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IF YOU could time travel, which period from history would you choose to visit? I recently took a trip back to the Palaeocene-Eocene thermal maximum 鈥� a time period about 56 million years ago during which a huge amount of carbon dioxide was released into Earth鈥檚 atmosphere, causing the global temperature to rise by about 5掳C.

Many scientists are interested in how this , not least because knowing more about it could help us better predict the impacts of current and future climate change. You can assist them by taking the same trip I did, and participating in the .

Head to the and search for 鈥淔ossil Atmospheres鈥� to get started. You will be asked to label cells in microscope images of modern and fossilised leaves from plants. These unique, non-flowering seed plants have been around for hundreds of millions of years. A handy tutorial on the website can guide you on how to distinguish the different types of leaf cell, so don鈥檛 worry if you aren鈥檛 a cell biology whizz.

Your labelling will help researchers to calculate what proportion of leaf cells are made up of stomata 鈥� pores on the leaf surface that allow uptake of CO₂. They can then use this to estimate what the environmental CO₂ levels were at the time the plant lived.

鈥淧lant leaves are extremely sensitive to the composition of the atmosphere,鈥� says Richard Barclay, a palaeobotanist on the Fossil Atmospheres project. That is because plants use CO₂ to make the sugar they need to grow.

When the concentration of CO₂ in the atmosphere changes, it affects plant productivity and they adjust the shape of their leaves and the size and number of pores on the leaf surfaces.

Barclay and his colleagues have collected hundreds of leaves, producing many images of each 鈥� far more than they can analyse on their own. Since Fossil Atmospheres first started in 2016, citizen scientists have helped the team analyse almost 128,000 images. The researchers are using the data to create a record of how Earth鈥檚 atmosphere has changed over time, as well as how it is continuing to change.

鈥淲e study climate change events in the geologic past, and they have eerily similar patterns to what we observe happening today,鈥� says Barclay. The key difference is that this time, the climate is changing much faster, he says.

Important information about how plants will fare in this uncharted world may be hidden among the thousands of leaf images online. What is needed now is for as many citizen scientists as possible to log on and get counting.

For other projects visit听newscientist.com/maker.