How do you measure past climates, anyway?

It can be tricky to grasp the mechanics of paleoclimate research. In part, that’s because it usually involves taking something we can measure, and using it as a proxy for something we can’t. For example, maybe we’d like to know what temperatures were typical in the Midwest a million years ago, or how the Greenland Ice Sheet has changed in the last ten thousand years. We don’t have direct records of temperature, humidity, or other climate parameters that go back past human observations, so we use leftover clues to piece together a picture.

For example, in my research I am taking samples from a sediment core and measuring two parameters: the chain lengths, and the carbon and hydrogen isotope ratios, of the leaf wax biomarkers preserved in the sediment. But the reason I’m making these measurements is to use them as a proxy for the things we actually want to know.

The chain lengths of the leaf waxes—AKA the number of carbon atoms in a row—vary based on the type of plant they came from. So, by knowing what chain lengths are preserved in a sediment core, I can picture what plants were growing around the lake when the sediment was deposited.

 

The isotope ratios—AKA the ratio of 13C to 12C and of 3H to 2H—vary based on a number of factors. Isotope ratios of leaf waxes are affected by the type of plant that made the waxes, where that plant got its water, the temperature while the plant was growing, and a few other factors. So, by measuring the isotope ratios of the leaf waxes preserved in the core, I can understand patterns of temperature and precipitation from when the plants grew.

That’s the general idea of my project: I’m analyzing what is there (the leaf waxes that have been preserved) to get the information that isn’t.