Scientists all over the world regularly assess the rich body of scientific literature, contributing to an ever-strengthening understanding of how the climate system works, and how it is changing in response to human activity.

The scientific topics assessed by Working Group I include: greenhouse gases and aerosols in the atmosphere; temperature changes in the air, land and ocean; the hydrological cycle and changing precipitation (rain and snow) patterns; extreme weather; glaciers and ice sheets; oceans and sea level; biogeochemistry and the carbon cycle; and climate sensitivity. The Working Group I assessment combines observations, palaeoclimate, process studies, theory and modelling into a complete picture of the climate system and how it is changing, including the attribution (or causes) of change.

The Working Group I assessment provides scientific information relevant for the global community to meet the challenge of climate change. As well as the global scale, Working Group I looks at variability and changes happening at a regional level, which is closely tied to how impacts and risks to human and natural systems are changing over time. This is, in turn, relates to the assessment of Working Group II and the need to design effective climate change adaptation policies. Other important policy dimensions of Working Group I are the assessment of the amount of carbon emissions compatible with climate and energy targets; interactions between land and the climate; and links between climate and air quality. All of these aspects are closely connected with the assessment of Working Group III and strategies to mitigate against the impacts of climate change.