A Picture is worth a thousand words. Below see timelines of global parameters, many of them approaching tipping points.
Methane Arctic Time Bomb.
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Methane is 25x more powerful than CO2. Temperatures are changing faster in the Arctic than anywhere else on the planet. There are two potential threats from the Arctic methane:
1. Permafrost is defined as land that has been frozen for more than 2 years. The NSIDC estimates that there are 23 million square miles of Arctic permafrost. This permafrost acts as a croygenic lid keeping huge stores of methane contained. This cryogenic lid is beginning to degrade. When temperatures hit a temperature, enormous releases of methane can enter the atmosphere in a very short period of time.
2. Clathrate is defined as a cage compound, a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type. Methane clathrates are those in which crystalline water traps methane molecules. These clathrates exist in enormous quantities and are stable as long as the low temperature/high pressure conditions exist. What is happening in the Arctic, however, is that due to global warming, the temperature of the shallow Arctic Ocean (one 50 meters deep) are warming and the clathrates are breaking apart, releasing the methane. The shallowness of the ocean means that there is no time for it to be absorbed in the water column and it is released directly into the atmosphere. The clathrate gun theory postulates that when a temperature is reached, huge volumes of methane can be released directly into the atmosphere,, increasing the feedback effect of global warming.
James E. Hansen has suggested in his 2008 paper Climate Threat to the Planet that the Earth could experience a runaway greenhouse effect and adopt a climate like that of Venus if fossil-fuel use continues until reserves are exhausted.