The idea of injecting sulfates into the atmosphere is based on the observation that large volcanic eruptions can cause short-term global cooling. But in addition to the usual problems with geo-engineering (for example, it does nothing to stop ocean acidification from excess CO₂), scientists have found a new one. Sulfate geo-engineering could endanger food and water supplies for billions of people in Africa and Asia, according to a recent paper in the Journal of Geophysical Research [PDF].

A Study with Some Scary Findings

Continuous sulfate injections into the stratosphere might cool the Earth, but does it matter where you do it? And what else might it do? To address these questions, the researchers ran a global climate model under four scenarios:

1. Business-as-usual with no geo-engineering
2. Low-level sulfate injections in the Arctic
3. Medium-level sulfate injections in the tropics
4. High-level sulfate injections in the tropics

In all the geo-engineering scenarios, sulfate was injected continuously for 20 years, and then abruptly turned off. The medium-level is roughly equivalent to a Pinatubo eruption every four years, the high-level every two years.

Compared to business-as-usual, the geo-engineering strategies slowed or even reversed global warming and the loss of Arctic summer sea ice. So far so good. But scientists found serious drawbacks when they looked at (1) regional effects and (2) the consequences of suddenly ending sulfate injections.

One of the most prominent dangers was the effect on summer monsoons in Africa and Asia, on which billions of people rely for food and water supplies. Both monsoon systems decreased markedly when sulfate was injected into the atmosphere – regardless of how much or where.

And what about when sulfate injections stopped, as might happen if a real-world geo-engineering strategy encountered technical difficulties or lost political support? The resulting warming rebound and sea ice loss was "more rapid … than has occurred in the past century or than is projected with business as usual." Since the rate of warming can be as damaging as temperature alone, warming rebound could have tremendous environmental and social consequences.

Best to Tackle the Root of the Problem

This paper adds to the growing list of geo-engineering risks (for example, see 20 reasons why geoengineering may be a bad idea [PDF]). As RealClimate scientists put it in their coverage of the latest paper, geo-engineering is like methadone: "an emergency treatment to substitute one addiction (carbon emissions) with another." What we really need to do is tackle the root of the problem and decrease greenhouse gas emissions.

This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense Fund.

Our own Scott Anderson is one of the experts featured in a new video on Carbon Capture and Sequestration (CCS). The video explains why CCS is an important tool in cutting emissions, and gives an animated description of how it works. It's a good companion to Scott's blog post on CCS.

1. Biological Sequestration
2. Geological Sequestration
3. Ocean Sequestration

To stop global warming, the U.S. must substantially move away from carbon-emitting fossil fuels to clean renewable energy. But a transition of this magnitude takes time. Right now this country is heavily dependent on coal for electricity, and traditional coal plants are none too clean.

How do we stop global warming while renewable technologies to meet our energy needs are still under development? Part of the answer may lie in an emerging transition technology called Carbon dioxide (CO₂) Capture and Storage (CCS). The idea behind CCS is to capture the CO₂ from industrial processes like coal plants, and then store it in deep geological formations.

CCS can dramatically reduce carbon emissions from coal plants. The IPCC's Special Report on Carbon Dioxide Capture and Storage says that "a power plant with CCS could reduce CO₂ emissions to the atmosphere by approximately 80-90% compared to a plant without CCS." This is after taking into account the extra energy needed to capture and compress the CO₂.

There is plenty of room for geologic storage. According to the Pew Center on Global Climate Change:

• The United States has the geological capacity to store the emissions from its coal-fueled plants in depleted oil and gas reservoirs for several decades.
• Capacity in other geological reservoirs is estimated to be in the hundreds of billions of tons (500 billion tons of capacity), enough to store current levels of domestic emissions for over 300 years.

Worldwide, according to the IPCC, there is a potential capacity in geological formations to store at least 545 gigatons.

How Geo-Sequestration Works

In geological sequestration, compressed CO₂ is injected into porous rock formations. It's very much like pouring water into a glass of marbles, where the water represents liquefied CO₂ and the space between the marbles represents available pore space in the rock found in deep saline formations or depleted oil and gas reservoirs.

The technology for injecting CO₂ into depleted oil and gas reservoirs already exists because of a process called Enhanced Oil Recovery (EOR), which has been in use for over 30 years. With EOR, oil not recovered in the initial withdrawal process is pushed to the surface by injecting CO₂.

Safety and Regulation

But the CO₂ in EOR isn't necessarily stored for the long-term – it's just injected. For geologic sequestration, the carbon ought to be stored for 1000 years or more without leaking back into the atmosphere. Not all depleted gas and oil reservoirs can meet this requirement.

Suitable sites, for example, must lie below and isolated from fresh water supplies, and ideally should have one or more layers of sealing caprock to prevent the CO₂ from seeping to the surface. The requirements are still being explored in dozens of ongoing projects.

There is also ongoing work on developing regulations to select and monitor suitable sites. On January 31, I testified before the Senate Committee on Energy and Natural Resources on this topic (see full testimony [PDF]). Although CCS is ready to begin deployment today, widespread adoption won't take place until the regulatory and commercial framework has been fully implemented, and this could take quite a few years. We have called upon policymakers to develop a sound regulatory framework as rapidly as possible.

A Critical Transition Technology

Some argue that CCS diverts attention from the development of clean energy resources, but in fact CCS is a critical transition technology. Coal plants and other industries will continue to emit CO₂ while alternative energy sources are developed and adopted. Earlier this year, the Commission of the European Communities issued a directive [PDF] emphasizing the importance of CCS in halting climate change:

Energy efficiency and renewables are in the long term the most sustainable solutions both for security of supply and climate. However, we cannot reduce EU or world CO2 emissions by 50% in 2050 if we do not also use the possibility to capture CO2 from industrial installations and store it in geological formations (carbon dioxide capture and storage, or CCS).

We're not champions of coal at EDF, but we are realists. Coal will be used to produce electricity for the foreseeable future, so we need technologies that allow coal to be used in a manner that avoids significant greenhouse gas emissions.

1. Biological Sequestration
2. Geological Sequestration
3. Ocean Sequestration

Global warming is occurring because – day after day, hour after hour – human activities pump large amounts of greenhouse gases into the atmosphere. One way to decrease emissions is to store carbon or CO₂ someplace other than the atmosphere.

There are two vastly different ways of sequestering carbon: biological and geological. The topic of this post is biological sequestration, which is among the biggest of the "low hanging fruits" for making quick, substantial cuts in emissions.

Our lands and forests have huge potential for storing carbon – they are nature's "carbon sinks". Green plants take CO₂ out of the atmosphere and convert it into organic carbon as they grow – a process called photosynthesis. Organic carbon is converted back to CO₂ when it is eaten or decomposed – a process called respiration.

Farmers and foresters can do many things to increase photosynthesis and/or decrease respiration. For example, they can replant forests or delay timber harvests to increase photosynthesis. Tilling increases the respiration of microbes in the soil by improving the conditions for decomposition, so no-till farming reduces respiration.

Are trees the only plants that can store carbon? No, but trees store the most carbon because they're large and long-lived. As long as the wood doesn't decompose or burn, it stores carbon away from the atmosphere. Still, restoring tilled fields to grasslands can help. Since the grass isn't harvested and the land isn't tilled, more of its organic carbon remains in the soil.

How do we get farmers and foresters to use better land management practices? The answer is a carbon market in which farmers and foresters can participate. In an inclusive carbon market, those who emit carbon pollution pay for credits, whereas farmers and foresters sell them through agricultural offsets.

Agricultural offsets require a robust monitoring and verification process – a reliable way to measure the carbon stored in soils and wood. This is challenging but doable, as Bill Chameides and his colleagues demonstrated in their technical manual on the topic. (Duke University, which published the book, offers excerpts online [PDF].)

Globally, soil carbon sequestration alone could offset as much as 15 percent of fossil fuel emissions. In addition, thoughtful offset projects can have side benefits such as improved soil quality, increased crop yields, and better wildlife habitat. And all that's keeping us from these benefits is a lack of economic incentives – a carbon market where farmers can sell their carbon credits.

In 2004, the USDA published an economic analysis of biological carbon sequestration in the U.S. agricultural sector. The report evaluated how landowners would respond if they could be paid to sequester carbon.

Not surprisingly, the results showed that the higher the price for carbon, the more farmers would do to enhance carbon sinks. At the highest price considered in the analysis, U.S. farmers and landowners would implement practices that could sequester as much as 160 million additional tons of carbon in forests and agricultural soils every year. That's equivalent to nearly 10 percent of America's CO₂ emissions in 2005! Even at lower carbon prices, agriculture can play a very important role in our fight against global warming.

When people talk about reducing emissions, they often focus on high-tech solutions. But let's not forget the low-tech strategies that can bring immediate results, like improved agricultural practices and increased energy efficiency.

There's always something new in climate change research. This week, scientists described the risks of geo-engineering, proposed an efficient way to reduce Arctic climate change, and discussed options for decreasing deforestation in developing countries.

• Matthews, HD & K Caldeira (2007) Transient climate-carbon simulations of planetary geoengineering. Proceedings of the National Academy of Sciences, 10.1073/pnas.0700419104

• Using geo-engineering to reduce the amount of sunlight reaching the Earth's surface is extremely dangerous. A large, rapid "warming rebound" could result if the technology fails or is stopped, especially if there are no parallel efforts to reduce greenhouse gas emissions. (For more on geo-engineering, see this earlier post.)

• Reddy, MS & O Boucher (2007) Climate impact of black carbon emitted from energy consumption in the world's regions. Geophysical Research Letters 34: L11802.

• Most of the black carbon (soot) that is causing rapid warming in the Arctic comes from Europe, so reducing black carbon emissions in Europe would be a fast and efficient way to slow Arctic climate change.

• B Schlamadinger & DN Bird, editors (June 2004) Special Issue: Options for including agriculture and forestry activities in a post-2012 international climate agreement. Environmental Science and Policy 10 (4): 269-394. (Paid subscription required)

• This special issue reports the results of a workshop in which participants discussed policy options for decreasing greenhouse gas emissions from deforestation in developing countries. More information is available on this website.

So here's an idea for fighting global warming. Instead of trying to reduce greenhouse gas pollution – the root cause of the problem – why not use technology to counteract the effect of the pollution? For example, we could artificially add to the planet's reflectivity so that the warming is cancelled by the cooling.

Using planetary-scale engineering to counteract climate change is called "geo-engineering". A number of geo-engineering ideas have been put forward recently. You can find many of them in this special issue of Climatic Change. One idea would place large, highly-reflective rafts on the ocean. Another would position mirrors in orbit around the Earth.

Yet another idea is based on the phenomenon of global dimming – cool the earth by adding reflective particles to the atmosphere. Deliberately polluting the lower atmosphere, where we live and breathe, isn't such a good idea because of the nasty health consequences. But there is another way: add reflective particles to the upper atmosphere.

The idea is to fly airplanes into the stratosphere, 10 miles or more above the Earth’s surface, and release sulfur dioxide. The sulfur dioxide will be converted into tiny sulfate particles and, presto chango, you're cooling the planet.

Doing this in the stratosphere has two advantages: (1) The particles would be in the upper atmosphere and not in the air we breathe, and (2) While particles in the lower atmosphere remain suspended only for a couple of weeks, particles in the stratosphere stay there for years, so much less sulfur dioxide is needed.

This idea may seem far-fetched, but it has technical merit. One of its staunchest proponents is Paul Crutzen, winner of the Nobel Prize for Chemistry for his work on stratospheric ozone depletion (and also, incidentally, a colleague of mine). But although it has technical merit, and it's great that scientists like Paul are investigating every possibility, I think it's premature to contemplate such drastic measures. The technologies needed to reduce greenhouse gas pollution are already in hand (see my post on Green Technologies), and geo-engineering is a risky proposition.

For one thing, adding particles to the stratosphere may exacerbate stratospheric ozone depletion. Plus we just don’t know what may happen when we start tinkering with the planet. Ed Tenner has a great book on this subject titled Why Things Bite Back. One of the many examples he gives is refrigeration technology. We wanted to use something other than toxic ammonia for cooling so we turned to chlorofluorocarbons, but these turned out to have a dangerous effect on the ozone layer.

The most important reason, however, is this. Even if we could completely cancel the warming effects of greenhouse gas pollution, we would still have another profoundly dangerous consequence to contend with: ocean acidification. And that is the topic of my next post, so be sure to tune in.

The five-year challenge was announced February 9 by British entrepreneur Richard Branson, with Al Gore. The judges, among them several eminent scientists, will meet annually to evaluate submissions. At the end of five years, if no one has won the prize, they'll decide whether to extend the challenge another five years.

Sound impossible? Well, in fact, it is possible. It turns out that CO₂ is absorbed into any alkaline (or non-acidic) material. The problem is making the absorption fast and efficient enough that it's commercially viable.

So, wanna save the world and win $25 million in a single bound? Check out the Virgin Earth Challenge.