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GHGs - 

According to widely accepted scientific theory, a greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. These gases are thought to be responsible for the greenhouse effect. Hence the name greenhouse gas.

Different greenhouse gases have different global warming potentials. These differences can be expressed as a Carbon dioxide equivalent. If we assign a factor of one to CO2 then Methane would have a factor of 21; Nitrous oxide( N2O) would have a factor of 310; Hydrofluorocarbons (HFCs) would have a factor between 140-12,100; SF6 23,900 and PFCs 6,500-9,200

 
Global Natural Climate Change Cycles - 

The idea that Global Warming is a natural cycle is well understood from paleo data covering the past 1 million years. Is there a difference between current climate, and the natural cycle? For the past million years the natural climate has oscillated between warm periods and ice ages. This shifting in and out of warm periods and ice ages is correlated strongly with Milankovitch cycles. In order to understand the difference between natural cycle and human-caused/influenced global warming, one needs to consider changes in radiative forcing and how this affects systems on Earth such as the atmosphere, vegetation, ice and snow, ocean chemistry and ocean heat content overturn cycles and related effects. The current radiative forcing levels are clearly outside of the natural cycle range.