Dr. Hansen described the three categories of evidence used to understand climate change. The historical record of temperature and carbon dioxide concentration is first in line, whether that record comes from instruments deployed around the world for which 130-years of data are available, or air trapped in glacial ice, for which data goes back 800,000 years. I would add that the historical record for CO2 and temperature now goes back 20 million years with the publication by Aradhna Tripati of her work with the foraminifera.
The next line of evidence includes current atmospheric and climate conditions, such as temperature data from around the world, glacier conditions in mountains and at the poles, and ocean chemistry.
Third-ranked by Hansen are computer-based climate simulations.
Dr. Hansen went to some length to explain the causes of historical climate variability. In addition to the climate forcings related to the Milankovitch cycles, he mentioned plate tectonic activity. When India was an island continent south of Asia, it was moving north through the Indian Ocean. During this time period, Hansen suggested that large amounts of carbon dioxide were released by volcanic activity triggered by this plate movement. This corresponds to a very warm period on the planet, much warmer than today, when sea levels were considerably higher as there were no large glaciers.
Some mention was also made of the oceans as a sink for atmospheric carbon dioxide, but I missed the reference (2009) and have not been able to find it. The new finding was of carbon measurements down to a depth of 2 km below the ocean's surface, and Hansen was quite excited about it. If any readers out there know of this study, please advise!
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