We offer simulations that help to understand the relationship between GHG emissions and concentrations, and the relative role of long-lived (e.g., CO2) and short-lived (e.g., CH4) emissions. We show that, absent technologies to remove CO2 from the atmosphere, the 350 CO2 ppm target is out of reach in this century, even if all emissions drop to zero almost immediately (i.e. in 2015). A 350 ppm CO2-equivalent target is potentially achievable, but would require CH4 concentrations falling below preindustrial levels, and thus elimination of emissions from human activities such as rice and livestock agriculture. More realistically, even some of the most aggressive targets proposed through 2035 would lead to concentrations (CO2 or CO2-eq) in the 415–450 ppm range. This is only feasible if after 2035 emissions continued a downward path toward zero. Only in these cases would the temperature target of no more than 2 °C above preindustrial be achieved, and only after peaking above that level before declining.