A leading environmentalist explains why drastically reducing carbon dioxide emis: The awareness that humans could alter the climate of Earth has dawned slowly on our consciousness. In 1896, Swedish chemist Svante Arrhenius deflected his anguish over a failed marriage into remarkably tedious and, as it turned out, accurate calculations about the effect of CO2 emissions on climate. It was an oddly therapeutic thing to do, but it had no more effect on public attention than the smallest cloud on a distant horizon. Another 69 years would pass before scientists warned a …