Biofuels are held up as a relatively easy solution to the twin problems of petroleum dependence and carbon emissions. But new research suggests that fuels such as corn-based ethanol may cause as many problems as they solve.

Silicate weathering reactions remove carbon dioxide from the atmosphere and store it in carbonate minerals. During the high atmospheric carbon dioxide conditions of the Early Eocene Climatic Optimum, rates of chemical weathering, physical erosion and denudation in the western USA were equivalent to the highest recorded rates in the non-glacial Quaternary.

An index that both climate scientists and policy makers anxiously keep track of is national emission rates of carbon dioxide, particularly for China, which has quickly been catching up with the US, hitherto the world's largest emitter of carbon dioxide. Despite all eff orts to the contrary, the rates for both China and the world are rapidly going up, rather than down.

Anyone clinging to the notion that we can wipe the slate clean of all our climate mistakes by deflecting the sun's rays with space mirrors is in for a disappointment. Dan Lunt of the University of Bristol, UK, and colleagues carried out the most detailed climate-modelling study to date on the impact of a sunshade.