It’s tough to separate out human activity from natural variability and other causes, but most scientists believe increased  levels play a part in the gradual warming we have experienced.   is a trace gas (only .04% of the atmosphere) and has been steadily rising since 1940 thanks to increasing industrialization and the world’s use of coal, oil, and natural gas But temperature changes since 1910 have not correlated with the rise of , and historical data show  levels lagging temperature changes, not leading them.  And some studies suggest solar activity correlates much better with global temperatures.