India, Dec. 13 -- 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 evide...