How can scientists compare today's climate to conditions of the distant past, long before humans started happily jotting down records of the weather? The climate of the past is kept like a secret in objects all around us. Ice from glaciers and sediments beneath our feet keep a catalog of temperatures hidden in their chemical composition.

Most of these measures can tell researchers the average yearly temperature over a series of years. But clamshells may do even better than that. A study this March of clamshells taken from Iceland found that the levels of oxygen isotopes in the shell layers varied depending on the temperature of the water in which they formed. By slicing tiny layers for analysis, the team found that it could determine temperature variation down to weeks, not just years.