How are human activities changing the amount of greenhouse gases in our atmosphere?
It is a well-documented and straightforward observation that levels of carbon dioxide and other greenhouse gases are increasing in our atmosphere. The six key greenhouse gases included in the Administrator’s Findings are at essentially unprecedented levels compared to the recent and distant past. Their concentrations are climbing, and this is projected to continue well into this century. The two most important directly emitted greenhouse gases, carbon dioxide and methane, are well above the natural range of atmospheric concentrations compared to at least the last 650,000 years (see TSD EPA-HQ-OAR-2009-0171-11645). The most recent report of the NRC states that carbon dioxide levels are now at 388 parts per million and increasing by almost two parts per million per year. The fact that greenhouse concentrations are now at such high levels is absolutely central to the Administrator’s Endangerment Finding. Without such a large and ever-increasing buildup of atmospheric levels of greenhouse
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