And another thing...

We know about fossils and rocks, ice cores and seabeds... but it turns out the Earth has been storing its memories in a range of other places too.

- Gowri S

Some are completely unexpected - even the air, it turns out, has memory. Some are intricate, and only now being discovered (these include moisture data in tree rings and temperature logs in deep-sea cores).There are ancient secrets about our planet hidden where one would least expect: in tree rings, deep in ocean beds, in the shells of crustaceans.

Together, these form a vast archive of Earth's "climate memories". "These 'climate proxies' can be studied today, to learn about things like past temperature or composition of atmosphere, details of which are otherwise lost to us," says Paul Pearson, a palaeoclimatologist, geoscientist and professorial research associate at University College London.

So what do they look like, and how do they work?

Such proxies generally fall into three categories: physical, chemical and biological.

Physical proxies include tree rings, ice cores and coral reefs, which preserve chemical evidence of past climates, and are helping researchers reconstruct timelines of temperature, precipitation and atmospheric conditions from millions of years ago. Chemical climate proxies include compounds found in shells and sediments, that serve the same purpose. Along similar lines, biological proxies such as pollen, spores and traces of algae found in lake or ocean sediment can offer clues to ancient ocean temperature, salinity and composition.

Tree rings in the southern hemisphere, for instance, have helped scientists learn more about climate transitions roughly 11,000 years ago, when a severe cold snap was followed by extended spells of warm and wet weather, marking the end of the last Ice Age.

A powerful proxy for long-term changes in humidity, tree rings have also helped track droughts through the late 1200s, and explain why ancient settlements such as the Mesa Verde cliff dwellings in modern-day Colorado were abandoned.