Climate change looms as this century’s primary long-term environmental challenge. Human actions – burning fossil fuels – have already raised the carbon dioxide concentration higher than it has ever been in the last 500,000 years, and it is rising by about 0.5 % a year.
即便核威胁被遏制住，21世纪还是可能出现严重的全球性新危机。气候变化已隐隐成为本世纪主要的长期环境问题。人类的行为——燃烧化石燃料——已经将二氧化碳浓度提高到50万年来最高的水平，而且还在以每年约0.5%的速度上升。