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Climate explained: what the world was like the last time carbon dioxide levels were at 400ppm - The Conversation AU
Jul 07, 2020 1 min, 11 secs

What was the climate and sea level like at times in Earth’s history when carbon dioxide in the atmosphere was at 400ppm.

The last time global carbon dioxide levels were consistently at or above 400 parts per million (ppm) was around four million years ago during a geological period known as the Pliocene Era (between 5.3 million and 2.6 million years ago)?

We know how much carbon dioxide the atmosphere contained in the past by studying ice cores from Greenland and Antarctica.

We can sample ice cores to reconstruct past concentrations of carbon dioxide, but this record only takes us back about a million years.

Beyond a million years, we don’t have any direct measurements of the composition of ancient atmospheres, but we can use several methods to estimate past levels of carbon dioxide.

The density of these stomata is related to atmospheric carbon dioxide, and fossil plants are a good indicator of concentrations in the past.

From this we can work out the level of carbon dioxide in the ocean.

Also during the Pliocene, we know the West Antarctic Ice Sheet was significantly smaller and global average temperatures were about 3℃ warmer than today.

It will take more than a few years or decades of carbon dioxide concentrations at 400ppm to trigger a significant shrinking of the West Antarctic Ice Sheet

In fact, if the whole of the West Antarctic melted it could raise sea levels by about 3.5 metres

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