Goals

• Recognize the great diversity of energy options currently available to us
• Explain scientific concepts in language non-scientists can understand

Learning Objectives

By the end of this module, you should be able to:

• Recall the various geoengineering strategies that have been suggested to mitigate climate change
• Recognize that geoengineering alone is unlikely to be sufficient to mitigate climate change
• Assess what you have learned in Unit 2

Questions?

If you have any questions, please post them to Help Discussion. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

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.

Changing the Surface Albedo

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).

Cloud Albedo Enhancement

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.

Stratospheric Aerosols

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:

1. Effects on regional climates. Following large volcanic eruptions like Mt. Pinatubo, climate scientists noted changes in precipitation leading to droughts and problems in food production. Most climate modelers studying this problem agree that these kinds of regional changes would be impossible to avoid and impossible to control.
2. Continued ocean acidification. Blocking sunlight in this way may limit the rise in temperature, but it would not address the problem of ocean acidification. A large fraction of the excess CO₂ added to the atmosphere by burning fossil fuels is absorbed by the oceans, and this increases the acidity of the oceans, endangering the entire marine ecosystem. This problem would continue if we only blocked the sunlight.
3. Ozone depletion. The sulfate additions to the stratosphere would promote the kinds of chemical reactions that destroy ozone, and ozone is a critically important part of our atmosphere in the sense that it blocks harmful UV radiation from reaching the Earth’s surface.
4. Effects on plants. The scattering of light caused by the aerosols may create more diffuse light that is beneficial to plants, but the reduced total amount of sunlight could be detrimental — the balance of these effects is not well understood.
5. More acid rain. The sulfate aerosols combine with atmospheric water vapor to form tiny droplets of sulfuric acid, which eventually fall out of the sky and land on the Earth’s surface. The global average increase in acid precipitation may not be alarming, but it is likely that there will be some regions that are noticeably impacted.
6. Effects on cirrus clouds. The injected aerosols are likely to affect the formation of high cirrus clouds, which can impact the energy balance and thus temperature of the Earth, but this is not well-understood at present — we don’t know whether this would increase or decrease cirrus clouds.
7. Whitening of the sky. The sulfate aerosols would make our sky more white than blue, which we might get used to after a while, but it would be unsettling to most people in the short term. We would, however, get much more vibrant sunsets.
8. Less sun for solar power. Reducing the amount of sunlight reaching the surface will necessarily reduce the amount of solar energy we can harvest — not a good thing.
9. Rapid warming if stopped. If we were to decide that this was a bad idea and stopped, a dramatic, potentially catastrophic warming would ensue (as shown in the model results above). This assumes that we continued to raise CO₂ concentrations in the atmosphere.
10. Human error. This is always a reality in any human endeavor, and an error in a system such as this could have serious consequences for billions of people.
11. Undermining sustainability efforts. Any geoengineering scheme is problematic in the sense that it lets us off the hook and just delays the inevitable transition to a more sustainable energy system. We will definitely run out of fossil fuels and the longer we wait to transition to a system based on renewable energy, the harder it will be.
12. Who is in control? Who has the power over the global temperature? This is a huge problem — it cannot be left to one nation, or one group of nations to make the decisions that will affect all of the people on Earth. Altering the climate this way will clearly create some winners and some losers. Island nations threatened with complete submergence in the future will want as much cooling as possible. The US and China and much of Europe will also want a lot of cooling to combat the rising sea levels that will force the relocation of major cities. But the Russians may want even more warming. So, who decides?
13. Unintended consequences. Even instance in which humans have undertaken to modify the environment, a whole host of unintended consequences arise. Our world is complex enough, with so many interconnections, that it is impossible for us to foresee all of the consequences of our actions, even with the best of intentions. As just one example of this, the first people to discover the usefulness of fossil fuels did not foresee the potential for global environmental harm.

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:

Carbon Capture and Sequestration

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.

Enhanced Rock Weathering

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.

Ocean Fertilization

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 C O₂, 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:

• It can be pumped into a greenhouse to be used by plants. The plants will use this CO₂, but when the plants are harvested, that CO₂ will be returned to the atmosphere, so the CO₂ is not sequestered for long.
• It can be bottled and used to carbonate beverages, but as soon as the beverages are consumed, the CO₂ returns to the atmosphere, so this does not really sequester the CO₂.
• It can be combined with hydrogen to make a synthetic fuel similar to gasoline or jet fuel, but when the fuel is burned, it releases the CO₂ back to the atmosphere. So again, this does not really sequester the CO₂.
• It can be mixed with water and injected underground into a geologic material that will undergo a chemical reaction with the carbonated water to precipitate carbonate minerals that effectively lock up or sequester the carbon. This is sometimes called direct air capture and carbon sequestration (DACCS). DACCS leads to negative emissions of CO₂, which would lead to lowering the concentration of CO₂ in the atmosphere, cooling the planet.

The general scheme of a DACCS system is illustrated in the figure below.

Some DACCS systems consist of relatively small (1 m cube) units with a fan on one side, and exhaust on the other, and filters inside that remove the CO₂. The CO₂ is then concentrated, mixed with water and injected into a geologic material below the surface where the carbonated water reacts with the rock to form carbonate minerals that lock up the carbon.

Click for a text description of the Direct Air Capture and Carbon Sequstion diagram.

The image is a schematic diagram illustrating a process for carbon dioxide (CO2) removal using a system powered by solar energy. Here's the breakdown of the components and flow:

• Background: The background is black, providing contrast for the elements of the diagram.
• Atmosphere: Represented by a blue arrow labeled "atm w/ CO2" (atmosphere with CO2) coming from the left side of the image, indicating the source of CO2.
• Solar Panels: At the top of the diagram, there are two blue solar panels labeled "solar energy," providing the power for the removal process.
• Removal Unit: A gray rectangular box labeled "Removal Unit" is centrally located, which is powered by the solar energy from the panels. This unit is responsible for extracting CO2 from the atmosphere.
• Flow of CO2:
  • CO2 enters the removal unit from the atmosphere via a blue arrow.
  • After processing, pure CO2 is directed out of the removal unit via a yellow arrow labeled "Pure CO2."
• Mixer: Below the removal unit, there's a circular component labeled "mixer." This mixer combines the pure CO2 with water, indicated by a blue arrow labeled "water" entering from the right.
• Basaltic Rock: At the bottom of the diagram, there's a label "Basaltic Rock," indicating that the mixture of CO2 and water is directed towards this geological formation.
• Flow to Basaltic Rock: A green arrow labeled "CO2" shows the flow of the CO2-water mixture downward towards the basaltic rock, suggesting a process where the CO2 is intended to be sequestered or reacted with the rock.
• Vegetation: There are green patches representing vegetation or grass at various points around the removal unit and along the flow paths, possibly indicating the natural environment or areas where CO2 might be absorbed by plants.

This diagram visually represents a conceptual system for capturing atmospheric CO2, purifying it, mixing it with water, and then sequestering it in basaltic rock, all powered by solar energy.

Credit: David Bice © Penn State is licensed under CC BY-NC-SA 4.0

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, 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, 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).

These results from a model of the global carbon cycle and the climate system shows what would happen if we used a DACCS system, starting in the year 2020 to counteract all of the carbon emissions from burning fossil fuels. The carbon removed by DACCS is shown in the upper panel along with the fossil fuel carbon emissions; it rises rapidly in the year 2020 and then tracks the carbon emissions, which effectively cancels out the carbon emissions. This results in a drop in atmospheric CO₂, a drop in temperature, and a rise in pH (higher pH values mean lower acidity).

Click for a text description of the DACCS figures with global carbon cycle model.

Top Graph: Human Emissions vs. DACS (Direct Air Carbon Capture and Storage).

• Axes:
  • X-axis: Labeled "Years," ranging from 1880 to 2100.
  • Y-axis: Labeled "GT of Carbon," ranging from 0.00 to 30.00 GT (Gigatons).
• Data Series:
  • Human Emissions: Represented by a solid red line. This line shows a steady increase in human carbon emissions from 1880, with a significant rise post-1950, peaking around 2050, then slightly declining but still remaining high through to 2100.
  • DACS: Represented by a blue dotted line starting around 2045, indicating the introduction of direct air capture and storage technology. This line shows a gradual increase in carbon removal via DACS, which helps to mitigate some of the human emissions but does not reduce them to zero by 2100.
• Legend: Located on the right side, identifying:
  • Red solid line: "human emissions"
  • Blue dotted line: "DACS"

Bottom Graph: pCO2 Atm, pH, and Global Temperature Change

• Title: Not explicitly provided but inferred from the context.
• Axes:
  • X-axis: Labeled "Years," ranging from 1880 to 2100.
  • Y-axis: Three different scales for three variables:
    • Left Y-axis for pCO2 (partial pressure of CO2 in the atmosphere) in ppm (parts per million), ranging from 280.00 to 800.00 ppm.
    • Right Y-axis for pH, ranging from 7.80 to 8.20.
    • Right Y-axis for global temperature change in °C, ranging from -1.00 to 7.00°C.
  • Data Series:
    • pCO2 Atm: Represented by a red line. This line shows an increase in atmospheric CO2 concentration from around 280 ppm in 1880, with a sharp rise post-1950, peaking around 2050, then slightly decreasing but still elevated by 2100.
    • pH: Represented by a blue line. This line shows a decrease in ocean pH (acidification) from around 8.20 in 1880, with a notable drop starting around 1950, continuing to decrease until around 2050, then leveling off but not recovering to pre-1950 levels by 2100.
    • Global Temp Change: Represented by a green line. This line shows the global temperature change, starting near 0°C in 1880, with a significant increase post-1950, peaking around 2050, then slightly decreasing but remaining high by 2100
  • Legend: Located at the bottom, identifying:
    • Red line: "pCO2 atm"
    • Blue line: "pH"
    • Green line: "global temp change"

Credit: David Bice © Penn State is licensed under CC BY-NC-SA 4.0

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.

A schematic representation of a BECCS system, which utilizes fast-growing plants to remove CO2 from the atmosphere, turning it into biomass that is harvested and burned in a power plant to produce electricity. The combustion emissions are scrubbed to remove the CO₂, which is then sequestered in a suitable geologic formation below the surface.

Click for a text description of Bio-Energy with Carbon Capture and Sequestration diagram.

The image is a flowchart diagram illustrating the process of Bio-Energy with Carbon Capture and Storage (BECCS). Here's the step-by-step description:

1. Atmospheric CO₂: Represented by a green arrow labeled "Atmospheric CO₂" pointing downwards, indicating that CO₂ from the atmosphere is absorbed by plants.
2. Biomass (Trees): On the left side, there is an illustration of a forest or group of trees, representing biomass. These trees absorb CO₂ from the atmosphere through photosynthesis.
3. Harvest: A green arrow labeled "Harvest" leads from the trees to the next step, indicating that the biomass (trees) is harvested.
4. Biomass Burning Power Plant: The harvested biomass is directed into a blue structure labeled "Biomass burning power plant." This plant burns the biomass to generate electricity.
5. Electricity to Grid: From the power plant, there is an arrow leading off to the right labeled "Electricity to grid," indicating that the electricity generated is supplied to the electrical grid for distribution.
6. CO2 Scrubbed: A yellow arrow labeled "CO₂ scrubbed" exits from the power plant, showing that the CO₂ produced during the burning of biomass is captured instead of being released into the atmosphere.
7. Mixer: The CO₂ is then directed to a gray oval labeled "mixer." Here, the captured CO₂ is mixed with water, indicated by a blue arrow labeled "water" entering from the right side.
8. CO2-Water Mixture: A green arrow labeled "CO₂ + water" leads from the mixer downwards.
9. Storage Rock: At the bottom, there is a label "Storage Rock," indicating that the mixture of CO₂ and water is sequestered into geological formations (storage rock) for long-term storage.

This diagram visually represents the process where biomass (trees) absorbs CO2, is harvested, burned for energy in a power plant, and then the CO2 emissions from this process are captured, mixed with water, and stored underground, effectively removing CO₂ from the atmosphere.

Credit: David Bice © Penn State is licensed under CC BY-NC-SA 4.0

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 than 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.

Learning Outcomes Survey

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 3-5 objectives for a given module, you should mark one with a 1, one with a 2, one with a 3, one with a 4, and one with a 5. All items must be ranked. For each Module, indicate what was difficult about the objective you have marked at the lowest confidence level.

Module 6

Rank the following 1-3

• Recognize the advantages and limitations of solar and wind energy.
• Recall the basic science behind solar and wind power generation.
• Evaluate who is responsible for maintaining "the grid" as home generation grows in popularity.

What did you find most challenging about the objective you ranked the lowest?

Module 7

Rank the following 1-3

• Recognize the advantages and limitations of geothermal, hydroelectric, and nuclear energy.
• Recall the basic science behind geothermal, hydroelectric, and nuclear power generation.
• Analyze why even people who rely heavily on energy resources tend to want those resources to be exploited far from their own homes.

What did you find most challenging about the objective you ranked the lowest?

Module 8

Rank the following 1-5

• Recognize that all energy technologies are inefficient.
• Compare wealth and energy intensity in developed countries.
• Identify options for improving energy efficiency in developed countries.
• Analyze why we don't always conserve as much as we should, despite the double benefits for the climate and our wallets.
• Use a model to calculate the effects of various strategies such as the use of renewable energy sources, conservation, and population control on reducing emissions

What did you find most challenging about the objective you ranked the lowest?

Module 9

Rank the following 1-3

• Recall the various geoengineering strategies that have been suggested to mitigate climate change.
• Recognize that geoengineering alone is unlikely to be sufficient to mitigate climate change.
• Assess what you have learned in Unit 2.

What did you find most challenging about the objective you ranked the lowest?

Scoring Information and Rubric

The self-assessment is worth a total of 25 points.

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.

Reminder - Complete all of the Module 9 tasks!

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.