Bob Carter of Harrisonville asks: "How much has Earth's atmosphere declined, and in which amounts, to cause climate change? Keep it simple!" Cliff Notes version: Carbon dioxide levels in the atmosphere have increased 34 percent since 1800, from 280 parts per million to 377 ppm. Methane concentrations are up 145 percent. Our emissions of these "greenhouse gases" have jumped 70 percent since 1970. Scientists agree these higher concentrations are "very likely" responsible for most observed climate warming.