Controversial climate claim exonerates carbon dioxide. The impact of cosmic rays on our climate might outweigh that of the greenhouse gas carbon dioxide, a controversial new report suggests1. "It's no excuse to ignore sensible resource use," says one of the report's authors, physicist Nir Shaviv of the Hebrew University of Jerusalem in Israel. "But the bottom line is that carbon dioxide is not the bad boy that people claim it is." The suggestion has met with scepticism, however: "I don't buy it," says climate-change expert Wallace Broecker of Columbia University in New York. Shaviv and climatologist Ján Veizer of...