Estimating natural and anthropogenic responses of the water cycle in the Earth system using comprehensive coupled climate models

Lehner, Flavio (2013). Estimating natural and anthropogenic responses of the water cycle in the Earth system using comprehensive coupled climate models (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Strong and growing quantitative evidence is at hand that the Earth's climate has undergone distinct changes in the past. During the last millennium the influence of external and internal processes rendered both warmer and colder periods, the most pronounced of which are referred to as the Medieval Climate Anomaly and the Little Ice Age, respectively. To disentangle the different influences and to determine the importance of individual forcing factors during such periods is a key task of climate research. In view of the current anthropogenically induced climate change, a robust understanding of the climate system's sensitivity to different forcings represents a crucial prerequisite for reliable projections of future climate change. However, achieving this understanding is not always straightforward as the climate system's internal variability is often large and different forcings act at the same time, making the detection and attribution of climate change a delicate endeavor. Further, most information of past climate variations comes from stationary proxy archives that can only draw an incomplete picture of the dynamics associated with those climate variations.
At this point climate models become an indispensable tool, offering unique possibilities to tackle these issues. Unlike in the real world, natural and anthropogenic forcing can be kept constant or applied separately in models and thereby permit the study of their isolated influence on climate. Additionally, ensembles of simulations that apply identical forcing but are started from different initial conditions are used to separate a forcing signal from internal variability. Ultimately, these approaches are applied across different models to reduce the risk that a single model lacks a crucial process. When thoroughly validated against observations and reconstructions, these models can be used to project future climate change, based on an expected evolution of the natural and anthropogenic forcing.
This thesis embraces all of these model approaches to address past and future changes in the hydrological cycle. The model studies in this thesis are to a large part motivated by the temporal and spatial limitations of observations and reconstructions and aim at closing the gap between the signal from such data sources and the dynamical understanding of it. Given the potential for advancing our knowledge of past climate change, collaborations between the observational and modeling community are still too few. Often, reconstructions are founded solely on the author's interpretation and are weakly supported by process understanding and model results, thereby increasing the chance for misinterpretation. Growing availability of state-of-the-art model output offers an excellent opportunity to test and back up the interpretation of reconstructions.
The hydrological cycle receives increasing attention in climate research as its future fate is of great importance for society and for the climate system itself. Freshwater stored in reservoirs of the polar regions is an important component of the global hydrological cycle, and a redistribution of this freshwater can have climatic impacts beyond the polar region. Therefore, the sensitivity of these reservoirs to climate change has been of interest for quite a while, yet, the scarceness of observations and reconstructions in the polar regions complicates the analysis of it. Using model simulations from 1500-2100 AD, we are able to put past changes in the polar freshwater cycle into context of ongoing and projected future climate change in these regions. The different reservoirs sea ice, ocean, and atmosphere exhibit different response behavior to preindustrial changes in external forcing such as low solar activity or volcanic eruptions. While being significant in some cases, these variations are dwarfed compared to future changes, when both polar regions are projected to become a strong source of freshwater for lower latitudes with implications for ocean circulation.
A second study investigates the issue of stationarity in a millennial proxy reconstruction of the North Atlantic Oscillation (NAO). This reconstruction has received a considerable level of attention as it suggests that the NAO has remained in a positive state for several centuries during the Medieval Climate Anomaly, potentially exhibiting a strong control on European climate during that time. Using pseudo-proxies from model simulations the explanatory power of the precipitation proxies used in this reconstruction is estimated. In the particular case, the proxy locations were found to be insufficient to robustly describe the NAO over the last millennium. Recommendations are made as to the improvement of the reconstruction concept.
Following from the criticism of this NAO reconstruction, alternative explanations for the North Atlantic-European climate variations during the last millennium are explored in an ensemble of model simulations from the Medieval Climate Anomaly to the Little Ice Age. A feedback mechanism between sea ice, ocean, and atmosphere is discovered in the region of the subpolar North Atlantic and Nordic Seas that can account for much of the climate variations during the transition from the Medieval Climate Anomaly to the Little Ice Age without relying on significant excursions of the NAO. This feedback mechanism is thoroughly tested with sensitivity experiments in which sea ice growth is artificially enhanced in certain locations of the Arctic Ocean and the northern North Atlantic. This represents a new approach to study the impact of sea ice on climate in the coupled system. Potential future applications of this approach are discussed in the outlook.
In the appendix of this thesis, preliminary results of a last millennium simulation with a comprehensive coupled Earth system model are presented. This simulation offers new opportunities to address a variety of research questions and will allow us to improve our understanding of past and future climate change.

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