Each of these sea-shells is in fact really, really tiny, because each was once home to a tiny single-celled marine life form called a foram. Though they don’t have eyes or brains or limbs, forams somehow manage to pull ingredients from seawater and stack them together into houses made out of the mineral calcium carbonate.
Even cooler, each little house ends up with a number written into it that tells us how much ice and snow there is on Earth. That’s right a single-celled sea creature knows how much-combined snow and ice there is on all the mountaintops and ice sheets and glaciers on our entire planet! Crazy, but it is true.
How it Happens
The number in the forams’ shells comes from the two types of oxygen they pull from seawater, the regular kind, with eight protons and eight neutrons, and the heavy kind, with eight protons and ten neutrons.
H2O molecules with regular oxygen are slightly lighter and less sluggish than H2O’s with heavy oxygen, so they’re more likely to evaporate from the ocean’s surface and go gallivanting around in clouds and since all the ice in the ice sheets comes directly from clouds, the ice sheets act as a kind of storage facility for regular oxygen.
The colder the global climate, the more regular oxygen the ice sheets store and the less is left behind in the oceans. As they build their shells, forams effectively capture the ratio of regular oxygen to heavy oxygen in the seawater, and we can read that ratio back to figure out how much ice there is at the poles, and what the average global temperature is. Of course, we can also just measure those things directly, but our thermometers and satellites can only tell us what’s going on right now.
Forams, on the other hand, have been recording this data for hundreds of millions of years, and their archives have been slowly piling up on the seafloor. So by drilling down and pulling up a big long core of ancient sediment, we can recover an almost continuous record of how Earth’s temperature has gone up and down and up and down and up and down etc over time. In fact, we owe most of what we know about our planet’s past climate to these tiny brainless seafarers and their tiny, beautiful homes.