 Because IPCC reports usefully synthesize a huge body of scientific research, they will be referenced frequently in what follows. The discussion will focus primarily on questions about the physical climate system, rather than questions about the impacts of climate change on societies or about climate policy options; the latter, which are also addressed by the IPCC, call for expertise that extends well beyond climate science (see IPCC 2014).  Section 5.1  discusses detection and attribution research, which is concerned with the following questions: In what ways has Earth’s climate changed since pre-industrial times? How much have human activities, especially human emissions of greenhouse gases, contributed to the changes?  Section 5.2  considers the projection of future climate change, with special attention to debates about the interpretation of model-based projections.  Section 5.3  provides a brief overview of several recent controversies related to the methods and conclusions of climate change research. Finally,  Section 5.4  notes some of the ethical questions and challenges raised by the issue of anthropogenic climate change.