New evidence from analyzing fossil plankton shells has revealed that CO2 concentration in the Earth’s atmosphere was about triple current levels around 52 million years ago. It then declined to levels close to current atmospheric CO2 concentration 34 million years ago when Antarctica began to glaciate.
Based on the trend that greenhouse gases are increasing rapidly whereas solar irradiance is changing much more slowly, the evidence reveals that the climate impact of changes in solar irradiance are much smaller in magnitude than the increase in warming due to greenhouse gases.
In this study the global average surface temperature over the past 2 million years has been derived from deep sea cores. The results reveal that global cooling occurred about 300,000 years before the rapid ice sheet growth and the development of the first 100,000-year glacial/deglacial cycle about 800,000 years ago.
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