This lecture explains why net anthropogenic CO2 emissions need to be reduced to zero to halt the rise in global temperature, putting paid to the once-popular idea of “contraction and convergence” to a “sustainable” per-capita CO2 emission rate. We’ll also discuss how other climate pollutants behave, including methane, nitrous oxide and aerosols, and why the concept of “CO2-equivalent emissions”, although deeply embedded in almost all climate policy discussions, can be misleading.