Carbon Cycle Feedbacks and Climate Tipping Behavior

Abstract

The climate of the past millions of years is strongly coupled to the carbon cycle and has shown a lot of variability. This variability might be caused by tipping points, relatively quick and large changes in an element in the climate system compared to their forcing. The coupling between temperature and the carbon cycle suggests an important role for the carbon cycle by either showing tipping behavior or by providing positive feedbacks to the climate while interacting with a tipping element. A better understanding of these relations is important since they might play a role under future climate change. In this thesis, we study whether there are tipping points in the marine carbon cycle and how the marine carbon cycle interacts with large changes in the Atlantic Meridional Overturning Circulation (AMOC), a potential tipping element. By using simple models we have looked for tipping points in the marine carbon cycle, but we did not find these, making it unlikely they are present. We did find a supercritical Hopf bifurcation with a stable internal oscillation with a period of 5,000 – 6,000 years that might be relevant when studying past climates. By varying the AMOC strength in the same models we were able to study its effect on atmospheric pCO2. We found that the AMOC strength had little effect on atmospheric pCO2 on long timescales. When we used a similar model with an AMOC that is able to tip from an on- to an off state and vice versa, we found that when the AMOC tips, atmospheric pCO2 changes by 25 to 40 ppm. Furthermore, we found that the marine carbon cycle can influence the window that the AMOC has multiple equilibria when a coupling between atmospheric pCO2 and atmospheric freshwater transport is introduced. When studying future climate change, different processes are relevant compared to the main processes relevant on long timescales. Because we study the system on much shorter timescales, i.e. multi-decadal to centennial, we can use a much more complex model. We found that under climate change, the marine ecosystem can provide a relatively strong positive feedback to atmospheric pCO2 through a phytoplankton composition shift. We found that large phytoplankton (diatoms) are replaced with smaller phytoplankton in the North Atlantic under climate change, resulting in a lower efficiency of the biological carbon pump in transporting carbon to the deep ocean. As a consequence, the uptake capacity of the ocean decreases, and more carbon remains in the atmosphere to raise atmospheric pCO2. We also found a positive feedback between atmospheric pCO2 and the AMOC. We found that as the AMOC weakens, atmospheric pCO2 increases creating a positive feedback loop. However, the found feedback was very small because of compensating effects within the carbon cycle and between different regions. Even though the global response was small, locally large changes occurred in climate and carbon cycle variables.