UC Professor discusses the impacts of a carbon dioxide level of 400ppm in new series Climate Explained on The Conversation
UC Professor James Shulmeister answers the question “what was the climate and sea level like at times in Earth’s history when carbon dioxide in the atmosphere was at 400ppm?”
Recent research shows that west Antarctica is now melting. Elaine Hood/NSF
We know how much carbon dioxide the atmosphere contained in the past by studying ice cores from Greenland and Antarctica. As compacted snow gradually changes to ice, it traps air in bubbles that contain samples of the atmosphere at the time. 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. One method uses the relationship between plant pores, known as stomata, that regulate gas exchange in and out of the plant. The density of these stomata is related to atmospheric carbon dioxide, and fossil plants are a good indicator of concentrations in the past.
Another technique is to examine sediment cores from the ocean floor. The sediments build up year after year as the bodies and shells of dead plankton and other organisms rain down on the seafloor. We can use isotopes (chemically identical atoms that differ only in atomic weight) of boron taken from the shells of the dead plankton to reconstruct changes in the acidity of seawater. From this we can work out the level of carbon dioxide in the ocean.
The data from four-million-year-old sediments suggest that carbon dioxide was at 400ppm back then.