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?'"