Fossils of a group of plants which includes banksias, proteas and macadamias, show that when past climates were high in carbon dioxide, the pores that plants use to absorb CO2 were smaller and more abundant, according to Australian research.

Fossilised plant pores, known as stomata, have been used to estimate past climate and CO2 levels.

The Australian research results challenge current models that predict larger and less densely packed stomata under the higher atmospheric carbon dioxide prevalent in warm climates.  Thus, the size and abundance of stomata are significantly affected by environmental factors other than atmospheric carbon dioxide

The researchers say this means future models may need to consider other environmental factors that might influence the size of these plant pores, not just CO2.

They used plant fossils from the major southern hemisphere family Proteaceae to show that stomata (the tiny adjustable pores that allow plants to absorb carbon dioxide) varied through the Cenozoic period (the last 65 million years).

In particular, stomata were smaller and more abundant when climates were warm and rainforest was abundant.

The research involved scientists from the University of Tasmania, The University of Adelaide and the CSIRO. It was supported by Australian Research Council Discovery Grants (grant nos DP140100307 and DP160100809).