The important question is what is causing the current, rapid warming? We cannot dismiss it as natural variation just because the planet has been warmer at various times in the past. Many studies suggest it can only be explained by taking into account human activity. Nor does the fact that it has been warmer in the past mean that future warming is nothing to worry about. The sea level has been tens of metres higher during past warm periods, enough to submerge most major cities around the world (see box at end of this article). Les mer her