evidence: They reveal there would have been little global warming - and possibly some cooling - over the 20th and 21st Centuries, if only natural factors had been influencing the climate. [...] There have been several hot periods during the Earth's past. [...] That should not comfort anyone, however, because humans were not around. At times in the past, sea level was 25m (80ft) higher than the present. A rise of 5-8m (16-26ft) is considered enough to submerge most of the world's coastal cities.