THE POLICY FORUM BY J. D. GLOVER ET AL. ("INCREASED FOOD AND ecosystem security via perennial grains," 25 June, p. 1638) highlights environmental advantages of perennial relative to annual bioenergy crop systems but omits potentially important consequences related to hydrology and climate. They categorize greater perennial leaf area index and rooting depth (relative to annual crops) as "utiliz[ing] more precipitation," but the work cited provides no evidence for increased rainfall recycling.

The direct climate impact of land-use change associated with bioenergy expansion (such as a shift from annual to perennial cropping systems) has received little attention. The impacts of changing fundamental biogeophysical surface properties associated with bioenergy crops may have significant implications for local and regional climate. Changes to local hydrology caused by large-scale perennial systems may be complex, and thus require careful evaluation. For example, the drawdown of soil water and enhanced evapotranspiration from perennial relative to annual cropping systems could lead to long-term depletion of the soil-water column, as well as changes in clouds and rainfall in downwind locales. Quantifying local and remote consequences for hydrology and climate resulting from a shift from annual to perennial bioenergy crops is therefore required if longterm sustainability of biomass production is to be attained.

Steinfeld, head of the livestock sector analysis and policy branch of FAO, joined the FSE team in June as FSE's new visiting scholar for the summer. He has been working on agricultural and livestock policy for the last 15 years, in particular focusing on environmental issues, poverty and public health protection. Prior to that, he has worked in agricultural development projects in different African countries.

While at Stanford Steinfeld is exploring technical and policy options suited to reduce the environmental impact of livestock and associated food chains, at global and regional levels. Livestock are the world's largest user of agricultural land, they play a large role in carbon, nitrogen and water cycles, and are a major determinant of biodiversity. Technical and policy options will be grouped into a small number of "scenarios" that feature different assumptions about production modes and levels of consumption.  His work aims at providing broad strategic options for policy makers to address livestock's environmental consequences, but within a context of growth and development.

We find your conclusion premature that there is no longer a direct correlation between food production in Brazil and deforestation in the Amazon (Nature 466, 554-556; 2010).

An increase in demand by international markets for export commodities such as soya beans and beef will mean more rainforest clearance. There is still potential for a huge increase in productivity, given that large producers of export goods are encouraged by government loans at favourable rates and fiscally exempt debt relief, which in turn attract investment in research and development.

Moreover, Brazil's Congress has proposed large structural changes to the Forest Code that could lead to further deforestation and threaten the preservation of the most important Brazilian biomes.

Brazil's own staple crops - rice, beans and cassava - account for very little deforestation. The small farmers producing these still suffer low credit and heavy debts, fragile land tenure, scant investment in crop research, and inferior storage conditions for their products.

A global farm should be socially fair as well as environmentally friendly. Although Brazilian agricultural policy is on the way to meeting these conditions, we are not yet there.

Global agricultural expansion cut a wide swath through tropical forests during the 1980s and 1990s. More than half a million square miles of new farmland - an area roughly the size of Alaska - was created in the developing world between 1980 and 2000, of which over 80 percent was carved out of tropical forests, according to Stanford researcher Holly Gibbs.

"This has huge implications for global warming, if we continue to expand our farmland into tropical forests at that rate," said Gibbs, a postdoctoral researcher in the Department of Environmental Earth System Science and in the Program on Food Security and the Environment, who led the study.

Gibbs and colleagues at several other universities analyzed Landsat satellite data and images from the United Nations to reach their conclusions. Theirs is the first study to map and quantify what types of land have been replaced by the immense area of new farmland developed across the tropical forest belt during the 1980s and 1990s.

While this huge increase was happening within the tropics, agricultural land in the non-tropical countries actually decreased in area.

The study was published this week in the online Early Edition of the Proceedings of the National Academy of Sciences.

The United Nations Food and Agriculture Organization estimates that to keep pace with increasing demand, global agricultural production will have to keep increasing, possibly even doubling by 2050. That would likely lead to millions of additional acres of tropical forest being felled over the next 40 years.

Direct impact on carbon released into atmosphere

"Every million acres of forest that is cut releases the same amount of carbon into the atmosphere as 40 million cars do in a year," Gibbs said.

Most of the carbon released comes from burning the forests, but even if the trees are simply cast aside, the bulk of the carbon from the plants makes its way into the atmosphere during decomposition, she said.

Gibbs and her colleagues found that about 55 percent of the tropical forests that had been cut between 1980 and 2000 were intact forests and another 28 percent were forests that had experienced some degradation, such as some small-scale farming, logging or gathering of wood and brush for cooking or heating fuel.

"The tropical forests store more than 340 billion tons of carbon, which is 40 times the total current worldwide annual fossil fuel emissions," Gibbs said. "If we continue cutting down these forests, there is a huge potential to further contribute to climate change."

The increasing demand for agricultural production stems in part from the ever-growing number of people on the planet, who all want to eat. Additionally, members of the growing middle class in emerging economies such as China and India are showing interest in eating more meat, which further intensifies demand. And incentives to grow crops for biofuel production have increased.

But Gibbs and her colleagues also observed some encouraging signs. The patterns of change in the locations they analyzed made it clear that during the 1990s, less of the deforestation was done by small family farms than was the case in the 1980s and more was done by large, corporate-run farms. Big agribusiness tends to be more responsive to global economic signals as well as pressure campaigns from advocacy organizations and consumer groups than individual small farmers.

In Brazil, where a pattern had developed of expanding soy production by direct forest clearing and by pushing cattle ranching off pastureland and into forested areas, a campaign by Greenpeace and others resulted in agreements by key companies to rein in their expansion. Instead, they worked to increase production on land already in agricultural use.

'Seeing positive changes'

"These farmers effectively increased the yield of soy on existing lands and they have also increased the head of cattle per acre by a factor of five or six," Gibbs said. "It is exciting that we are starting to see how responsive industry can be to consumer demands. We really are seeing positive changes in this area."

Along with wiser use of land already cleared, Gibbs said, improvements in technology and advances in yield intensification also could slow the expansion of farming into the forests.

Other studies that analyzed land use changes between 2000 and 2007 have shown that the pace of cutting down the tropical forests has begun to slow in some regions.

But as long as the human population on the planet continues to grow, the pressure to put food on the table, feed in the barnyard and fuel in the gas tank will continue to grow, too.

"It is critical that we focus our efforts on reducing rates of deforestation while at the same time restoring degraded lands and improving land management across the tropics," Gibbs said. "The good news is that pressure from consumer groups and nongovernmental organizations combined with international climate agreements could provide a real opportunity to shift the tide in favor of forest conservation rather than farmland expansion."

In addition to her position at the Department of Environmental Earth System Science and the Program on Food Security and the Environment, Gibbs is affiliated with Stanford's Woods Institute for the Environment. Jon Foley, a professor of ecology, evolution and behavior, and director of the Institute on the Environment at the University of Minnesota, was Gibbs' PhD adviser when the research was begun. He is a coauthor of the paper.

Initial funding for the project was provided by NASA. Gibbs is currently funded by a David H. Smith Conservation Research Fellowship.

A new issue brief by Scott Rozelle and fellow researchers Jinxia Wang and Jikun Huang concludes that climate change will have a significant effect on China's crop yields and impact its economy, including the grain trade. It concludes that China's government is responsible for responding in ways that will help the country adapt to and mitigate the effects of climate change. The issue brief was jointly published by the International Centre for Trade and Sustainable Development and the International Food and Agricultural Trade Policy Council.

The heat wave in Russia has captured international media attention, breaking temperature records left and right. It has also captured the attention of commodity traders. You see, in a typical year Russia produces about as much wheat as the United States, and is among the top exporters of wheat flour in the world. But this year, wheat has been decimated in the areas around Moscow, with yield expected to be 30 percent or so below normal. This week Russia announced they are banning all exports of wheat from August 15 through the end of the year. Since late June, wheat prices on the Chicago Board of Trade have risen by 50 percent, to more than $7 a bushel.

It is, and always will be, impossible to say whether a single event is caused by climate change. But we can ask, is this the type of thing we expect to be more common? In terms of warming, we can say with little doubt that heat waves like this will become more common with global warming. Exactly how much more common is tough to say, but it is likely that the average summer in 2050 will be as warm as the warmest summer in the 20th century. I am not aware of anyone who has done the calculation of exactly how common the type of heat experienced this year will be, but based on projections in the UN Intergovernmental Panel on Climate Change (IPCC) reports one can suspect this type of heat wave will be relatively common in Russia in a few decades.