The greatest risk from global warming to humanity and all life
is the multiple combined 'cascading' Arctic climate feed backs.
These are the following.
o Melting back of subarctic spring-summer snow cover.
o Melting decline of Arctic summer sea ice extent.
o Warming far north peat lands - emits methane.
o Thawing permafrost - emitting methane & nitrous oxide.
o Melting sea floor Arctic methane hydrate - methane.
Methane and nitrous oxide are much more powerful greenhouse gases than CO2.
Methane from of sub sea floor methane hydrates.
Positive feedback from warming may cause the
release of carbon or methane from the terrestrial biosphere and oceans (See Meehl et
al., 2007), which would add to the mitigation required.
Quote from IPCC AR4 2.2.4 Risk
of catastrophic or abrupt change.