean acidification, researchers can estimate the amount of CO2 responsible for warming. However, one of the great mysteries of the PETM has been why ocean acidification was not evenly spread throughout the world’s oceans but was so much worse in the Atlantic than anywhere else.

This pattern has also made it difficult for researchers to assess exactly how much CO2 was added to the atmosphere, causing the 5°C rise in temperatures. This is important for climate researchers as the size of the PETM carbon release goes to the heart of the question of how sensitive global temperatures are to greenhouse gas emissions.

Solving the mystery of these remarkably different patterns of sediment dissolution in different oceans is a vital key to understanding the rapid warming of this period and what it means for our current climate.

A study recently published in Nature Geoscience shows that my co-authors Katrin Meissner, Tim Bralower and I may have cracked this long-standing mystery and revealed the mechanism that led to this uneven ocean acidification.

We now suspect that atmospheric CO2 was not the only contributing factor to the remarkably corrosive Atlantic Ocean during the PETM. Using global climate model simulations that replicated the ocean basins and landmasses of this period, it appears that changes in ocean circulation due to warming played a key role.

55 million years ago, the ocean floor looked quite different than it does today. In particular, there was a ridge on the seafloor between the North and South Atlantic, near the equator. This ridge completely isolated the deep North Atlantic from other oceans, like a giant bathtub on the ocean floor.

In our simulations this “bathtub” was filled with corrosive water, which could easily dissolve calcium carbonate. This corrosive water originated in the Arctic Ocean and sank to the bottom of the Atlantic after mixing with dense salty water from the Tethys Ocean (the precursor to today’s Mediterranean, Black, and Caspian Seas).

Our simulations then reproduced the effects of the PETM as the surface of the Earth warmed in response to increases in CO2. The deep ocean, including the corrosive bottom water, gradually warmed in response. As it warmed it became less dense. Eventually the surface water became denser than the warming deep water and started to sink, causing the corrosive deep water mass to spill over the ridge – overflowing the “giant bath tub”.

The corrosive water then spread southward through the Atlantic, eastward through the Southern Ocean, and into the Pacific, dissolving sediments as it went. It became more diluted as it travelled and so the most severe effects were felt in the South Atlantic. This pattern agrees with sediment records, which show close to 100% dissolution of calcium carbonate in the South Atlantic.

If the acidification event occurred in this manner it has important implications for how strongly the Earth might warm in response to increases in atmospheric CO2.

If the high amount of acidification seen in the Atlantic Ocean had been caused by atmospheric CO2 alone, that would suggest a huge amount of CO2 had to go into the atmosphere to cause 5°C warming. If this were the case, it would mean our climate was not very sensitive to CO2.

But our findings suggest other factors made the Atlantic far more corrosive than the rest of the world’s oceans. This means that sediments in the Atlantic Ocean are not representative of worldwide CO2 concentrations during the PETM.

Comparing computer simulations with reconstructed ocean warming and sediment dissolution during the event, we could narrow our estimate of CO2 release during the event to 7,000 – 10,000 GtC. This is probably similar to the CO2 increase that will occur in the next few centuries if we burn most of the fossil fuels in the ground.

To give this some context, today we are emitting CO2 into the atmosphere at least 10 times faster than than the natural CO2 emissions that caused the PETM. Should we continue to burn fossil fuels at the current rate, we are likely to see the same temperature increase in the space of a few hundred years that took a few thousand years 55 million years ago.

This is an order of magnitude faster and it is likely the impacts from such a dramatic change will be considerably stronger.

Written with the help of my co-authors Katrin and Tim, as well as our lab’s communications manager Alvin Stone.

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