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Plants may give us fewer of the nutrients we need to survive if global warming is not controlled, a visiting expert says.

Fossil expert Dr Scott Wing, who was in New Zealand to speak at the Greenhouse Earth Symposium at Te Papa last week, said a study suggested ancient plants may have made less protein as CO2 levels rose.

If the theory is correct, insects were left hungrier when plants made less of the protein they needed to live.

The phenomenon could affect humans if plants begin cutting protein again.

Fossil records show insects began eating more plants about 55 million years ago, when the planet suddenly warmed up.

Dr Wing, who is the curator of fossil plants at the Smithsonian Institution in Washington, acknowledged there may have been more insects around to eat the plants.

"We don't know this yet, but it could be that the increase in feeding was a response to both the warming and a change in atmosphere itself."

People, animals and insects need protein, which plants make to help them get energy from the sun. But Dr Wing said plants needed to make less of the vital substance when CO2 levels were higher.

He told the Herald that one way plants responded to higher CO2 levels was by cutting protein production.

Dr Wing studies fossil records from the early Eocene period, a time when natural causes poured greenhouses gases into the atmosphere at about the same rate CO2 is rising today, to predict what could happen this century.

People had already seen changes in the flowering times of plants and changes to where plants could grow, but they needed to realise that greenhouse gases would only reach the levels they did in the Eocene period if humans did not get them under control.

During the Eocene, mammals moved between continents to cope with the changing environment, and plants and animals made homes in places they had not lived in before.