Reconstructing the past
How do we know what the Earth's climate was like in the past?
Over the past few decades, sophisticated instruments have gathered huge amounts of data about the Earth’s climate. For earlier periods data are sketchier, but some long time-series exist – created, for example, by Victorian naturalists and others who recorded temperatures, the arrival of migrants and plant flowering times.
Before the 1800s things get trickier, as few reliable measurements were recorded. Instead, scientists may infer what was going on from other sources, such as incidental details in written historical records – one recent study used diaries kept by people in the Netherlands and Belgium that recorded years when canals froze.
Further back, tree rings can be dated; the size of a ring is an indication of a tree’s growth for a particular year, which depends on climate.
Perhaps the most widely used method for exploring distant history is analysis of ice cores – columns of ice extracted from Greenland and Antarctic ice sheets. The ice at particular depths can be dated precisely and analysed chemically to provide clues about the climate prevailing at that time. The deepest ice cores go down more than 3 km, equivalent to around 800,000 years of Earth history.
Remarkably, it is even possible to infer what the ancient climate was like from analysis of cave stalagmites. For older periods still, chemical analysis of rocks can provide clues about the climate millions of years ago.Lead image:
Illustration © Glen McBeth