Editing Ice age (section)

===Changes in Earth's atmosphere===
There is evidence that [[greenhouse gas]] levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.

The [[Snowball Earth]] hypothesis maintains that the severe freezing in the late [[Proterozoic]] was ended by an increase in {{CO2}} levels in the atmosphere, mainly from volcanoes, and some supporters of Snowball Earth argue that it was caused in the first place by a reduction in atmospheric {{CO2}}. The hypothesis also warns of future Snowball Earths.

In 2009, further evidence was provided that changes in solar [[insolation]] provide the initial trigger for Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change.<ref>{{Cite journal |first1=Peter U. |last1=Clark |first2=Arthur S. |last2=Dyke |first3=Jeremy D. |last3=Shakun |first4=Anders E. |last4=Carlson |first5=Jorie |last5=Clark |first6=Barbara |last6=Wohlfarth |first7=Jerry X. |last7=Mitrovica |first8=Steven W. |last8=Hostetler |first9=A. Marshall |last9=McCabe |name-list-style=amp|author-link6=Barbara Wohlfarth |year=2009 |title=The Last Glacial Maximum |journal=Science |volume=325 |issue=5941 |pages=710–714 |doi=10.1126/science.1172873 |pmid=19661421 |bibcode = 2009Sci...325..710C|s2cid=1324559 }}</ref>