Modelling Geoengineering

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?


19 thoughts on “Modelling Geoengineering

  1. Thanks, great summary. Here’s hoping you don’t wind up becoming a geoengineer, though I realize at some point we will have little to lose

  2. Did the climatologists’ models predict what geoengineering would cost in dollars and health? No, I didn’t think so. Geoengineering would run up $Trillions more in debt, most of it going into the pockets of Iron Triangle companies, and as you said, after we’re all sick and broke even more than today, then what?

    I think there may be a better solution that won’t cost much, and politicians and armies can’t stop it. Scientists know that vines absorb ten times more CO2 than trees, so vines are very effective at reducing CO2 . Furthermore, vines tend to thrive even in moderately dry regions where other plants don’t grow very well. So how about everyone planting kudzu and poison ivy everywhere?

    These vines may even have a beneficial side effect. Once they have overgrown all the garages and roads, people won’t be driving their cars, and this will eliminate even more CO2 emissions.

    Now kudzu and poison ivy may have a few undesirable side effects, but surely they can’t be as bad as the unknown horrors of geoengineering.

  3. The consequences of climate change include: Panic.

    As much as fear the option of geoengineeering may delay mitigation, I fear that sooner or later governments will react in panic and order the deployment of some form of geoengineering.

    If you have some of the answers ready it may be possible to avoid the worst consequences. There maybe currently unknown thresholds, maybe a mixture of techniques will be best. If it is to happen then it is far better to do it knowledgeably.

    That still does not address ocean acidification a problem that is only just beginning to sink into general awareness. And just maybe the knowledge of the consequences will make mitigation more attractive.

  4. “Later in my career as a climate modeller…”

    How cool is it to say that? :-)

    @ 4°C/decade – holy crap!

    I’m guessing we’ll probably re-post this one at SkS too if you don’t mind, Kate. Fascinating stuff.

    Yes, go ahead and repost it if you’d like. -Kate

  5. Here in Central Illinois the weather has been dry and hot and the prospects for this summer don’t look good.

    While our precipitation the past fourteen months has been substantially below normal, that is not the case everywhere. Recently, roughly a single county about fifty miles south of Chicago and one-hundred miles northeast of Peoria, received more than eleven inches of rain in about a twelve-hour period. Most of the immediate area surrounding this county received one to two inches. I watched the radar, and saw that a strong storm cell continuously evolved just to the west of the deluged area, moved east, dumped its moisture, and then dissipated. If climatologists can show a model that will accurately predict conditions leading up to this phenomena and model it and the surrounding areas, then I might believe their ability to predict the unknown horrendous effects of geoengineering.

    During that time interval, some southern Chicago suburbs also received five inches or so of rain with local flooding, but the immediate area around the deluged county did not. Perhaps some form of residual heating effects from Chicago’s large metropolitan area had earlier drifted southwest and was spawning this outbreak. Or maybe some government agency was secretly experimenting with geoengineering and not caring what impacts their clandestine research might be having on local residents.

    If climatologists are unable to identify the causes for such an event and develop models to predict and evaluate it, then I doubt they would ever be able to identify the potential causes of local disasters associated with geoengineering and develop models to predict and evaluate them.

  6. The precipitation change over Amazonia looks to be the most extreme change over land. Any idea on the implications of that?

  7. In your conclusion you state that, “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.”

    This is not true, and the confusion derives from your equating “geoengineering” solely with Solar Radiation Management (SRM). There is a second large class of geoengineering technologies known as Carbon Dioxide Removal (CDR), which includes things like direct air capture, bioenergy with CCS, ocean iron fertilization, etc. Any responsible use of SRM would be paired with the use of CDR strategies to achieve “negative emissions.” As atmospheric CO2 concentrations declined, SRM deployment would be dialed down as well.

    • There’s still debate in the literature as to whether carbon capture ought to be classified as “geoengineering”. Regardless, it’s too glib to just say that SRM will be paired with CDR. If CDR were that easy, we’d be doing it now, geoengineering or not. It’s still a massive undertaking (on par with extracting all the fossil fuels in the first place, except without so many economic benefits), and would have to be spread out over a very long time period until CO2 declines to pre-industrial (during which time the “failure of SRM” arguments would still apply). And I feel I’m not as optimistic as you as to the certainty with which a “responsible” strategy would be employed.

  8. Since you mentioned Dr. Robock’s GeoMIP project, I thought I’d link you to another one of his publications – ( It’s interesting to study geo-engineering – for instance, in my own work in aerosol-cloud interactions/processes, studying marine cloud whitening and that general area of technologies and ideas is an attractive sort of case study or thought experiment. However, beyond just sheer cost, engineering effort, and logistical issues, it’s worth bearing in mind that at this juncture in time, any sort of large-scale geo-engineering strategy is likely illegal under the 1976 ENMOD treaty.

    Geo-engineering is just too risky and too expensive. Unless it’s proposed by a pessimist who is frustrated with current efforts to implement serious mitigation strategies, I think it’s worth having a quizzical attitude when someone proposes geo-engineering as a solution to global warming. It’s right up there with thorium reactors (still decades away from commercialization) and nuclear fusion – too expensive and the technology won’t exist for quite some time.

  9. Geoengineering thinkers tend to resist acknowledging that we have been performing a real geoengineering experiment all along. It is just unintentional. So the easiest, and most proven method is to just reverse the experiment – halting all emissions and sequestering CO2. We know that works.

    The unacceptability of this approach only reveals the true problem: Global warming is a human social problem – not really a climate science one, Climate change will unfold in physical ways you can nicely describe. More difficult is to say how we can unify human will to meet adaptation and mitigation problems.

    I am glad you are looking in this direction, and heartened that you are pondering the ethics of the field. Indeed, once geoeingineering is started, it cannot be stopped, but what about unilateral actions? Should we stop a rogue nation from proceeding? Or even a wealthy individual? What about collateral damages? If warmer weather crops are suddenly shocked by geoengineered cold weather, is there a liability? What about increased flooding or drought? Who is liable for damages caused by intentional climate change? [since we are unable to assign liability to UNintentional change, I doubt this can be solved]

    If you give an open sense of decision making to your models, what if they decide that human population should be radically decreased?

    I would hope that you continue to study climate modeling, and we are blessed that you share so much of what you learn.

  10. So are you hoping to specialize in particularly policy-relevant science in your future career? Geoengineering is certainly one (very controversial) candidate. Regional/decadal prediction, extremes, sea level rise, and climate or carbon cycle thresholds are other currently popular areas. The first few are aimed more at adaptation than prevention. (Arguably we’re more likely to have to do that than either GHG emissions abatement or geoengineering.) I wouldn’t set too strong expectations about what you’ll be doing in the future, though; the convergence of interesting science and what policy-makers are interested in changes every 5 years.

    Damon has done a lot of excellent work on Earth system modeling, especially with the terrestrial carbon cycle. I can also attest that he’s a great guy to collaborate with. (Unfortunately, his department only supports M.Sc. students, not Ph.D. students.)

    By the way, a couple papers on geoengineering that may interest you. The first one, concerning sea level rise, was also done with UVic. The second one looks at the economic impacts the of “failure to geoengineer forever” scenario you mention.

    “Tension between reducing sea-level rise and global warming through solar-radiation management”
    “The economics (or lack thereof) of aerosol geoengineering”

  11. I am very concerned about Weather Modification that has been taking place by our Gov’t. Global Dimming is stopping Sun on Water Natural Evaporation for natural rain clouds. The Sun’s rays are being reflected back into the Universe and now there is talk of other Planets heating up, too. All Oil Activities are heating up the Arctic; Ice Breakers work 24/7 to chip up what should remain frozen. DOE, DARPA, HAARP, etc., for control & profit is a serious threat against all life on Earth. It is the epitome of Gov’t pollution. Weather is a Commodity traded on the Chicago Mercantile Exchange… You can stop Climate Change by keeping Gov’t, Business, profiteers, etc., from screwing with Nature.

  12. Interesting observations. I however still have a stance against geoengineering, but we have to be clear about different kinds of methods. For instance by starting to paint houses and buildings white to reflect sunlight I see more as a building technique and something we for sure should start implementing on all buildings new and old. But when it comes to stuff like putting mirros up into space I’m highly critical of it, the truth is we dont really know what would happen and if it backfires it could be extremely risky. Still think geo-engineering should be the very last resort and it’s also very expensive.

  13. Richard Pauli touched on some of what I wanted to mention. I’ll just add what happens if some nation or nations suffer because larger, richer nations geoengineered the climate? We may stop heat waves and drought in our nations, but suppose that creates problems in other nations? They may have wanted the climate regime shift as it might have meant more reliable precipitation events. Instead, they’re now suffering drought and famine which brings in ethical issues along with all the geopolitical and economic issues (not to mention complications arising if the suffering nations possess nuclear weapons—how far would you go to provide food to your family and protect your people?).

    • Excellent points–that’s why we should scale back on our current geoengineering by reducing greenhouse gas emissions. Kim Stanley Robinson has written a trilogy on global warming (first book is “Forty Signs of Rain”). The two takeaways for me were: 1) Nothing happens on the policy front until the temperature in DC falls to 50 degrees F below 0 (due to a shift in ocean currents) and 2) Ultimately we do geoengineering, not because it is safe (it definitely isn’t), but because we have nothing left to lose.

  14. Kate,

    The 21 August 2012 post at Science of Doom has some images of a physical “pan” model of polar atmospheric circulation. I thought you might find these of interest.


  15. Pingback: At what point do we admit that geoengineering is an option? – TechCrunch

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