Assessing the observed impact of anthropogenic climate change

Assessing the observed impact of anthropogenic climate change Gerrit Hansen Global climate change is unequivocal, and greenhouse gas emissions continue rising despite international mitigation efforts. Hence whether and to what extent the impacts of human induced climate change are already being felt around the world is a timely question. The thesis assesses the observed impact of anthropogenic climate change across systems, sectors and world regions. A clear framework for impact attribution studies is laid out and applied to several examples from the literature. The development of the evidence of climate change impacts over the last two decades is documented, and an overview of the status of knowledge is provided with a focus on recent trends in human and managed systems. The role of human influence, compared to natural variability, for climate related impacts is assessed for a large range of individual observations that have been reported in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report. The analysis is based on a novel method that explicitly considers information on the quality and appropriateness of observational and model data for each observation. The thesis discusses caveats and challenges in attributing observed effects to climate change, and points out the consequence of those limitations for scientific policy advice. Key issues concern the question whether extreme weather events and their impacts can be attributed to human induced climate change and the limited availability of long-term monitoring records in many vulnerable regions. The impact of anthropogenic climate change is confirmed for a broad range of natural system effects, and to a lesser degree for human systems. Confident conclusions are mostly limited to direct temperature effects while precipitation effects remaining more uncertain.