In one sense, humans have been altering or “engineering” different aspects of the Earth since the earliest civilizations — but this has mainly been on a small scale. Now, the threats presented by climate change are leading to the development of a whole new set of schemes that seek to alter the global climate — a fairly ambitious task. Geoengineering is the term used to describe these schemes to intentionally modify or control Earth’s climate system, in order to reduce global warming. There are two general categories of geoengineering schemes — CO2 removal and insolation (sunlight) reduction.
By the end of this module, you should be able to:
To Read | Materials on the course website (Module 9) | |
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To Do | Quiz 9 Unit Self-Assessment |
Due Sunday Due Sunday |
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These projects seek to reduce the insolation (incoming solar radiation) — the energy input for our climate — absorbed by the Earth. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by the excess CO2. Insolation management projects appear to have the advantage of speed, and in some cases, costs. There are a variety of ways that we might achieve a reduction in insolation and thus cool the planet.
Ocean acidification is one of the more serious consequences of emitting carbon dioxide into the atmosphere. The atmosphere and the oceans exchange gases like oxygen and carbon dioxide to achieve a kind of equilibrium or balance in terms of concentration. So, if we put more CO2 into the atmosphere, the oceans will try to absorb a lot of that CO2. Indeed, we now know that the oceans absorb something like 25% of the CO2 we have emitted through the burning of fossil fuels. This is a good thing in terms of moderating the greenhouse gas forcing of our climate, but it has the result that the oceans become more acidic — just as carbonated water is more acidic than tap water. The problem with this is that the phytoplankton that make up the base of the food chain in the oceans cannot tolerate acidic conditions. The oceans are in fact becoming more acidic, and while we humans would never sense this change, the far more sensitive phytoplankton definitely feel it. If further acidification occurs, the phytoplankton will decline and because they are the base of the food chain, most other life in the oceans will also decline, putting the whole ocean ecosystem in peril. This is yet another reason why we need to stop emitting CO2 and even reduce the amount of CO2 in the atmosphere. If we pull CO2 out of the atmosphere, the oceans will release some of its CO2 into the atmosphere, reducing the acidity of the oceans.
Directly changing the albedo of the surface through the use of light colored or reflective materials on buildings, glaciers, etc. For buildings, this has the added benefit of reducing the cooling costs, but it is not likely to be as effective on a global scale as some of the other schemes. Buildings in cities represent something like 0.1% of the Earth’s surface, so by changing the albedo of the building tops, we cannot make much of a change in the global albedo and thus the global temperature. To calculate this, we need to do some very simple math. As a whole, our planet has an albedo of 0.3, so if we change the albedo of 0.1% (which, as a fraction, is 0.001) of the whole planet to an albedo of 0.9 (very reflective), then we get the new albedo by adding 0.3*0.999 + 0.9*0.001 — this give us the new albedo of 0.3006. This would lower the temperature by 0.06°C, which is clearly not going to be enough! However, this approach does hold promise for individual cities, which suffer from a phenomenon called the “urban heat island effect” — they are hotter than the surrounding countryside where there are more plants. Plants cool an environment by releasing water in the form of vapor — this is called transpiration, and just like evaporation, it cools the surface. So, it is a good idea to whiten up building tops, but this is not going to solve our global warming problem. There are few, if any, risks associated with these kinds of operations. But, the cost of doing this could be as high as \$300 billion per year based on a study by the Royal Society — a lot of money for a small reduction in temperature!
There are a couple of ideas for making the atmosphere reflect more sunlight, including brightening clouds and adding aerosols (tiny particles, either solids or liquids, that stay suspended in the atmosphere for a relatively long period of time).
The basic idea here is to make clouds brighter by increasing the concentration of tiny droplets of water that make up the clouds. It has long been recognized that in parts of the world that are dustier, the clouds tend to be brighter because of a higher concentration of water droplets. The tiny water droplets in clouds form around even tinier particles called Cloud Condensation Nuclei (CCN) — the more CCNs you have, the more water droplets form, the brighter the clouds are, the more sun they reflect. It has been suggested that spraying tiny salt crystals derived from the oceans would serve this purpose, and this, along with the fact that the oceans cover 75% of the Earth, means that this would be done over the oceans. The troposphere (lower part of the atmosphere where clouds form) is a dynamic place, which makes the effectiveness of this approach somewhat difficult to predict, but in theory, it could provide enough of an albedo increase to accomplish the cooling we might want (a couple of degrees C). It is estimated that something like 1500 ships equipped to extract the salt particles and inject them into the atmosphere would be needed — these ships do not exist at present, and they would have to be custom-made. The cost of this approach is a bit uncertain, but is probably not excessive. The main drawbacks of doing something like this include the uncertainty about how this would affect weather in cities near the oceans and the fact that this would not address the problem of ocean acidification.
We could reduce the amount of solar energy reaching the surface and thus cool the planet by making the atmosphere more reflective through the injection of sulfur aerosols into the stratosphere (above the troposphere). We know that this works because of the cooling that follows large, explosive volcanic eruptions that inject tiny aerosols (particles) of sulfate (SO4) into the stratosphere. Based on the volcanic eruptions, we can estimate how much sulfur is needed to counteract a doubling of CO2 — about 5 Tg of S per year (one Tg or teragram is 1012 g), which is about half the amount that injected into the atmosphere by the eruption of Mt. Pinatubo in the Philippines in 1991.
The estimated cost of this would be on the order of \$50 billion per year (consider that the US military expenditures are about \$750 billion per year). These particles have a limited residence time (1-2 years) in the stratosphere, so this would require continual injection via airplanes or balloons. The costs of doing this are surprisingly small (as low as \$50 billion per year), but it would have to be maintained — if we were to start down this path and then suddenly realize that it was a mistake and stop, we would face a truly shocking period of rapid warming. This is because we would probably continue to burn fossil fuels and emit more CO2 into the atmosphere.
This scenario is illustrated in the figure below, from a simple climate model like the one we used in Module 4, modified to include a sulfate aerosol geoengineering scheme.
Alan Robock, a volcanologist and climatologist from Rutgers University has made a list of reasons to not embark on this kind of geoengineering. Here is the list of some of the major ones:
Nevertheless, the fact remains that this mode of geoengineering would work — we could cool the planet, and the cost would be relatively small. So, perhaps it is something we should carefully study and consider in the event of unexpectedly severe climate damages — a parachute to be deployed only when the plane is going down!
Reducing insolation could also be accomplished with space-based mirrors or other structures. One proposal here involves the placement of roughly 16 trillion small disks at a stable position 1.5 million km above the Earth. Each disk would have a diameter of 60 cm and would weigh just one gram. They would not be true mirrors, but would scatter enough sunlight to reduce the insolation by 2%, which be sufficient to cool the planet by 2°C. Getting these disks into place and then keeping them there would be a challenge, and it is estimated that it would take 10 years to put them into place using a special type of gun that could transport up to 10 million of them at a time. The total cost could be 5 trillion dollars every 50 years (the lifetime of the disks). This sounds a bit like science fiction, but it has been developed by a group of prominent astronomers and physicists, so we should assume it is viable, but nevertheless very costly and not something we could easily control. As with all of the insolation reduction schemes, this would do nothing to deal with the problem of ocean acidification.
Carbon dioxide removal projects address the root cause of warming by removing greenhouse gases from the atmosphere. These kinds of geoengineering schemes have the added benefit that by lowering the CO2 concentration in the atmosphere, they also prevent (and can reverse) ocean acidification. CO2 removal projects are generally slower, more expensive, and less developed than some of the insolation reduction schemes. There are a variety of ways that this could be done, including:
You may have heard of this in the context of “clean coal”, which refers to technologies that would allow us to burn coal without emitting CO2 into the atmosphere. Carbon capture and sequestration from power plants that burn fossil fuels involves the chemical removal (sometimes called “scrubbing”) of CO2 from the emissions of power plants, and then the injection of the concentrated CO2 into deep aquifers. In the best case, the sequestered CO2 interacts with minerals in the aquifer to lock the CO2 into a mineral form such as CaCO3 (calcite) which would prevent its release back into the atmosphere. A number of pilot projects of this type have already begun, and it does seem to be technically feasible, but it is not cheap. This would more than double the cost of fossil fuel-generated electricity, making this an expensive proposition, but that in itself would encourage developing clean, renewable energy sources like wind and solar that are also cheaper. Note that this would reduce emissions of CO2, but it would not lower the CO2 concentration in the atmosphere — just prevent it from further increases.
Other means of carbon capture have been proposed, including the promotion of natural chemical weathering reactions of some rocks in which atmospheric CO2 is consumed. These natural rock weathering reactions could be accelerated by crushing up the rock, which would increase the surface area of the minerals. This would have minimal environmental side effects, but it would also be quite slow and is limited by the availability of the right kinds of rocks. As such, this is not considered as a viable solution to our climate problems.
The surface waters in the southern oceans are depleted in iron, which is an important micronutrient for photosynthesizing plankton, so the plankton in this part of the oceans are under-performing. Adding powdered iron promotes an increase in plankton growth, thus drawing more CO2 from the surface oceans, which in turn enables the oceans to absorb more CO2 from the atmosphere. A few small-scale experiments have been conducted and they appear to work in the short-term, but scaling this up would be challenging and the iron would have to be continuously applied, just as fertilizer is continuously applied to crops. This would be a very expensive solution, and as such, is not considered as a realistic option.
Carbon dioxide can be chemically extracted from the atmosphere, and a couple of projects led by universities and private companies have developed systems to do this. These systems involve using natural winds or fans to pass air through filters that are coated with chemicals — either amines (organic molecules derived from ammonium), or a sodium hydroxide solution — that react with CO2, causing it to attach to the filter material. When the filters are full, they are closed off and subjected to either high humidity or temperatures of 100°C, which releases the CO2 — it is then drawn off and eventually concentrated into nearly pure CO2. Once the CO2 has been concentrated, there are several options:
The general scheme of a DACCS system is illustrated in the figure below.
These DACCS systems can be relatively small, and they can be deployed anywhere near a site where the CO2 can be injected into a suitable underground geologic storage site. Climeworks [2], a Swiss company, has already deployed several of these units; one is located in Iceland where they use waste heat from a geothermal power plant to provide energy to run the system and then inject the carbonated water into basaltic rock, which is an ideal geologic storage unit. A Canadian company, Carbon Engineering [3], has even gotten some of the major oil companies to invest heavily in this new technology, which is meant to be deployed in larger facilities.
At the moment, these systems are quite expensive. Climeworks is removing carbon for about \$600 per ton of CO2, and they are confident that they can quickly get down to \$200 per ton, and, if they greatly expand their manufacturing process, they might get it down below \$100 per ton. Carbon Engineering says that they will be able to do it for less than \$100 per ton. The lesson we take away from wind and solar energy is that the prices for these technologies are likely to continue to decrease as more units are produced. But, if we use \$100 per ton as a good near-term estimate, it would cost \$1 trillion to remove 10 Gt of CO2 (remember that our current global emissions are in the range of 37 Gt CO2 in 2018). This sounds like a lot of money, but it is only 1% of the global GDP and just a shade more than what we spend in the US on our military. Deploying this on a large scale also requires a lot of energy, but if that energy came from solar or wind power, there would still be a net removal of CO2 from the atmosphere.
One of the attractive features of DACCS technologies is that they could help solve the problem of ocean acidification at the same time as lowering the temperature (or preventing it from getting too high).
If we wanted to use DACCS to get to zero carbon emissions, we would have to remove as much as we emit from burning fossil fuels. Doing this would allow the carbon cycle to begin to return to normal; the temperature would decrease slightly, and ocean acidification would be reversed.
The figure below shows the results of a little experiment where a DACCS system is added to a global carbon cycle model to show what would happen if, starting in 2020, we began to remove carbon through DACCS to match the carbon emissions from burning fossils. The model begins in 1880 and runs up to 2014 using the actual human emissions of carbon, and then switches to a projection made by the IPCC for future carbon emissions.
The upper panel shows the gigatons of C from human emissions and the gigatons of C removed by DACCS, which begins in the year 2020. The lower panel shows how this change affects the global temperature change (green, in °C relative to the start), the atmospheric CO2 concentration (red, in parts per million), and the ocean pH, which is inversely related to the acidity).
If we continue to burn fossil fuels as we have been (the scenario shown in the figure above), the cost of using DACCS to negate the emissions would be immense — a total of perhaps \$600 trillion by the end of the century. But if we also make drastic reductions in our carbon emissions, the cost of DACCS would be more manageable. This raises an important point — the cheapest thing to do is to switch to renewable energy (mainly wind and solar) and thus dramatically reduce our emissions. And, as we will see later, less money spent on geoengineering means more money to be spent on things like education, healthcare, and other things that improve our quality of life.
BECCS encompasses a wide range of different plans, but what they all share in common is the utilization of plants to draw CO2 from the atmosphere (which they have perfected over millions of years) and then using the biomass to generate power. In one version, the plant material is fermented to yield biofuels like ethanol, but when the ethanol is burned, it releases the CO2 back into the atmosphere — this is not going to result in negative carbon emissions. But in another form, a BECCS scheme combusts the biomass to electrical energy in a power plant equipped with CO2 scrubbers on their emissions.
The captured CO2 from these power plants is then injected into a deeply buried geologic layer where it is sequestered — just as with the DACCS approach. A BECCS system will reduce the amount of CO2 in the atmosphere while at the same time producing energy, the sale of which helps offset the costs. Some estimates suggest that a system such as this could remove carbon at a net cost of \$15 per ton of CO2 — significantly cheaper that the DACCS systems (which might get to \$100/ton in the near future).
Deploying BECCS on a large enough scale to make a serious reduction in CO2 would require a lot of land and water to grow the biofuels, and this imposes a limit since we will also need the land and water resources to grow food crops for a growing population. One estimate suggests that in order to remove 12 GT of CO2 from the atmosphere each year, we would need to commit an area equal to one third of the present cropland area to BECCS and we would need perhaps one half of the water currently used by agriculture. These are some pretty serious environmental constraints!
Nevertheless, BECCS holds great promise for being an important part of a negative emissions strategy that we will need to dramatically lower our net carbon emissions.
After completing your Discussion Assignment, don't forget to take the Module 9 Quiz. If you didn't answer the Learning Checkpoint questions, take a few minutes to complete them now. They will help you study for the quiz and you may even see a few of those questions on the quiz!
We have now come to the end of Unit 2. The purpose of this exercise is to encourage you to think a little bit about what you have learned well, and just as importantly, about what you feel you have not learned so well. Think about the learning objectives presented at the beginning of the Unit, and repeated below. What did you find difficult or challenging about the things you feel you should have learned better than you did? What do you think would have helped you learn these things better?
For each Module in Unit 2, rank the learning outcomes in order of how well you believe you have mastered them. A rank of 1 means you are most confident in your mastery of that objective. Use each rank only once - so if there are four objectives for a given module, you should mark one with a 1, one with a 2, one with a 3, and one with a 4. All items must be ranked. For each Module, indicate what was difficult about the objective you have marked at the lowest confidence level.
The self-assessment is worth a total of 25 points.
Description | Possible Points |
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All options are ranked | 10 |
Questions are answered thoughtfully and completely | 15 |
Geoengineering is the deliberate manipulation of the earth’s atmosphere, with the objective of controlling the rate of warming or otherwise mitigating the rate of climatic change. Geoengineering options that would directly reduce the amount of radiation trapped within the atmosphere range from controlling emissions through the capture and long-term storage of greenhouse gasses in geologic formations to the deployment of satellites or other devices aimed at reflecting sunlight back into space. Geoengineering options that would affect the climate through modification of land and sea include reforestation and deploying chemicals in the ocean that would cause oceans to absorb greater amounts of radiation. Geoengineering is a controversial proposition, and geoengineering activities are not currently regulated by any major international agreements. There are two primary reasons for the controversy. First, with few exceptions, most geoengineering options exist only in theory or in the realm of science fiction. The exceptions (options with which we have some real-world experience) include cloud seeding and the injection of carbon dioxide into oil and gas wells to get even more oil and gas out. None of these applications of geoengineering technologies are related to climate change – they have been employed for short-term weather modification or to make fossil fuel production activities even more productive. Second, geoengineering is often perceived as a fix to the climate problem that can (might?) work when all other options have been exhausted. The “bathtub” analogy of the greenhouse effect tells us that most geoengineering options alone will not be sufficient to reverse or mitigate any ill effects from climate change.
You have reached the end of Module 9! Double-check the Module Roadmap to make sure you have completed all of the activities listed there before you begin Module 10.