what what is the evidence that global warming is either not occurring or is not caused by humans?
Atmospheric carbon dioxide concentrations are higher today than at any time in at least the past 650,000 years. [ They are about 35% higher than before the industrial revolution, and this increase is caused by human activities, primarily the burning of fossil fuels. Carbon dioxide is a greenhouse gas, as are methane, nitrous oxide, water vapor, and a host of other trace gases. They occur
naturally in the atmosphere. Greenhouse gases act like a blanket for infrared radiation, retaining radioactive energy near the surface that would otherwise escape directly to space. An increase in atmospheric concentrations of carbon dioxide and of other greenhouse gases augments the natural greenhouse effect; it increases the radioactive energy available to Earth’s surface and to the lower atmosphere. Unless compensated for by other processes, the increase in radioactive energy available to the surface and the lower atmosphere leads to warming. This we know. How do we know it?
The concentrations of carbon dioxide and other greenhouse gases in atmospheric samples have been measured continuously since the late 1950s. Since then, carbon dioxide concentrations have increased steadily from about 315 parts per million (ppm, or molecules of carbon dioxide per million molecules of dry air) in the late 1950s to about 385 ppm now, with small spatial variations away from major sources of emissions. For the more distant past, we can measure atmospheric concentrations of greenhouse gases in bubbles of ancient air preserved in ice (e.g., in Greenland and Antarctica). Ice core records currently go back 650,000 years; over this period we know that carbon dioxide concentrations have never been higher than they are now. Before the industrial revolution, they were about 280 ppm, and they have varied naturally between about 180 ppm during ice ages and 300 ppm during warm periods (Fig. 1). Concentrations of methane and nitrous oxide have likewise increased since the industrial revolution (Fig. ]
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