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A trio of graphs showing how low incoming sunlight must fall at different atmospheric carbon dioxide levels in order to trigger an ice age<p>Yellow lines show changes in incoming sunlight in the Northern Hemisphere due to Milankovitch cycles over the next 500,000 years. (l<strong>eft panel</strong>) At pre-industrial levels carbon dioxide levels of around 280 parts per million (ppm), insolation must drop below about 455 watts/m<sup>2</sup> (red line) to trigger an ice age, a threshold that will be reached a