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 the 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, 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 and reflected in simple climate
models.