Argon Argon Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust. Nearly all of the argon in
Regulation of greenhouse gases under the Clean Air Act in the climate system. Once emitted into the atmosphere, GHGs influence the Earth's energy balance for a period of decades to centuries. Consistent with their long-lasting impacts, the IPCC found that even if the concentrations of all GHGs had been kept constant at year 2000 levels, a further warming of approximately 0.1C per decade would be expected. The fact that GHGs are long-lived in the atmosphere also means that they become well mixed across the globe – hence the global nature of the problem. After considering the scientific evidence before her, Administrator Jackson found that greenhouse gases could be reasonably