evidence: But scientists often use a different measure, known as "equilibrium climate sensitivity". This is defined as the global mean warming caused by a doubling of pre-industrial carbon dioxide (CO₂) levels in the atmosphere. [...] But the predicted range of rising temperature has remained stubbornly wide, somewhere between 2°C and 5.5°C of warming, as assessed in several generations of reports issued by the Intergovernmental Panel on Climate Change. This is despite concerted efforts to narrow it down. [...] As highly reflective ice melts on land or sea, the underlying surface is exposed and less sunlight reflected back into space. This increases the amount of warming for a given amount of greenhouse gases. It’s what scientists refer to as a positive feedback loop.