evidence: The first task was to create a database of climate proxies — bits of fossil evidence that hint at how the world once was. For example, the variety of oxygen found in the teeth of extinct, eel-like creatures known as conodonts reflects the water temperature in the oceans where they lived. The chemical composition of fats from ancient algae indicates how they constructed their cell walls to deal with the heat. Yet the database was restricted to evidence from the oceans, which cover only 70 percent of the planet’s surface. And each proxy could reveal the temperature only at a particular spot at a single point in time. Even with 150,000 data points, Judd said, it was like trying to assemble a jigsaw puzzle with only 1 percent of the pieces. [...] Wing acknowledged that there is a lot more work to do. He and his colleagues aim to continue to refine the timeline by adding data from land-based proxies, such as fossil leaves. They also hope it will help researchers trying to model future climate change by allowing them to examine warmer periods from Earth’s past.