Does the pace of carbon emissions matter in an atmospheric general circulation model?

Abstract. The concentration of carbon dioxide in the atmosphere changes our climate and its variability. It impacts all parts of nature and society. Consequently, there is an ongoing societal discussion about speeding up the transition to net-zero carbon emissions. The faster emissions are reduced, the less carbon dioxide will accumulate in the atmosphere where it largely remains influencing climate for hundreds of years. What has not yet been broadly studied, is the question whether the rate of the emissions themselves in addition to the resulting concentration has significant impact on climate and weather. To address this question, we run simulations with the Geophysical Fluid Dynamics Laboratory Atmospheric Model version 2 (GFDL-AM2), with different CO₂ forcing rates. In order to investigate mainly the atmospheric role, the oceanic boundary condition is supplied by a slab ocean. We find that for the the same warming level (2 °C) but different warming rates (e.g. annual rates of 1 % CO₂ increase compared to 4 %), the differences in the annual average temperature and precipitation and day-to-day variability patterns are of the same order of magnitude between different simulations with the same rate and between different simulations with different rates. Thus, we find that without a significant influence of ocean circulation changes, the fast mixing times within the atmosphere and thereby the lack of an atmospheric memory, inhibits a significant role of the rate of CO₂ emissions for weather variability. This result is not unexpected, but needed confirmation. In summary, the atmospheric dynamics alone do not allow for an influence of the rate of carbon emissions on the annual average and day-to-day variability in temperature and precipitation.