 On a standard characterization, Earth’s climate system is the complex, interactive system consisting of the atmosphere, hydrosphere, cryosphere, lithosphere and biosphere (IPCC-glossary: 1451). Some definitions appear to be narrower, limiting the scope of the climate system to those aspects of the atmosphere, hydrosphere, etc. that jointly determine the values of paradigmatic climate variables, such as average surface temperature and rainfall, in response to external influences (see, e.g., American Meteorological Society 2017a). In either case, it would seem that some human activities, including those that release greenhouse gases to the atmosphere and those that change the land surface, are part of the climate system. In practice, however, climate scientists often classify such human activities as external influences, along with volcanic eruptions (which eject reflective aerosols and other material into the atmosphere) and solar output (which is the primary source of energy for the system) (see IPCC-glossary: 1454). Classifying human activities as external to the climate system seems to be a pragmatic choice—it is easier, and a reasonable first approximation, to represent anthropogenic greenhouse gas emissions as exogenous variables in climate models—though it may also reflect a deeper ambivalence about whether humans are part of nature. 