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melting permafrost could add an additional 2 degrees C to estimate for 2100 of 2 degrees C - IPCC
although the IPCC didn't include melting permafrost impact in basic estimate of warming they did comment on it as a positive feedback element:
http://www.climateemergencyinstitute.com/ipcc_2014.html
Amplifying positive feedback Adds up to an extra 2.0C by 2100
The feedback between climate and the carbon cycle is positive in the 21st century and beyond. Models indicate a loss of carbon of 59 [20 to 98] PgC and 17 [13 to 21] PgC per °C warming from the land and the ocean, respectively [this is about 0.5C by 2100, but does not include large forest die-back, peat-lands and large Arctic feedback sources].
Permafrost feedback Until the year 2100, up to 250 PgC [picograms of carbon] could be released as CO2, and up to 5 Pg as CH4. Given methane's stronger greenhouse warming potential, that corresponds to a further 100 PgC of equivalent CO2 released until the year 2100 [so 350 PgC and about 1.5°C]. (AR5 WG1 FAQ 6.1)
Quite some feedback - possible increase in warming by 2100 of 100% over original estimate. There must be quite a wide confidence interval around this estimate though, as they do not have a whole lot of data on the rate of warming of the permafrost at this time.
