Positive (bad) feedbacks to the climate system happen when global warming (or CO2 emissions) have an effect on some component of the planet that increases the global warming even more.
There are a number of positive carbon feed backs by which components of the warmed planet add more carbon emissions as CO2 and/or methane. An obvious one would be forest fires - global warming increases forest fires that emit carbon dioxide (and some methane) when they burn.
These dangerous carbon feed backs were not included in the 2007 IPCC assessment of the 'most likely' temperature increases. They are inevitable results of more GHGs in the atmosphere and the resulting global warming. The IPCC has always acknowledged (since the very first 1990 IPCC assessment) they are real and inevitable, even though they are excluded from the warming porjections.
Greenhouse gas feedbacks (1st
IPCC assessment 1990 SPM p. 18)
The net emissions of carbon dioxide
from terrestrial ecosystems will be elevated at higher temperatures.
flux of carbon dioxide to the atmosphere may be particularly evident in warmer
conditions in tundra and boreal legions where there are large stores of carbon.
It the oceans become warner, their net uptake of carbon dioxide may decrease
because of changes in (i) the chemistry of carbon dioxide in seawater, (ii)
biological activity in surface waters, and (iii) the rate of exchange of carbon
dioxide between the surface layers and the deep ocean.
from natural wetlands and rice paddies are particularly sensitive to temperature
and soil moisture. Emissions are significantly larger at higher
temperatures and with increased soil moisture.
Higher temperatures could
increase the emissions of methane at high northern latitudes from decomposable
organic matter trapped in permafrost and methane
The soil and vegetation emit more CO2 when warmed and this is called a terrestrial carbon feedback. The IPCC estimates that for a high emissions scenario the terrestrial carbon feedback causes an extra 1.0 to 1.5C by 2100 to the global temperature increase in the assessments.
Most of the planet's carbon is stored cold or frozen in the Arctic. These are subArctic peat rich wetlands, permafrost (permanently frozen peat) and subsea floor frozen solid methane gas hydrate. Permafrost thawing is estimated to add another 0.8C to the global warming by 2100. If global emissions continue on today's worst case scenario terrestrial carbon feedback will add 1.5C and Arctic feedbacks another similar amount by 2100- adding 3.0C to what the assessments call the 'most likely' temperature increase.