<P> Maureen Raymo, William Ruddiman and others propose that the Tibetan and Colorado Plateaus are immense CO "scrubbers" with a capacity to remove enough CO from the global atmosphere to be a significant causal factor of the 40 million year Cenozoic Cooling trend . They further claim that approximately half of their uplift (and CO "scrubbing" capacity) occurred in the past 10 million years . </P> <P> 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 . </P> <P> The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO 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 CO . The hypothesis also warns of future Snowball Earths . </P> <P> In 2009, further evidence was provided that changes in solar insolation provide the initial trigger for the earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change . </P>

After the ice age ended what was different on earth