Carbon-Cycle Feedbacks Operating in the Climate System

Climate change involves a direct response of the climate system to forcing which is amplified or damped by feedbacks operating in the climate system. Carbon-cycle feedbacks alter the land and ocean carbon inventories and so act to reduce or enhance the increase in atmospheric CO 2 from carbon emissions. The prevailing framework for carbon-cycle feedbacks connect changes in land and ocean carbon inventories with a linear sum of dependencies on atmospheric CO 2 and surface temperature. Carbon-cycle responses and feedbacks provide competing contributions: the dominant effect is that increasing atmospheric CO 2 acts to enhance the land and ocean carbon stores, so providing a negative response and feedback to the original increase in atmospheric CO 2 , while rising surface temperature acts to reduce the land and ocean carbon stores, so providing a weaker positive feedback for atmospheric CO 2 . The carbon response and feedback of the land and ocean system may be expressed in terms of a combined carbon response and feedback parameter, λ carbon in units of W m − 2 K − 1 , and is linearly related to the physical climate feedback parameter, λ climate , revealing how carbon and climate responses and feedbacks are inter-connected. The magnitude and uncertainties in the carbon-cycle response and feedback parameter are comparable with the magnitude and uncertainties in the climate feedback parameter from clouds. Further mechanistic insight needs to be gained into how the carbon-cycle feedbacks are controlled for the land and ocean, particularly to separate often competing effects from changes in atmospheric CO 2 and climate forcing.