Disentangling Drivers and Patterns of European Climate Change

Abstract

Combined observations of the earth’s atmosphere, oceans, and land surface show a clear change in the global climate system over the past 100 years. The planet’s continuously rising temperature, which is shown by these observations, is predominantly driven by persistent human greenhouse gas emissions. The global warming trend is expected to continue throughout the 21st century as long as greenhouse gases accumulate in the atmosphere. Increased global temperatures threaten humans, life, and ecosystems on the planet by potentially triggering illnesses such as heat strokes, food insecurity, biodiversity loss, and natural hazards. In regions where precipitation is decreasing together with increasing temperature, temperature-related hazards are amplified and additional hazards connected to water scarcity arise. Model simulations of the future climate show substantial regional variations in both temperature and precipitation changes. It is critical to gain further confidence in such projected spatial climate change patterns, especially for vulnerable regions, where there is a risk of amplified warming or strong changes in precipitation. To gain further confidence, there is a need to understand which changes in the climate system cause these spatial patterns. Scientific confidence in the projected patterns is essential for developing effective regional strategies to mitigate and adapt to climate change. The goal of this thesis is to determine the causes of spatial patterns of future temperature and precipitation changes in climate simulations. The focus is on prominent patterns of climate change in Europe, namely on the projected amplified summer warming pattern in the Mediterranean and the projected year-round precipitation decline in the same region. We find that the amplified Mediterranean summer warming is predominantly caused by spatially different lapse-rate changes. Lapse-rate changes are substantial throughout the continent but weakest in the Mediterranean. The influence of Hadley circulation changes on the European summer climate is found to be negligible. The Mediterranean summer precipitation decline can largely be explained by thermodynamic changes including warming contrasts between land and ocean, lapse-rate changes, and changes in atmospheric humidity. Circulation changes further contribute to the summer precipitation decline but are of secondary importance. The situation in winter is different, where circulation changes and connected changes in the atmospheric state are the main cause for the precipitation decline in the Mediterranean. Our results also indicate that lapse-rate changes are not exclusively important for the pattern of summer land temperature changes in Europe but also in many other mid-latitude and tropical regions on the northern and southern hemispheres during summer. The connection of lapse-rate changes to amplified summer warming patterns increases the confidence in the regional projections since lapse-rate changes are a consequence of the well-understood higher moisture-holding capacity of warmer air. The confidence in the Mediterranean precipitation decline should be considered larger in summer than in winter since the influence of uncertain circulation changes is smaller in summer compared to winter.