The time lag between a carbon dioxide emission and maximum warming increases with the size of the emission

In a recent letter, Ricke andCaldeira (2014Environ. Res. Lett. 9 124002) estimated that the timing between an emission and themaximum temperature response is a decade on average. In their analysis, they took into account uncertainties about the carbon cycle, the rate of ocean heat uptake and the climate sensitivity but did not consider one important uncertainty: the size of the emission. Using simulations with an Earth SystemModel we show that the time lag between a carbon dioxide (CO2) emission pulse and themaximumwarming increases for larger pulses. Our results suggest that as CO2 accumulates in the atmosphere, the full warming effect of an emissionmay not be felt for several decades, if not centuries.Most of thewarming, however, will emerge relatively quickly, implying that CO2 emission cuts will not only benefit subsequent generations but also the generation implementing those cuts. In a recent letter, Ricke and Caldeira [1] estimated that the time lag between a carbon dioxide (CO2) emission and the maximum warming response is a decade on average. This is an important finding as it indicates that the full climate damages expected to occur in response to a CO2 emission will already be felt by the generation responsible for those emissions. Conversely, the relatively short response timescale implies that CO2 emission cuts implemented today have the potential to influence the rate of warming in the short term. Thus, their finding corroborates the notion that the rate of warming over the next decades is not inevitable, butwill bedetermined by futureCO2 emissions [2]. A range of previous studies has explored the warming commitment of past CO2 emissions [3–9]. A robust finding of these studies is that the CO2-induced warming persists for many centuries. The novel aspect of the letter by Ricke and Caldeira [1] (henceforth referred to as R&C) is its focus on the time lag between a CO2 emission and the resulting maximum warming response. In their study, R&C used simple representations of the carbon cycle and the physical climate system. The carbon cycle was simulated by impulse response functions derived from the response of a range of Earth System Models to a 100 GtC emission pulse [10]. The physical climate system was represented by onedimensional diffusion or two-box models fitted to Coupled Model Intercomparison Project phase 5 (CMIP5) simulations with an abrupt quadrupling of atmospheric CO2. Combinations of these models made it possible to sample 6000 realizations of the coupled climate-carbon cycle system. For a 100GtC pulse of CO2 released into the atmosphere with a background CO2 concentration of 389 ppm, R&C found the median time between an emission and maximum warming to be 10.1 years, with a 90%probability range of 6.6–30.7 years. A pulse emission of CO2 results in an abrupt increase in atmospheric CO2 concentrations, followed by a slow decline as CO2 is taken up by the ocean and terrestrial biosphere. The radiative forcing associated with the CO2 increase causes warming, but the warming is delayed due to the long timescales of ocean heat uptake. Initially, the ocean takes up a large amount of heat, but this heat uptake decreases over time, allowing the atmosphere to warm. The timing of peak warming is determined by the balance between two opposing processes: the decline of radiative forcing of atmospheric CO2 following the pulse emission (which has a cooling effect on the atmosphere), and the decrease of ocean heat uptake (which has awarming effect). OPEN ACCESS