Time of emergence of extreme floods and droughts over the north-eastern United States

Natural climate variability is known to be an important source of uncertainty in climate risk assessment. This can pose a substantial obstacle to the implementation of adaptation strategies because it may mask the signal of climate change. In this study, the authors investigate how extreme flows in 133 catchments in the eastern and northeastern United States are affected by internal climatic variability. They evaluate the ratio of internal climate variability to anthropogenic climate change on projected future extreme streamflow using temperature and precipitation data from a single model initial-condition large ensemble (SMILE) at high spatial and temporal resolution. To better understand the role of internal climate variability and its impacts on the climate change signal, the authors use three different parametric and non-parametric tests to evaluate the time of emergence (TOE) of the climate change signal. The results are presented for three classes of catchment area: small (500 km²), medium (500-1000 km²), and large (>1000 km²). The findings suggest that the future intensity of both floods and droughts will gradually increase, with the expected increases in flood and drought signals being strongly influenced by catchment size. Small catchments are likely to see higher increases in flooding than the other catchment sizes, but weaker increases in extremely severe droughts. The size of the catchment also affects TOEs, with smaller catchments seeing earlier TOE for floods and later ones for droughts. These findings provide significant information on adaptation timelines.