Rising levels of atmospheric CO2 do not necessarily cause global warming, which contradicts the core thesis of human-caused climate change. Earth's climate record shows that warming has preceded, not followed, a rise in CO2. According to a 2003 study published in Science, measurements of ice core samples show that over the last four climactic cycles (past 240,000 years), periods of natural global warming preceded global increases in CO2. [117] In 2010 the Proceedings of the National Academy of Sciences published a study of the earth's climate 460-445 million years ago which found that an intense period of glaciation, not warming, occurred when CO2 levels were 5 times higher than they are today. [4] According to ecologist and former Director of Greenpeace International Patrick Moore, PhD, "there is some correlation, but little evidence, to support a direct causal relationship between CO2 and global temperature through the millennia." [60]