ALBERT

Description: Palaeontologists increasingly use large datasets of observations collected from museum specimens to address broad-scale questions about evolution and ecology on geological timescales. One such question is whether information from fossil organisms can be used as a robust proxy for atmospheric carbon dioxide through time. Here, we present the citizen science branch of ‘Fossil Atmospheres’, a project designed to refine stomatal index of Ginkgo leaves as a palaeo-CO 2 proxy by involving citizen scientists in data collection through the Zooniverse website. Citizen science helped to overcome a barrier presented by the time taken to count cells in Ginkgo samples; however, a new set of challenges arose as a result. A beta-testing phase with Zooniverse volunteers provided an opportunity to improve instructions to ensure high fidelity data. Exploration of citizen scientists' estimates shows that volunteer counts of stomata are accurate with respect to counts made by the project's lead scientist. However, counts of epidermal cells have a wide range, and mean values tend to underestimate expert counts. We demonstrate a variety of approaches to reducing the inaccuracy in the calculated stomatal index that this variation causes. Zooniverse serves as an ideal tool for collection of palaeontological data where the distribution of fossils would be impossible, but where specimens can be easily imaged. Such an approach facilitates the collection of a large palaeontological dataset, as well as providing an opportunity for citizens to engage with climate research. This article is part of the theme issue ‘Biological collections for understanding biodiversity in the Anthropocene’.