The researchers also highlight another result, which has implications for numerical simulations of climate change. By using a pair of experiments, both of which explore the climate impacts of black carbon and differ only in whether black-carbon changes can also adjust to atmospheric-circulation responses, Sand et al. demonstrate the role of the two-way black carbon–atmosphere interactions in driving the full climate response. Their findings are unexpected because these interactions seem to be the dominant cause of the climate response to changes in black carbon. The change in global surface temperature varies by a factor of two between the two experiments, with considerably larger differences at altitude. Indeed, many rainfall responses appear only when feedbacks of black carbon-to-atmosphere-to-black carbon are included.