In its special report on global warming of 1.5°C the Intergovernmental Panel on Climate Change (IPCC) concluded that “Human  activities  are  estimated  to  have  caused  approximately  1.0°C  of  global  warming  above  pre-industrial level”. Considering the significant uncertainties that are inherent to every scientific analysis of such a complex system as the global climate, this lecture highlights the way the IPCC works to come to such conclusions.