Climate change: Warning from 'Antarctica's last forests'

I have seen people claim that since the earth has warmed in the past, it somehow means that our actions can't cause the same. This makes the same amount of sense as saying that since in the past, natural forces made the ground wet, it proves that watering the yard can't make it wet.

Greenhouse gases have the same effects whether they are created by nature or by human actions.

By Jonathan Amos BBC Science Correspondent

This plant material isn't much to look at, but scientists say it should serve as a warning to the world about where climate change could take us if carbon emissions go unchecked.

The time period is an epoch geologists call the Pliocene, 2.6-5.3 million years ago.

It was marked by temperatures that were significantly warmer than today, perhaps by 2-3 degrees globally.

These were conditions that permitted plant growth even in the middle of the White Continent.

Higher, too, were sea-levels. It's uncertain by how much, but possibly in the region of 10-20m above the modern ocean surface.

What's really significant, though, is that the amount of carbon dioxide in the atmosphere was very similar to what it is today - at around 400 CO2 molecules for every million molecules of air.

Indeed, the Pliocene was the last time in Earth history that the air carried this same concentration of the greenhouse gas.

And it tells you where we're heading if we don't get serious about addressing the climate problem, cautions Prof Martin Siegert from the Grantham Institute at Imperial College London.

Temperatures may currently be lower than in the Pliocene, but that's only because there is a lag in the system, he says.

"If you put your oven on at home and set it to 200C, the temperature doesn't get to that level immediately; it takes a bit of time," he told reporters.

"And it's the same with Earth's climate. If you ratchet up the level of CO2 at 400 parts per million (ppm), it won't suddenly get to an equilibrium overnight. It will take maybe 300 years or something.

"So, the question to us is: what is the equilibrium state; what is Earth's climate going to look like with 400ppm, all things being settled?"

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