Feeds:
Posts
Comments

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.

The latest brainchild of John Cook is the free online edX course Denial 101x. This course, tackling basic climate science as well as the science of climate change denial, had amassed 15,000 students as of May 12.

You can access the video lectures and interviews on their YouTube channel without even signing up for the course. The best video content, in my opinion, is the full interviews with scientists. They are such interesting people with so many stories to tell, and the conversational style of the interviews makes the quality of communication absolutely top-notch. I like to listen to these interviews while I wash the dishes (our apartment has been extra clean recently thanks to John Cook).

Here are my favourites from Week 1 alone (there are multiple hours of interviews released each week, it’s kind of overwhelming):

  1. Naomi Oreskes, who is such a strong and smart and feisty person – I want to be just like her. “My husband always says that when [climate change deniers] started attacking me, they really didn’t know what they were up against.”

  2. Katharine Hayhoe, who brings a new perspective I haven’t really thought about much: how to reach people of faith. “I received so much support from colleagues who say, ‘I do not share your faith or don’t even understand your faith, but I support what you’re doing.'”

  3. Michael Mann, who has probably been through more than anyone else in this community, but has nonetheless kept an incredibly positive attitude. “What I’m concerned about, more concerned about, is the fact that there are now many younger scientists out there who are doing research on climate change, and when they publish a paper that gets a lot of attention, they are finding themselves subject to these orchestrated attacks. In many cases, they’re just graduate students. They have absolutely no experience in dealing with this sort of issue.”

It turns out that when you submit a paper to a journal like Nature Geoscience “just in case, we have nothing to lose, they’ll probably reject it straight away”…sometimes you are unexpectedly successful.

Read it here!

Assorted media coverage:

More detailed post to come…

Almost four years ago I took a job as a summer student of Dr. Steve Easterbrook, in the software engineering lab of the University of Toronto. This was my first time taking part in research, but also my first time living away from home and my first time using a Unix terminal (both of which are challenging, but immensely rewarding, life skills).

While working with Steve I discovered that climate model output is really pretty (an opinion which hasn’t changed in the four years since) and that climate models are really hard to install (that hasn’t changed either).

At Steve’s suggestion I got a hold of the code for various climate models and started to pick it apart. By the end of the summer I had created a series of standardised diagrams showing the software architecture of each model.

These diagrams proved to be really useful communication tools: we presented our work at AGU the following December, and at NCAR about a year after that, to very positive feedback. Many climate modellers we met at these conferences were pleased to have a software diagram of the model they used (which is very useful to show during presentations), but they were generally more interested in the diagrams for other models, to see how other research groups used different software structures to solve the same problems. “I had no idea they did it like that,” was a remark I heard more than a few times.

Between my undergrad and PhD, I went back to Toronto for a month where I analysed the model code more rigorously. We made a new set of diagrams which was more accurate: the code was sorted into components based on dependency structure, and the area of each component in a given diagram was exactly proportional to the line count of its source code.

Here is the diagram we made for the GFDL-ESM2M model, which is developed at the Geophysical Fluid Dynamics Laboratory in Princeton:

We wrote this all up into a paper, submitted it to GMD, and after several months and several rounds of revision it was published just yesterday! The paper is open access, and can be downloaded for free here. It’s my first paper as lead author which is pretty exciting.

I could go on about all the interesting things we discovered while comparing the diagrams, but that’s all in the paper. Instead I wanted to talk about what’s not in the paper: the story of the long and winding journey we took to get there, from my first day as a nervous summer student in Toronto to the final publication yesterday. These are the stories you don’t read about in scientific papers, which out of necessity detail the methodology as if the authors knew exactly where they were going and got there using the shortest possible path. Science doesn’t often work like that. Science is about messing around and exploring and getting a bit lost and eventually figuring it out and feeling like a superhero when you do. And then writing it up as if it was easy.

I also wanted to express my gratitude to Steve, who has been an amazing source of support, advice, conversations, book recommendations, introductions to scientists, and career advice. I’m so happy that I got to be your student. See you before long on one continent or another!

An ice sheet forms when snow falls on land, compacts into ice, and forms a system of interconnected glaciers which gradually flow downhill like play-dough. In Antarctica, it is so cold that the ice flows right into the ocean before it melts, sometimes hundreds of kilometres from the coast. These giant slabs of ice, floating on the ocean while still attached to the continent, are called ice shelves.

For an ice sheet to have constant size, the mass of ice added from snowfall must equal the mass lost due to melting and calving (when icebergs break off). Since this ice loss mainly occurs at the edges, the rate of ice loss will depend on how fast glaciers can flow towards the edges.

Ice shelves slow down this flow. They hold back the glaciers behind them in what is known as the “buttressing effect”. If the ice shelves were smaller, the glaciers would flow much faster towards the ocean, melting and calving more ice than snowfall inland could replace. This situation is called a “negative mass balance”, which leads directly to global sea level rise.

Photo by Tas van Ommen

Respect the ice shelves. They are holding back disaster.

Ice shelves are perhaps the most important part of the Antarctic ice sheet for its overall stability. Unfortunately, they are also the part of the ice sheet most at risk. This is because they are the only bits touching the ocean. And the Antarctic ice sheet is not directly threatened by a warming atmosphere – it is threatened by a warming ocean.

The atmosphere would have to warm outrageously in order to melt the Antarctic ice sheet from the top down. Snowfall tends to be heaviest when temperatures are just below 0°C, but temperatures at the South Pole rarely go above -20°C, even in the summer. So atmospheric warming will likely lead to a slight increase in snowfall over Antarctica, adding to the mass of the ice sheet. Unfortunately, the ocean is warming at the same time. And a slightly warmer ocean will be very good at melting Antarctica from the bottom up.

This is partly because ice melts faster in water than it does in air, even if the air and the water are the same temperature. But the ocean-induced melting will be exacerbated by some unlucky topography: over 40% of the Antarctic ice sheet (by area) rests on bedrock that is below sea level.

bedmap2

Elevation of the bedrock underlying Antarctica. All of the blue regions are below sea level. (Figure 9 of Fretwell et al.)

This means that ocean water can melt its way in and get right under the ice, and gravity won’t stop it. The grounding lines, where the ice sheet detaches from the bedrock and floats on the ocean as an ice shelf, will retreat. Essentially, a warming ocean will turn more of the Antarctic ice sheet into ice shelves, which the ocean will then melt from the bottom up.

This situation is especially risky on a retrograde bed, where bedrock gets deeper below sea level as you go inland – like a giant, gently sloping bowl. Retrograde beds occur because of isostatic loading (the weight of an ice sheet pushes the crust down, making the tectonic plate sit lower in the mantle) as well as glacial erosion (the ice sheet scrapes away the surface bedrock over time). Ice sheets resting on retrograde beds are inherently unstable, because once the grounding lines reach the edge of the “bowl”, they will eventually retreat all the way to the bottom of the “bowl” even if the ocean water intruding beneath the ice doesn’t get any warmer. This instability occurs because the melting point temperature of water decreases as you go deeper in the ocean, where pressures are higher. In other words, the deeper the ice is in the ocean, the easier it is to melt it. Equivalently, the deeper a grounding line is in the ocean, the easier it is to make it retreat. In a retrograde bed, retreating grounding lines get deeper, so they retreat more easily, which makes them even deeper, and they retreat even more easily, and this goes on and on even if the ocean stops warming.

retrograde_bed

Diagram of an ice shelf on a retrograde bed (“Continental shelf”)

Which brings us to Terrifying Paper #1, by Rignot et al. A decent chunk of West Antarctica, called the Amundsen Sea Sector, is melting particularly quickly. The grounding lines of ice shelves in this region have been rapidly retreating (several kilometres per year), as this paper shows using satellite data. Unfortunately, the Amundsen Sea Sector sits on a retrograde bed, and the grounding lines have now gone past the edge of it. This retrograde bed is so huge that the amount of ice sheet it underpins would cause 1.2 metres of global sea level rise. We’re now committed to losing that ice eventually, even if the ocean stopped warming tomorrow. “Upstream of the 2011 grounding line positions,” Rignot et al., write, “we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin.”

They look at each source glacier in turn, and it’s pretty bleak:

  • Pine Island Glacier: “A region where the bed elevation is smoothly decreasing inland, with no major hill to prevent further retreat.”
  • Smith/Kohler Glaciers: “Favorable to more vigorous ice shelf melt even if the ocean temperature does not change with time.”
  • Thwaites Glacier: “Everywhere along the grounding line, the retreat proceeds along clear pathways of retrograde bed.”

Only one small glacier, Haynes Glacier, is not necessarily doomed, since there are mountains in the way that cut off the retrograde bed.

From satellite data, you can already see the ice sheet speeding up its flow towards the coast, due to the loss of buttressing as the ice shelves thin: “Ice flow changes are detected hundreds of kilometers inland, to the flanks of the topographic divides, demonstrating that coastal perturbations are felt far inland and propagate rapidly.”

It will probably take a few centuries for the Amundsen Sector to fully disintegrate. But that 1.2 metres of global sea level rise is coming eventually, on top of what we’ve already seen from other glaciers and thermal expansion, and there’s nothing we can do to stop it (short of geoengineering). We’re going to lose a lot of coastal cities because of this glacier system alone.

Terrifying Paper #2, by Mengel & Levermann, examines the Wilkes Basin Sector of East Antarctica. This region contains enough ice to raise global sea level by 3 to 4 metres. Unlike the Amundsen Sector, we aren’t yet committed to losing this ice, but it wouldn’t be too hard to reach that point. The Wilkes Basin glaciers rest on a system of deep troughs in the bedrock. The troughs are currently full of ice, but if seawater got in there, it would melt all the way along the troughs without needing any further ocean warming – like a very bad retrograde bed situation. The entire Wilkes Basin would change from ice sheet to ice shelf, bringing along that 3-4 metres of global sea level rise.

It turns out that the only thing stopping seawater getting in the troughs is a very small bit of ice, equivalent to only 8 centimetres of global sea level rise, which Mengel & Levermann nickname the “ice plug”. As long as the ice plug is there, this sector of the ice sheet is stable; but take the ice plug away, and the whole thing will eventually fall apart even if the ocean stops warming. Simulations from an ice sheet model suggest it would take at least 200 years of increased ocean temperature to melt this ice plug, depending on how much warmer the ocean got. 200 years sounds like a long time for us to find a solution to climate change, but it actually takes much longer than that for the ocean to cool back down after it’s been warmed up.

This might sound like all bad news. And you’re right, it is. But it can always get worse. That means we can always stop it from getting worse. That’s not necessarily good news, but at least it’s empowering. The sea level rise we’re already committed to, whether it’s 1 or 2 or 5 metres, will be awful. But it’s much better than 58 metres, which is what we would get if the entire Antarctic ice sheet melted. Climate change is not an all-or-nothing situation; it falls on a spectrum. We will have to deal with some amount of climate change no matter what. The question of “how much” is for us to decide.

A problem which has plagued oceanography since the very beginning is a lack of observations. We envy atmospheric scientists with their surface stations and satellite data that monitor virtually the entire atmosphere in real time. Until very recently, all that oceanographers had to work with were measurements taken by ships. This data was very sparse in space and time, and was biased towards certain ship tracks and seasons.

A lack of observations makes life difficult for ocean modellers, because there is very little to compare the simulations to. You can’t have confidence in a model if you have no way of knowing how well it’s performing, and you can’t make many improvements to a model without an understanding of its shortcomings.

Our knowledge of the ocean took a giant leap forward in 2000, when a program called Argo began. “Argo floats” are smallish instruments floating around in the ocean that control their own buoyancy, rising and sinking between the surface and about 2000 m depth. They use a CTD sensor to measure Conductivity (from which you can easily calculate salinity), Temperature, and Depth. Every 10 days they surface and send these measurements to a satellite. Argo floats are battery-powered and last for about 4 years before losing power. After this point they are sacrificed to the ocean, because collecting them would be too expensive.

This is what an Argo float looks like while it’s being deployed:

With at least 27 countries helping with deployment, the number of active Argo floats is steadily rising. At the time of this writing, there were 3748 in operation, with good coverage everywhere except in the polar oceans:

The result of this program is a massive amount of high-quality, high-resolution data for temperature and salinity in the surface and intermediate ocean. A resource like this is invaluable for oceanographers, analogous to the global network of weather stations used by atmospheric scientists. It allows us to better understand the current state of the ocean, to monitor trends in temperature and salinity as climate change continues, and to assess the skill of ocean models.

But it’s still not good enough. There are two major shortcomings to Argo floats. First, they can’t withstand the extreme pressure in the deep ocean, so they don’t sink below about 2000 m depth. Since the average depth of the world’s oceans is around 4000 m, the Argo program is only sampling the upper half. Fortunately, a new program called Deep Argo has developed floats which can withstand pressures down to 6000 m depth, covering all but the deepest ocean trenches. Last June, two prototypes were successfully deployed off the coast of New Zealand, and the data collected so far is looking good. If all future Argo floats were of the Deep Argo variety, in five or ten years we would know as much about the deep ocean’s temperature and salinity structure as we currently know about the surface. To oceanographers, particularly those studying bottom water formation and transport, there is almost nothing more exciting than this prospect.

The other major problem with Argo floats is that they can’t handle sea ice. Even if they manage to get underneath the ice by drifting in sideways, the next time they rise to the surface they will bash into the underside of the ice, get stuck, and stay there until their battery dies. This is a major problem for scientists like me who study the Southern Ocean (surrounding Antarctica), which is largely covered with sea ice for much of the year. This ocean will be incredibly important for sea level rise, because the easiest way to destabilise the Antarctic Ice Sheet is to warm up the ocean and melt the ice shelves (the edges of the ice sheet which extend over the ocean) from below. But we can’t monitor this process using Argo data, because there is a big gap in observations over the region. There’s always the manual option – sending in scientists to take measurements – but this is very expensive, and nobody wants to go there in the winter.

Instead, oceanographers have recently teamed up with biologists to try another method of data collection, which is just really excellent:

They are turning seals into Argo floats that can navigate sea ice.

Southern elephant seals swim incredible distances in the Southern Ocean, and often dive as far as 2000 m below the surface. Scientists are utilising the seals’ natural talents to fill in the gaps in the Argo network, so far with great success. Each seal is tranquilized while a miniature CTD is glued to the fur on its head, after which it is released back into the wild. As the seal swims around, the sensors take measurements and communicate with satellites just like regular Argo floats. The next time the seal sheds its coat (once per year), the CTD falls off and the seal gets on with its life, probably wondering what that whole thing was about.

This project is relatively new and it will be a few years before it’s possible to identify trends in the data. It’s also not clear whether or not the seals tend to swim right underneath the ice shelves, where observations would be most useful. But if this dataset gains popularity among oceanographers, and seals become officially integrated into the Argo network…

…then we will be the coolest scientists of all.

Rewinding the Clock

I don’t really care about the panda bears. But that’s not saying this problem [climate change] isn’t serious. This is a people problem, this is a billion dead people problem. This is a national security problem. This is rewinding the clock 300 years to a time we don’t want to go back to.

– Nick Wood (spoken at a presentation I attended, and possibly slightly paraphrased as I scrambled to write it down; his profile is here)

Follow

Get every new post delivered to your Inbox.

Join 399 other followers