Mitigating global climate change

The Energy Challenge 

Finding the energy to power human civilization without ruining the environment is the greatest challenge facing humanity this century. World energy demand is projected to double by mid-century and triple by the end of the century. The overwhelming majority of this power is provided by the combustion of the fossil fuels coal, oil, and natural gas. Oil and gas production are expected to peak before the middle of the century and it's not clear how much of the shortfall can be replaced by renewables instead of coal. There is plenty of coal, but among its drawbacks is the emission of large amounts of carbon dioxide (CO2), a greenhouse gas, per unit of energy recovered from coal. There is overwhelming scientific consensus1 that global climate change is occurring and that human CO2 emissions are highly likely to be a major contributor.

More alarming than the relatively small amount of climate change we've seen so far is the potential for drastic climate change in the future, perhaps not in our lifetimes but in our childrens' or grandchildrens'. If humanity stopped emitting CO2 today, global warming would continue due to the excess CO2 that's already in the atmosphere and won't come out for hundreds of years. And of course we can't stop emitting any time soon: we must find the energy to power human civilization while developing "clean" energy sources as aggressively as possible. But how aggressively is that? The inertia in the world's energy infrastructure is vastly underappreciated. The world economy is chugging along, full steam ahead, with little or no regard for the consequences because (among other things) the consequences aren't known for sure, and many people don't feel it's wise to incur certain costs now to reduce the risk of uncertain consequences later. Coal-fired power plants built today will still be discharging CO2 into the air 50 years from now. It will take many decades to deflect humanity's energy delivery system from its current trajectory, even after we start trying really hard. How much CO2 can we put into the atmosphere before inducing sufficient climate change to depress human civilization? Scientists make educated guesses1 but nobody knows for sure. As the chemist Nathan Lewis has stated so eloquently, each year that atmospheric CO2 levels set new records, humanity is undertaking the greatest experiment in the history of human civilization.

The House Process

We are working on an approach to capturing carbon dioxide from the atmosphere and storing it permanently in the ocean in the innocuous form of dissolved bicarbonate, while partially counteracting the ongoing acidification of the ocean from increased atmospheric carbon dioxide concentrations. The approach employs chemical and electrochemical processes whose net effect is a vast acceleration the earth's natural chemical weathering process.

In natural chemical weathering of silicate rock, atmospheric carbon dioxide dissolves into rain drops and fresh water forming a weak carbonic acid. The carbonic acid is neutralized as rain water percolates through continental rocks, producing an alkaline solution of carbonate salts. The dissolution products eventually flow into the ocean, where the added alkalinity enables the ocean to hold the dissolved carbon instead of releasing it back into the atmosphere. (In contrast, when carbonic acid rain falls directly into the ocean, most of the carbon out-gases back into the atmosphere, and the rest stays dissolved as carbonic acid, causing ocean acidification.) The more continental rock is dissolved by weathering, the more carbon is transferred permanently from the atmosphere to the ocean and ultimately to the ocean sediments. The ocean's pH balance has been set by a natural competition between the rate of acid neutralization by weathering and the rate of CO2 emission from volcanoes. This competition determines the alkalinity of the ocean and the amount of CO2 in the atmosphere and in the ocean. Currently, humans are overwhelming this process by emitting CO2 at nearly 100 times the natural volcanic rate.

A way to bring the rates back toward balancing was invented by Kurt House, a Ph.D. candidate in Harvard's Earth and Planetary Sciences Department, working with faculty members Michael Aziz, Daniel Schrag (Earth and Planetary Sciences Department and Harvard School of Engineering and Applied Sciences and Director of the Harvard University Center for the Environment), and Christopher House (Department of Geosciences, Penn. State University). This approach is reported on November 7, 2007 in Environmental Science and Technology. It is possible to speed up the planet's rate of chemical weathering by electrochemically removing a stronger acid from the ocean (hydrochloric acid) and using it, instead of carbonic acid, to weather silicate rocks. To minimize the potential for adverse side effects on the environment it is combined with other chemical and electrochemical processes, the net result of which is identical to the natural silicate weathering process. The result is an increase in ocean alkalinity, enabling the ocean to store more atmospheric CO2 as bicarbonate, which is the most plentiful and innocuous form of carbon already dissolved in the ocean. There is so much bicarbonate in the ocean that, in principle, taking half the CO2 out of the atmosphere and converting it to bicarbonate in the ocean would raise the bicarbonate concentration by only about 1%. 

Unlike climate engineering schemes to reflect more sunlight back into space to cool the planet, this process counteracts the continued ocean acidification that threatens coral reefs and their rich biological communities; indeed, the careful, dispersed, monitored and regulated application of the process in vulnerable regions may become necessary to preserve the reef ecosystems. Unlike schemes for the capture and storage of CO2 from fossil fuel power plants, this process works equally well on all sources of CO2— including the 2/3 of human emissions that do not emanate from power plants. Additionally, it would not be necessary to find and characterize the leakage rates of hundreds of underground geologic storage sites. Another advantage of the process is that it could be run in remote locations and powered by stranded energy, such as geothermal and flared natural gas, that is too remote to be harnessed to serve population centers. 

Implementation of the approach we are studying would be ambitious and costly, and would carry environmental risks that require further study. Replicating natural weathering would involve building dozens of facilities akin to large chlorine gas industrial plants on coasts of volcanic rock around the world. They might look like the plant in Fig. 1. As we report in the paper, if everything works out favorably, an ambitious effort might take care of ~15% of the carbon problem (one "stabilization wedge"2), but it is very difficult to see how it could possibly solve the entire problem - so don't get the idea that you can relax your efforts at conservation and clean energy R&D. 

The least risky trajectory is a significant cut in our carbon dioxide emissions, but quite possibility we won't be able to cut them rapidly enough to avoid unacceptable levels of climate change. If it looks like we're not going to make it, the House Process has the potential to let us rescind a portion of those emissions and get us back on track, while mitigating some of the chemical impacts that the excess CO2will have on the oceans. It won't be ready in time, though, if we wait until we're sure we'll need it before pursuing R&D on the technical and environmental issues involved as discussed below.

We have some very crude cost estimates (too crude to be publishable) but we have not done a rigorous cost analysis of the process. "The cost is likely to turn out to be in the affordable range, but it may or may not turn out to be cheap enough to be implemented", says Aziz. Out of a spectrum of measures to cut or offset carbon dioxide emissions, an economically optimized world committed to controlling the greenhouse effect would fully implement measures starting with the cheapest (e.g., turning off the lights in the rooms of your house as the last person leaves the room, which immediately saves money, but even if everyone in the world did this all the time it wouldn't be enough to solve the carbon problem) and continuing through the less-cheap (e.g. investing in a new high-efficiency furnace, which costs real money initially but saves money eventually because it pays for itself after a certain number of years) to the always-costly (e.g. capturing CO2 from fossil fuel power plant exhaust streams or replacing gasoline with bio-diesel fuel) until enough carbon emissions are saved that the global problem is solved. R&D is needed to see where on that "cost curve" the House Process would fall. At least two complications in the argument above are immediately apparent. Some of the economically cheapest solutions may not be implementable for political or other reasons. And the risk of unintended negative consequences to the environment (or, for example, to the price of corn tortillas due to the redirection of corn to ethanol production in the US) is another "figure of merit" independent of cost. It's not clear how to amalgamate all these "figures of merit" together to come up with an "optimal" policy. 

Where do we go from here?

If a technology based on this process is to be ready when and if needed, substantial laboratory and field research is needed to better understand its effect on biogeochemical cycles and other unintended consequences; and to improve efficiency and scalability of some of the chemical and electrochemical processes, including:
• Alternative hydrogen-chlorine fuel cell concepts;
• Novel polymer electrolyte and crystalline ion conductors for the hydrogen-chlorine fuel cell;
• One-electrochemical-step processes from salt water or sea water to acid and base;
• Inexpensive, stable catalysts for chlorine reduction in the hydrogen-chlorine fuel cell and for water splitting in the one-step processes;
• Treatment of impurities in sea water;
• Seepage flow of hydrochloric acid through natural rock formations;
• Options for chlorine end use at the gigatonne / year level;
• Evaluation of figure of merit for (moles of carbon removed per mole of NaOH produced) vs. time;
• Methods and kinetics for absorbing CO2 directly from the air to form bicarbonate
• Calcite saturation kinetics in perturbed ocean chemistries and biologies;

A parting thought
Here's a quote from a famous ex-politician. Can you guess who? "If you say 'yes if…' rather than 'no because…', it is amazing how much more you'll get done"3

Selected Publications

184. K.Z. House, C.H. House, D.P. Schrag, and M.J. Aziz, "Electrochemical Acceleration of Chemical Weathering as an Energetically Feasible Approach to Mitigating Anthropogenic Climate Change", Environmental Science and Technology (7 Nov. 2007).

Press Release and Coverage

Harvard's press release
Environmental Science & Technology's press release
Coverage in Science Daily: See also related stories about ocean acidification

Literature References
1See web site of the Intergovernmental Panel on Climate Change
2Pacala, S.; Socolow, R. "Stabilization Wedges: Solving the Climate Problem for the next 50 Years with Current Technologies," Science 305, 968 (2004).
3Surprise! This quote is not from Al Gore. It's from Newt Gingrich, at the Energy Future Coalition 25 x 25 Event, 3/8/06

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