9  Earth system feedbacks

9.1 Land as an element in the Earth system

The Earth system can be regarded as a coupled system in which its elements (atmosphere, ocean, cryosphere, biosphere, lithosphere) interact on various time scales. A primary goal of Earth System research is to understand the interactions occurring on time scales that are relevant for society in the context of anthropogenic climate change. It is now established with overwhelming evidence that anthropogenic CO₂ emissions from the combustion of fossil fuels have caused a rise in atmospheric concentrations beyond levels reached over the past 800,000 years, and that this concentration increase is the dominant driver of climate change as observed over the last decades (Arias et al. 2021).

Any prediction of climate change in the coming decades, centuries and millennia relies on an understanding of the processes that are key to the questions:

• How fast will anthropogenic CO₂ emissions and other greenhouse gases accumulate in the atmosphere?
• Has does the anthropogenic modification of the biosphere through land use change affect greenhouse gases, land-climate interactions, and the carbon cycle?
• What is the climate response to changes in atmospheric CO₂ and other drivers?
• What mechanisms does the rise in CO₂ and the change in climate set in motion and how do they feed back to climate change?

For understanding

This thesis focuses on the terrestrial biosphere and how it affects atmospheric CO₂ and climate change through feedbacks and in direct response to anthropogenic impacts. The land carbon cycle, nitrous oxide (N₂O ) emissions from soils, methane (CH₄ ) emissions from peatlands and inundated soils, and a multitude of other processes (arneth10ngeo?), respond to a changing climate and atmospheric composition. At the same time, emissions of these greenhouse-gases, as well as direct anthropogenic impacts on terrestrial ecosystems through land use change affect climate. In this thesis, I present results from Earth system modeling studies that addressed above mentioned impacts and mechanisms and provide a quantification of their relative potency in affecting future climate change.

The conceptual framework of forcings and feedbacks and the quantification formalism adopted and extended here will serve as a guide to describe and quantify the myriad interactions and feedbacks in the terrestrial biosphere. Before turning to the introduction of this framework in ?sec-concepts}, I will provide a brief introduction into the processes that shape the role of the terrestrial biosphere in the Earth system and will focus in particular on the interaction of the carbon and nitrogen cycle.

Arias, P. A., N. Bellouin, E. Coppola, R. G. Jones, G. Krinner, J. Marotzke, V. Naik, et al. 2021. “Technical Summary.” Book Section. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group i to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, edited by V. Masson-Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, et al. Cambridge, UK; New York, NY, USA: Cambridge University Press. https://doi.org/10.1017/9781009157896.002.