C4 refers to the metabolic pathway that certain plants use to conduct photosynthesis, as opposed to the method used by the C3 plants, which dominate cooler, higher-latitude regions. C4 organisms comprise about a quarter of the world's plant life today. In many hotter regions, they came to dominate about 6 million to 10 million years ago. They apparently evolved to photosynthesize under warm, dry, conditions, and low levels of atmospheric carbon dioxide, which developed at that time. They have a special ability to take advantage of strong summer rainfall.
"Our results suggest that there was not a single factor that drove the transformation of tropical ecosystems around the globe," said coauthor Pratigya Polissar, a research scientist at Columbia University's Lamont-Doherty Earth Observatory.
"Understanding how changing CO2 levels and other environmental factors affected these plants in the past can help us evaluate and plan for effects from increasing CO2 levels and climate change today," said Polissar.
The study's lead author is Jake Andrae, a PhD. candidate at the University of Adelaide. The other authors are S. Howard and P.A. Hall, also of Adelaide; J.M.K. Sniderman of the University of Melbourne; and Sam Phelps of Lamont-Doherty.