Carbon cycle response to temperature overshoot beyond 2 °C – an
analysis of CMIP6 models
Abstract
There is a substantial gap between the current emissions of greenhouse
gases and levels required for achieving the 2 and 1.5 °C temperature
targets of the Paris Agreement. Understanding the implications of a
temperature overshoot is thus an increasingly relevant research topic.
Here we explore the carbon cycle feedbacks over land and ocean in the
SSP5-3.4-OS overshoot scenario by using an ensemble of Coupled Model
Intercomparison Project 6 Earth system models. Models show that after
the CO2 concentration and air temperature peaks, land and ocean are
decreasing carbon sinks from the 2040s and become sources for a limited
time in the 22nd century. The decrease in the carbon uptake precedes the
CO2 concentration peak. The early peak of ocean uptake stems from its
dependency on the atmospheric CO2 growth rate. The early peak of the
land uptake occurs due to a larger increase in ecosystem respiration
than the increase in gross primary production, as well as due to a
concomitant increase in land-use change emissions primarily attributed
to the wide implementation of biofuel croplands. The carbon cycle
feedback parameters amplify after the CO2 concentration and temperature
peaks due to inertia of the Earth system so that land and ocean absorb
more carbon per unit change in the atmospheric CO2 change (stronger
negative feedback) and lose more carbon per unit temperature change
(stronger positive feedback) compared to if the feedbacks stayed
unchanged. The increased negative CO2 feedback outperforms the increased
positive climate feedback. This feature should be investigated under
other scenarios.