Tag Archives: geoengineering

Modelling Geoengineering, Part II

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Near the end of my summer at the UVic Climate Lab, all the scientists seemed to go on vacation at the same time and us summer students were left to our own devices. I was instructed to teach Jeremy, Andrew Weaver’s other summer student, how to use the UVic climate model – he had been working with weather station data for most of the summer, but was interested in Earth system modelling too.

Jeremy caught on quickly to the basics of configuration and I/O, and after only a day or two, we wanted to do something more exciting than the standard test simulations. Remembering an old post I wrote, I dug up this paper (open access) by Damon Matthews and Ken Caldeira, which modelled geoengineering by reducing incoming solar radiation uniformly across the globe. We decided to replicate their method on the newest version of the UVic ESCM, using the four RCP scenarios in place of the old A2 scenario. We only took CO2 forcing into account, though: other greenhouse gases would have been easy enough to add in, but sulphate aerosols are spatially heterogeneous and would complicate the algorithm substantially.

Since we were interested in the carbon cycle response to geoengineering, we wanted to prescribe CO2 emissions, rather than concentrations. However, the RCP scenarios prescribe concentrations, so we had to run the model with each concentration trajectory and find the equivalent emissions timeseries. Since the UVic model includes a reasonably complete carbon cycle, it can “diagnose” emissions by calculating the change in atmospheric carbon, subtracting contributions from land and ocean CO2 fluxes, and assigning the residual to anthropogenic sources.

After a few failed attempts to represent geoengineering without editing the model code (e.g., altering the volcanic forcing input file), we realized it was unavoidable. Model development is always a bit of a headache, but it makes you feel like a superhero when everything falls into place. The job was fairly small – just a few lines that culminated in equation 1 from the original paper – but it still took several hours to puzzle through the necessary variable names and header files! Essentially, every timestep the model calculates the forcing from CO2 and reduces incoming solar radiation to offset that, taking changing planetary albedo into account. When we were confident that the code was working correctly, we ran all four RCPs from 2006-2300 with geoengineering turned on. The results were interesting (see below for further discussion) but we had one burning question: what would happen if geoengineering were suddenly turned off?

By this time, having completed several thousand years of model simulations, we realized that we were getting a bit carried away. But nobody else had models in the queue – again, they were all on vacation – so our simulations were running three times faster than normal. Using restart files (written every 100 years) as our starting point, we turned off geoengineering instantaneously for RCPs 6.0 and 8.5, after 100 years as well as 200 years.

Results

Similarly to previous experiments, our representation of geoengineering still led to sizable regional climate changes. Although average global temperatures fell down to preindustrial levels, the poles remained warmer than preindustrial while the tropics were cooler:

Also, nearly everywhere on the globe became drier than in preindustrial times. Subtropical areas were particularly hard-hit. I suspect that some of the drying over the Amazon and the Congo is due to deforestation since preindustrial times, though:

Jeremy also made some plots of key one-dimensional variables for RCP8.5, showing the results of no geoengineering (i.e. the regular RCP – yellow), geoengineering for the entire simulation (red), and geoengineering turned off in 2106 (green) or 2206 (blue):

It only took about 20 years for average global temperature to fall back to preindustrial levels. Changes in solar radiation definitely work quickly. Unfortunately, changes in the other direction work quickly too: shutting off geoengineering overnight led to rates of warming up to 5 C / decade, as the climate system finally reacted to all the extra CO2. To put that in perspective, we’re currently warming around 0.2 C / decade, which far surpasses historical climate changes like the Ice Ages.

Sea level rise (due to thermal expansion only – the ice sheet component of the model isn’t yet fully implemented) is directly related to temperature, but changes extremely slowly. When geoengineering is turned off, the reversals in sea level trajectory look more like linear offsets from the regular RCP.

Sea ice area, in contrast, reacts quite quickly to changes in temperature. Note that this data gives annual averages, rather than annual minimums, so we can’t tell when the Arctic Ocean first becomes ice-free. Also, note that sea ice area is declining ever so slightly even with geoengineering – this is because the poles are still warming a little bit, while the tropics cool.

Things get really interesting when you look at the carbon cycle. Geoengineering actually reduced atmospheric CO2 concentrations compared to the regular RCP. This was expected, due to the dual nature of carbon cycle feedbacks. Geoengineering allows natural carbon sinks to enjoy all the benefits of high CO2 without the associated drawbacks of high temperatures, and these sinks become stronger as a result. From looking at the different sinks, we found that the sequestration was due almost entirely to the land, rather than the ocean:

In this graph, positive values mean that the land is a net carbon sink (absorbing CO2), while negative values mean it is a net carbon source (releasing CO2). Note the large negative spikes when geoengineering is turned off: the land, adjusting to the sudden warming, spits out much of the carbon that it had previously absorbed.

Within the land component, we found that the strengthening carbon sink was due almost entirely to soil carbon, rather than vegetation:

This graph shows total carbon content, rather than fluxes – think of it as the integral of the previous graph, but discounting vegetation carbon.

Finally, the lower atmospheric CO2 led to lower dissolved CO2 in the ocean, and alleviated ocean acidification very slightly. Again, this benefit quickly went away when geoengineering was turned off.

Conclusions

Is geoengineering worth it? I don’t know. I can certainly imagine scenarios in which it’s the lesser of two evils, and find it plausible (even probable) that we will reach such a scenario within my lifetime. But it’s not something to undertake lightly. As I’ve said before, desperate governments are likely to use geoengineering whether or not it’s safe, so we should do as much research as possible ahead of time to find the safest form of implementation.

The modelling of geoengineering is in its infancy, and I have a few ideas for improvement. In particular, I think it would be interesting to use a complex atmospheric chemistry component to allow for spatial variation in the forcing reduction through sulphate aerosols: increase the aerosol optical depth over one source country, for example, and let it disperse over time. I’d also like to try modelling different kinds of geoengineering – sulphate aerosols as well as mirrors in space and iron fertilization of the ocean.

Jeremy and I didn’t research anything that others haven’t, so this project isn’t original enough for publication, but it was a fun way to stretch our brains. It was also a good topic for a post, and hopefully others will learn something from our experiments.

Above all, leave over-eager summer students alone at your own risk. They just might get into something like this.

More on Phytoplankton

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On the heels of my last post about iron fertilization of the ocean, I found another interesting paper on the topic. This one, written by Long Cao and Ken Caldeira in 2010, was much less hopeful.

Instead of a small-scale field test, Cao and Caldeira decided to model iron fertilization using the ocean GCM from Lawrence Livermore National Laboratory. To account for uncertainties, they chose to calculate an upper bound on iron fertilization rather than a most likely scenario. That is, they maxed out phytoplankton growth until something else became the limiting factor – in this case, phosphates. On every single cell of the sea surface, the model phytoplankton were programmed to grow until phosphate concentrations were zero.

A 2008-2100 simulation implementing this method was forced with CO2 emissions data from the A2 scenario. An otherwise identical A2 simulation did not include the ocean fertilization, to act as a control. Geoengineering modelling is strange that way, because there are multiple definitions of “control run”: a non-geoengineered climate that is allowed to warm unabated, as well as preindustrial conditions (the usual definition in climate modelling).

Without any geoengineering, atmospheric CO2 reached 965 ppm by 2100. With the maximum amount of iron fertilization possible, these levels only fell to 833 ppm. The mitigation of ocean acidification was also quite modest: the sea surface pH in 2100 was 7.74 without geoengineering, and 7.80 with. Given the potential side effects of iron fertilization, is such a small improvement worth the trouble?

Unfortunately, the ocean acidification doesn’t end there. Although the problem was lessened somewhat at the surface, deeper layers in the ocean actually became more acidic. There was less CO2 being gradually mixed in from the atmosphere, but another source of dissolved carbon appeared: as the phytoplankton died and sank, they decomposed a little bit and released enough CO2 to cause a net decrease in pH compared to the control run.

In the diagram below, compare the first row (A2 control run) to the second (A2 with iron fertilization). The more red the contours are, the more acidic that layer of the ocean is with respect to preindustrial conditions. The third row contains data from another simulation in which emissions were allowed to increase just enough to offest sequestration by phytoplankton, leading to the same CO2 concentrations as the control run. The general pattern – iron fertilization reduces some acidity at the surface, but increases it at depth – is clear.

depth vs. latitude at 2100 (left); depth vs. time (right)

The more I read about geoengineering, the more I realize how poor the associated cost-benefit ratios might be. The oft-repeated assertion is true: the easiest way to prevent further climate change is, by a long shot, to simply reduce our emissions.

Feeding the Phytoplankton

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While many forms of geoengineering involve counteracting global warming with induced cooling, others move closer to the source of the problem and target the CO2 increase. By artificially boosting the strength of natural carbon sinks, it might be possible to suck CO2 emissions right out of the air. Currently around 30% of human emissions are absorbed by these sinks; if we could make this metric greater than 100%, atmospheric CO2 concentrations would decline.

One of the most prominent proposals for carbon sink enhancement involves enlisting phytoplankton, photosynthetic organisms in the ocean which take the carbon out of carbon dioxide and use it to build their bodies. When nutrients are abundant, phytoplankton populations explode and create massive blue or green blooms visible from space. Very few animals enjoy eating these organisms, so they just float there for a while. Then they run out of nutrients, die, and sink to the bottom of the ocean, taking the carbon with them.

Phytoplankton blooms are a massive carbon sink, but they still can’t keep up with human emissions. This is because CO2 is not the limiting factor for their growth. In many parts of the ocean, the limiting factor is actually iron. So this geoengineering proposal, often known as “iron fertilization”, involves dumping iron compounds into the ocean and letting the phytoplankton go to work.

A recent study from Germany (see also the Nature news article) tested out this proposal on a small scale. The Southern Ocean, which surrounds Antarctica, was the location of their field tests, since it has a strong circumpolar current that kept the iron contained. After adding several tonnes of iron sulphate, the research ship tracked the phytoplankton as they bloomed, died, and sank.

Measurements showed that at least half of the phytoplankton sank below 1 km after they died, and “a substantial portion is likely to have reached the sea floor”. At this depth, which is below the mixed layer of the ocean, the water won’t be exposed to the atmosphere for centuries. The carbon from the phytoplankton’s bodies is safely stored away, without the danger of CO2 leakage that carbon capture and storage presents. Unlike in previous studies, the researchers were able to show that iron fertilization could be effective.

However, there are other potential side effects of large-scale iron fertilization. We don’t know what the impacts of so much iron might be on other marine life. Coating the sea surface with phytoplankton would block light from entering the mixed layer, decreasing photosynthesis in aquatic plants and possibly leading to oxygen depletion or “dead zones”. It’s also possible that toxic species of algae would get a hold of the nutrients and create poisonous blooms. On the other hand, the negative impacts of ocean acidification from high levels of CO2 would be lessened, a problem which is not addressed by solar radiation-based forms of geoengineering.

Evidently, the safest way to fix the global warming problem is to stop burning fossil fuels. Most scientists agree that geoengineering should be a last resort, an emergency measure to pull out if the Greenland ice sheet is about to go, rather than an excuse for nations to continue burning coal. And some scientists, myself included, fully expect that geoengineering will be necessary one day, so we might as well figure out the safest approach.

Modelling Geoengineering

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Later in my career as a climate modeller, I expect to spend a lot of time studying geoengineering. Given the near-total absence of policy responses to prevent climate change, I think it’s very likely that governments will soon start thinking seriously about ways to artificially cool the planet. Who will they come to for advice? The climate modellers.

Some scientists are pre-emptively recognizing this need for knowledge, and beginning to run simulations of geoengineering. In fact, there’s an entire model intercomparison project dedicated to this area of study. There’s only a small handful of publications so far, but the results are incredibly interesting. Here I summarize two recent papers that model solar radiation management: the practice of offsetting global warming by partially blocking sunlight, whether by seeding clouds, adding sulfate aerosols to the stratosphere, or placing giant mirrors in space. As an added bonus, both of these papers are open access.

A group of scientists from Europe ran the same experiment on four of the world’s most complex climate models. The simulation involved instantaneously quadrupling CO2 from preindustrial levels, but offsetting it with a reduction in the solar constant, such that the net forcing was close to zero.

The global mean temperature remained at preindustrial levels. “Great,” you might think, “we’re home free!” However, climate is far more than just one globally averaged metric. Even though the average temperature stayed the same, there were still regional changes, with cooling in the tropics and warming at both poles (particularly in their respective winters):

There were regional changes in precipitation, too, but they didn’t all cancel out like with temperature. Global mean precipitation decreased, due to cloud feedbacks which are influenced by sunlight but not greenhouse gases. There were significant changes in the monsoons of south Asia, but the models disagreed as to exactly what those changes would be.

This intercomparison showed that even with geoengineering, we’re still going to get a different climate. We won’t have to worry about some of the big-ticket items like sea level rise, but droughts and forest dieback will remain a major threat. Countries will still struggle to feed their people, and species will still face extinction.

On the other side of the Atlantic, Damon Matthews and Ken Caldeira took a different approach. (By the way, what is it about Damon Matthews? All the awesome papers out of Canada seem to have his name on them.) Using the UVic ESCM, they performed a more realistic experiment in which emissions varied with time. They offset emissions from the A2 scenario with a gradually decreasing solar constant. They found that the climate responds quickly to geoengineering, and their temperature and precipitation results were very similar to the European paper.

They also examined some interesting feedbacks in the carbon cycle. Carbon sinks (ecosystems which absorb CO2, like oceans and forests) respond to climate change in two different ways. First, they respond directly to increases in atmospheric CO2 – i.e., the fertilization effect. These feedbacks (lumped together in a term we call beta) are negative, because they tend to increase carbon uptake. Second, they respond to the CO2-induced warming, with processes like forest dieback and increased respiration. These feedbacks (a term called gamma) are positive, because they decrease uptake. Currently we have both beta and gamma, and they’re partially cancelling each other out. However, with geoengineering, the heat-induced gamma goes away, and beta is entirely unmasked. As a result, carbon sinks became more effective in this experiment, and sucked extra CO2 out of the atmosphere.

The really interesting part of the Matthews and Caldeira paper was when they stopped the geoengineering. This scenario is rather plausible – wars, recessions, or public disapproval could force the world to abandon the project. So, in the experiment, they brought the solar constant back to current levels overnight.

The results were pretty ugly. Global climate rapidly shifted back to the conditions it would have experienced without geoengineering. In other words, all the warming that we cancelled out came back at once. Global average temperature changed at a rate of up to 4°C per decade, or 20 times faster than at present. Given that biological, physical, and social systems worldwide are struggling to keep up with today’s warming, this rate of change would be devastating. To make things worse, gamma came back in full force, and carbon sinks spit out the extra CO2 they had soaked up. Atmospheric concentrations went up further, leading to more warming.

Essentially, if governments want to do geoengineering properly, they have to make a pact to do so forever, no matter what the side effects are or what else happens in the world. Given how much legislation is overturned every time a country has a change in government, such a promise would be almost impossible to uphold. Matthews and Caldeira consider this reality, and come to a sobering conclusion:

In the case of inconsistent or erratic deployment (either because of shifting public opinions or unilateral action by individual nations), there would be the potential for large and rapid temperature oscillations between cold and warm climate states.

Yikes. If that doesn’t scare you, what does?

A Misplaced Ban

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The recent UN Convention on Biodiversity passed a ban on geoengineering. The journal Science gained access to the draft text of the protocol prior to its official release, parts of which they quoted in a recent news article. Here are the relevant passages:

Ensure…in the absence of science-based, global, transparent and effective control and regulatory mechanisms for geo-engineering, and in accordance with the precautionary approach…that no climate-related geoengineering activities that may affect biodiversity take place, until there is an adequate scientific basis on which to justify such activities and appropriate consideration of the associated risks for the environment and biodiversity and associated social, economic and cultural impacts, with the exception of small scale scientific research studies that would be conducted in a controlled setting…and only if they are justified by the need to gather specific scientific data and are subject to a thorough prior assessment of the potential impacts on the environment.

Any technologies that deliberately reduce solar insolation or increase carbon sequestration from the atmosphere on a large scale that may affect biodiversity (excluding carbon capture and storage from fossil fuels when it captures carbon dioxide before it is released to the atmosphere) should be considered as forms of geoengineering which are relevant to the Convention on Biological Diversity until a more precise definition can be developed.

The implications of this ban are staggering. As the Science article notes, it could “broadly affect a whole field of research still taking shape”, a field that could end up being vital to our survival. Nobody wants to have to use geoengineering before we do all we can to reduce fossil fuel emissions (well, except for some U.S. politicians, I’m sure). But if it’s 2100 and we’ve virtually eliminated fossil fuels but it’s still not enough, the planet still hasn’t reached radiative equilibrium, and the sea keeps rising and the temperatures keep going up and up…drastic measures to counteract the damage we’ve already done might be our only hope. I’ve heard geoengineering described as a tourniquet: the worst possible option, except for bleeding to death.

The convention leaves the door open to small-scale research, but what if small-scale isn’t enough to improve our understanding of geoengineering’s impacts? I believe that it’s more prudent to take small risks now so that we understand our future options, rather than jump blindly into full deployment when the time comes – and unless we get our act together in the next few years, a prospect that looks more unlikely by the day, that time might easily come sooner than we’d like.

Science interviewed Ken Caldeira, one of the world’s top environmental scientists, on the geoengineering ban, and he made some good points. He argued that “may affect biodiversity” is such a weak statement that it could be used to prevent almost any field research into geoengineering. Additionally, failing to specify negative effects could also prevent studies that aim to increase biodiversity for geoengineering – for example, increasing the productivity of an ecosystem in order to expand its capacity as a carbon sink.

I am also at a loss as to why expanded carbon sinks are given the same status as solar insolation techniques, such as giant mirrors in space or sulfates in the stratosphere to scatter sunlight. It was my understanding that the latter was seen to be riskier. However, I haven’t read much geoengineering research – does anyone have recommendations for good papers?

The Chemical & Engineering News article on the subject interviewed Bart Gordon, the outgoing chair of the U.S. Congress Science & Technology Committee. (I’m really scared to find out who the Republicans are going to replace him with. Initial prospects don’t look good.) He issued a geoengineering report the same day that the UN ban passed, and also had some great words to say:

A research moratorium that stifles science, especially at this stage in our understanding of climate engineering’s risks and benefits, is a step in the wrong direction and undercuts the importance of scientific transparency. If climate change is indeed one of the greatest long-term threats to biological diversity and human welfare, then failing to understand all of our options is also a threat to biodiversity and human welfare.

Science and knowledge isn’t the threat. What we do with that knowledge is the threat. Since the possibility of geoengineering is already out there, how could increasing our understanding around the topic be anything but the most proactive option?