Human activities currently add about nine gigatons of carbon to the atmosphere yearly. Photosynthetic organisms on land and in the ocean absorb about five of those gigatons through the natural uptake of CO2, leaving to humans the task of dealing with the rest. But no matter how much carbon there is, capturing it and preventing it from reentering the atmosphere is an immense engineering challenge; even today's best technology is orders of magnitude less effective than photosynthesis at trapping atmospheric carbon.

A new analysis published in the October issue of Bioscience suggests that by 2050 humans could offset between five and eight gigatons of the carbon emitted annually by growing plants and trees optimized via genetic engineering both for fuel production and carbon sequestration.

Bioenergy crops represent an opportunity to mitigate atmospheric carbon dioxide in two separate ways, says lead author Christer Jansson, a senior staff scientist at Lawrence Berkeley National Laboratory's Earth Sciences Division. First, they are a carbon-neutral energy source that could offset the burning of fossil fuels. Second, "if they are the right kind of plants, they have a chance to transfer a lot of carbon underground for long-term sequestration," he says.

Plants take up CO2 and store carbon in their biomasses. Carbon can stay for decades or centuries in leaves, stems, branches, seeds and flowers aboveground, whereas carbon allocated to underground root systems is more apt to be transferred into the soil, where it can stay sequestered for millennia. Therefore, an ideal bioenergy plant would produce lots of aboveground biomass for fuel as well as have an extensive root system. Preliminary research indicates that genetic engineering approaches could be employed to enhance both these traits.

Using genetic modification to enhance photosynthesis and thus biomass yield is a realistic approach, says Stephen P. Long, a professor of crop sciences at the University of Illinois at Urbana–Champaign who was not part of the study. Long notes that transgenic tobacco plants, with simple modifications applicable to other plants as well, have already been shown to be more productive. "We are in a position now where we certainly know enough to where we could engineer quite a few of these changes," he says.

Meanwhile, regarding the problem of coaxing plants to allocate more carbon to their root systems, Jansson says an important difference between perennial and annual plants is a good place to start. "Perennials are more efficient than annuals at hiding carbon underground," he says. That's because annuals, which make up most of the world's food crops, spend much more energy producing seeds, stems and leaves than for building their root systems. On the other hand, perennials like switchgrass and Miscanthus have more extensive root systems—necessary because they remain dormant for part of the year and then must grow up again from their roots.

Whereas it may be exciting to imagine a bioenergy or food crop that produces lots of aboveground biomass and has large, carbon-sequestering root systems, research into whether this goal is realistic is still in its early stages. "Perenniality is a complex trait," Jansson says. He suggests it may end up being easier to modify perennials so they possess desirable annual-like features, as opposed to the other way around—but it's too early to tell.

For the short term Jansson is confident that science can modify plants so they are more drought resistant and salt tolerant. Crops that could be maintained with brine or brackish water, such as industrial wastewater or seawater, would help preserve freshwater supplies. "These are important traits that need to be introduced into food and bioenergy crops," Jansson says, adding that "we will see this sooner" than enhanced photosynthesis or perennials with annual traits and/or vice versa.

The authors stress that genetic engineering should not be viewed as a cure-all, but rather part of a larger breeding effort. Further, Jansson says, "One problem is that the different aspects we mention—increasing photosynthesis, improving bioenergy crop yield, and putting more carbon into the root systems—are highly interlinked, and thus not necessarily additive." It could be, for example, that a modifying a plant to grow more roots takes away aboveground biomass production. Again, research in this area is too preliminary to tell.

Allison Thomson, who studies climate change and land use at the Joint Global Change Research Institute in College Park, Md., also expressed the need for caution when interpreting the study's projections. They are valuable in principle, she says, but also based on many assumptions regarding future economic conditions, land availability, and the size of bioenergy's role in a larger future energy strategy. For example, she says, "you can't really say how much bioenergy we are going use if you're not also considering other available energy sources and how much they emit." Furthermore, she points out, whether or not there is a price for carbon, which is hard to account for at this point, will figure heavily into future energy scenarios.

Also important to consider are potential land-use issues related to increasing demand for food. "When we do modeling, that's the one demand you can't ignore," Thomson says. "People want to eat before they want bioenergy."

Besides all the unknowns, there is also existing regulatory policy regarding genetically modified organisms, which imposes high costs of compliance, thereby making it difficult to assess whether the ideas discussed in the paper are all doable, Long says: "The bottleneck and damper on all this is really, 'How do you get transgenics out there, and meet all the regulatory requirements and costs?'"