Comments by Expert Reviewers on the Geoengineering Proposals

Background to the Geoengineering Contest #

When talking about geoengineering (which might be more appropriately called "climate engineering"), it is important to understand that the concepts require thinking in very large and broad terms. The primary greenhouse gas, both in terms of mass emitted and the overall climate effects over the long term, is carbon dioxide (CO2). Current emissions of CO2 to the atmosphere as a result of human activities (i.e. including emissions from combustion of fossil fuels and reduction of the carbon contained in the terrestrial biosphere) total on the order of 10 billion metric tons of carbon per year (or roughly 37,000 million metric tons of CO2, as international negotiators express it). This is all before the warming effects of other gases and aerosols resulting from human activities are counted. Reducing the amount of CO2 emitted is referred to as mitigation.

Recapturing the CO2 after it is emitted and sequestering it somewhere out of the atmosphere is called Carbon Dioxide Removal (CDR), which is one of the two types of climate engineering that are generally discussed as being potentially conceivable. Present total human emissions of CO2 add roughly 2.5 ppmv (parts per million by volume) to the global atmospheric CO2 concentration each year, pushing a roughly equivalent amount into the upper ocean and terrestrial biosphere together that will remain there only as long as the extra 2.5 ppmv stays in the atmosphere. If CDR is used to attempt to reduce the atmospheric CO2 concentration by 2.5 ppmv, the distribution of the temporarily stored equivalent in the upper ocean and terrestrial biosphere will readjust so that half returns to the atmosphere. Similar kinds of adjustments would occur if one took the CO2 out of any of the active carbon reservoirs on Earth. For this reason, if one intends to use CDR to try to keep the atmospheric concentration level, all of the CO2 emitted by human activities must be captured or stored somewhere, which leads to three fundamental considerations for approaches to CDR:

  1. To remove all of the emitted CO2, the scale of the infrastructure, and so the cost of the capture and removal system for the emitted CO2, would likely have to be comparable to the scale of the industrial system that presently exists to extract, process, distribute, and combust the global total of fossil fuel and biospheric carbon;
  2. If so much effort needs to be made to extract CO2 from the atmosphere, ocean, or biosphere, why not use it as the basis for producing biofuels that would replace extraction and combustion of fossil carbon from the ground?
  3. Were the cost of CDR comparable to the capital and operating costs of the present fossil fuel system, would it not be less expensive to invest the resources into direct capture and storage of CO2 at the point of combustion, making the present system more efficient, and/or greatly increasing the amount of non-carbon emitting energy systems?

Thus, while it is interesting to contemplate possible CDR proposals such as those submitted to this contest, the prospects for implementation in the near-term likely would depend on the strategy being less expensive than mitigation via biofuel replacement for fossil fuels. This is not only because processing the total global amount of fossil fuel emissions would be so costly, but also because such an approach would have to be preferable to the other alternatives for reducing greenhouse gas emissions.

On the other hand, if mitigation reduces the amount of carbon emissions over the next few decades to just its most essential uses (e.g., possibly as fuel for aircraft and ships) where substitutes are unlikely to be cost competitive, CDR could become a cost-effective means for keeping the CO2 concentration from rising higher and even possibly for pulling the CO2 concentration down toward its present, or even pre-industrial, value. Taking into account exchange with the ocean, CDR could also be used to reduce ocean acidification. Given these different potentials and time scales, starting research on such approaches might thus be considered a near-term priority.

The other approach to climate engineering is to offset the increased trapping of infrared energy caused by increasing concentrations of greenhouse gases. As a rough order of magnitude, raising the concentration of CO2 by 300 ppmv increases the trapping of infrared energy by 4 W/square meter at the tropopause; over time, such an increase would be projected to increase the global average temperature by about 3 degrees C (plus or minus ~50%). Ignoring the logarithmic dependence of radiative forcing on the CO2 concentration and accounting for the area of the Earth of 5.1 x 10^14 square meters, the annual increase of ~2.5 ppm in the CO2 concentration traps ~17 trillion watts, which is about as much as results each year from the combustion heat of the fossil fuels. The reason that the CO2-induced increment to the Earth's energy is more important than the heat addition is that the CO2-induced effect will be repeated each year for many centuries, and to a lesser extent over many millennia, whereas the heat of combustion for one year's fossil fuel use occurs only once.

Preventing the absorption of this much solar energy each year into the future (and then also doing so for each different year's CO2 addition) is the challenge posed for Solar Radiation Management (SRM) approaches to climate engineering. Even though the reduction in solar radiation each year to balance the CO2 increase only needs to be about 0.015%, this is a huge amount of energy. Proposals to do this, not submitted to this contest, include imitating a volcanic eruption by injecting compounds into the stratosphere that lead to a greater loading of sulfate aerosols and brightening marine stratus clouds by injecting a mist of cloud condensation nuclei. While projected to be less expensive than mitigation, there are limits to the magnitude of the CO2 warming that can be offset without likely inducing significant unintended consequences. In addition to questions relating to technical implementation, there are also very likely to be complex issues of governance, ethics, and equity that would need to be addressed.

Selection and Evaluation of Contest Finalists by Expert Reviewers #

The original call within the contest expressed a particular interest in proposals that might help to reduce the atmospheric methane (CH4) concentration. Methane (or natural gas) is the second most important greenhouse gas. Even though, in terms of mass, global emissions are small compared to those of CO2 (no one wants to let a potential fuel go to waste), the effect of the excess CH4 concentration (the CH4 concentration is now ~2.5 times its preindustrial value) on the global radiation balance is roughly 30-40% of the effect of the increase in the CO2 concentration. This is the case because, on an equal mass basis, each CH4 increment has roughly 100 times the ability to trap infrared radiation as a CO2 increment as a result of its molecular structure and relatively low concentration. Despite the importance of limiting or counterbalancing the influence of the increased methane emissions, the expert reviewers concluded that there were no proposals among those submitted to the contest that offered the potential for large global effects on methane concentration that would considerably reduce the radiative forcing from this gas.

From the contest entries, the expert reviewers selected three CDR- related proposals designed to moderate the influence of the human-induced CO2 emissions. The background information above is meant to give context to the type of challenge that geoengineering poses and this applies also to the proposals that were selected to move forward as finalists in the contest. Although the scale of their ambition can make them sound like science fiction, the proposals are indeed serious. As a first step in the evaluation process, the expert reviewers offered comments and expressed reservations based on the potential challenges of scaling up the proposed approaches to an extent that would make a difference as well as on potential limits that are imposed by physics and by engineering difficulties. The proposers have responded to these comments, and the expert reviewers have considered them in preparing their final comments (these comments from the experts are also posted in the Comments tab of each proposal):

  1. Saving the Planet v2.0 (for details, see To remove CO2 from the air as well as limit the effects of acidification of the ocean on the marine biosphere, this proposal envisions accelerating the natural weathering process. As a result of natural weathering, the Earth system limited extreme ocean acidification during past periods of Earth history that had a very high CO2 concentration. This proposal envisions accelerating this process by a large factor by subjecting rocks to acid produced in an engineered electrochemical reaction. The process produces hydrogen as a by-product, which is intended as a fuel. While the chemistry is relatively straightforward, implementing such a strategy over the large global ocean area would require very large amounts of energy and materials and a very extensive engineering effort, and it is not clear that this is feasible. It appears more likely any practical application would, if the approach worked as described in the scheme, necessarily be limited to reasonably small but ecologically important areas. While this is important, this alone would not have the large effect on the atmospheric CO2 concentration needed for this proposal to become a major part of a geoengineering approach to limiting climate change.
  2. Emergency 20-year drawdown of excess CO2 via push-pull ocean pumps (for details, see reduce the atmospheric CO2 concentration and thereby reduce CO2-induced global warming, this proposal envisions pulling excess nutrients up to the surface layer from up to a kilometer depth, using sunlight to power biomass growth in order to draw down and tie up CO2 present in the upper ocean before subsequently sinking the biomass to the ocean depths. The intention is that lower upper ocean CO2 concentration would lead to ocean uptake of CO2 from the atmosphere in areas where the biomass is being grown. (Furthermore, due to the natural redistribution of excess CO2 among fast reservoirs that would result from reducing the atmospheric CO2 concentration, this would also lead to uptake of CO2 from the terrestrial biosphere and other areas of the upper ocean). There are several fundamental problems that the proposal needs to overcome: (a) bringing up nutrient-rich water from depth brings up water supersaturated in CO2 that would normally be released to the atmosphere, especially as the water warms at the surface; (b) as the biomass grows and pulls down the upper ocean CO2, the transfer of CO2 from the atmosphere to the ocean can be slow compared to what would be required to implement such an approach to draw down atmospheric CO2 (it might seem that, once started, CO2 replenishment of CO2 taken up by the increased biological activity could come from bringing CO2-rich deep water to the surface at the right rate, but the goal is to be pulling CO2 from the atmosphere); (c) it requires energy to pull up and push down the waters, and doing so on a reliable basis from wind power may prove difficult, which would disrupt the important timing of the various steps; and (d) the benefit to offset the cost is, in essence, postulated to come from reducing the rate of climate change rather than generating revenue by selling the biomass as a fuel that would reduce the pace of fossil fuel extraction as a source of energy, and this would require a very large implementation to actually limit the pace of climate change, and the effectiveness of such a large effort would furthermore be limited by difficulties such as raised in (a) and (b) above. Whether clever engineering could work out these problems was not clear to the expert reviewers as it was to the proposer.
  3. Large scale ocean based algae production system (for details, see This proposal focuses on using presently unproductive regions of the ocean to grow algae that can be harvested for generation of energy, with the intent being to reduce the reliance on fossil fuels as a way to limit climate change. Strictly speaking, this type of approach is generally considered mitigation (in particular, generating renewable energy) and not CDR because the process is not pulling CO2 out of the system with the intent to sequester the CO2, but is instead focused on limiting extraction of fossil fuel CO2 (so leaving CO2 in the ground) and thus preventing the consequent additional injection of fossil fuel CO2 into the ocean. As for the proposal with the push-pull pumps, there is the problem that arises because lifting nutrient rich waters from depth to the surface can lead to the outgassing of the super-saturated CO2, especially in the warm sunlit regions that best grow algae. The proposer attempts to deal with this by isolating the biomass growing waters in floating containers; the more complicated such the approach gets, the harder and more expensive it is to scale up the biomass production to a significant level. Finally, there is the challenge of being cost effective compared to other options: the advantages of biofuels are that they are transportable and their energy can be concentrated and stored as compared to the electricity that is generated by wind and solar power systems. On the other hand, production of biofuels at sea seems likely to be more costly even though the solar energy is free and the areas of the ocean that would be used are not contributing to provision of other ecosystem services.

Overall, each of the proposals has challenges, which is not surprising given the scale of the problem that human activities have created and that need to be remedied. Mitigation is important and feasible, and for geoengineering to be seen as a part of the solution it must be cost-effective and affordable in addition to being able to be carried through with few unintended impacts. Given the worsening situation with respect to climate change and ocean acidification and the limited action by the international community to date, however, the alternative to proposals such as these is becoming more and more unacceptable. Thus, the choice becomes the lesser of two difficult pathways, and the prospects for having to make the choice is increasing because nations have not yet stepped forward to aggressively limit emissions.