In the context of climate change and possible responses to the risk associated with it, three variables strongly influence the levels and growth of greenhouse gas emissions: population, income (measured as per capita gross domestic product GDP]), and intensity of emissions (measured as tons of greenhouse gas emissions per million dollars of GDP). (Population) x (per capita GDP) x (Intensityghg) = Emissionsghg This is the relationship for a given point in time; over time, any effort to change emissions alters the exponential rates of change of these variables. This means that the rates of change of the three left-hand variables, measured in percentage of annual change, sum to the rate of change of the right-hand variable, emissions.