Carbon Cycle Feedbacks under Positive and Negative CO2 Emissions

Abstract:

Approximately half of current anthropogenic emissions are absorbed by land and ocean systems. However, this rate of absorption is sensitive to changes in atmospheric CO₂ levels and climate. As atmospheric CO₂ concentration increases, carbon sinks will sequester more carbon, resulting in a negative concentration-carbon cycle feedback. Changes in climate, in response to increasing CO₂ concentration, will decrease the efficiency of carbon sinks, resulting in a positive climate-carbon cycle feedback. Research has so far focused on carbon cycle feedbacks under increasing CO₂ concentration (positive emissions). Since the majority of scenarios achieving the 2°C climate target require negative emissions technologies, an understanding of carbon cycle feedbacks under decreasing CO₂ concentration (negative emissions) is required to produce reliable climate projections. This study uses an Earth system model to investigate carbon cycle feedbacks under positive and negative emissions. To this end, the study analyzes biogeochemically coupled, radiatively coupled and fully coupled climate model simulations with prescribed atmospheric CO₂ concentration increasing at 1% per year from preindustrial levels until it quadruples, followed by a 1% per year decline back to preindustrial levels. In response to CO₂ changes, the land and ocean initially take up carbon in the negative emissions phase before switching into sources of CO₂. In response to changes in climate, the ocean continues to outgas for over a century in the negative emissions phase, whereas the land outgases for a shorter time before becoming a sink of CO₂. The differences in the behaviour of carbon cycle feedbacks under positive and negative emissions are likely largely due climate system inertia: the carbon cycle response under negative emissions is a combination of the response to both negative emissions and the prior positive emissions. This study has implications for the reversibility of climate and CO₂ effects on carbon sinks under negative emissions.