Event-to-event intensification of the hydrologic cycle from 1.5 °C to a 2 °C warmer world

Abstract

The Paris agreement was adopted to hold the global average temperature increase to well below 2 °C and pursue efforts to limit it to 1.5 °C. Here, we investigate the event-to-event hydroclimatic intensity, where an event is a pair of adjacent wet and dry spells, under future warming scenarios. According to a set of targeted multi-model large ensemble experiments, event-wise intensification will significantly increase globally for an additional 0.5 °C warming beyond 1.5 °C. In high latitudinal regions of the North American continent and Eurasia, this intensification is likely to involve overwhelming increases in wet spell intensity. Western and Eastern North America will likely experience more intense wet spells with negligible changes of dry spells. For the Mediterranean region, enhancement of dry spells seems to be dominating compared to the decrease in wet spell strength, and this will lead to an overall event-wise intensification. Furthermore, the extreme intensification could be 10 times stronger than the mean intensification. The high damage potential of such drastic changes between flood and drought conditions poses a major challenge to adaptation, and the findings suggest that risks could be substantially reduced by achieving a 1.5 °C target.

Figure 1

Global spatial map and zonal mean of the 2 °C minus 1.5 °C E2E multi-model ensemble mean of the HAPPI data (95% significant level is stippled). Black boxes represent the IPCC AR5 reference regions (http://www.ipcc-data.org/guidelines/pages/ar5_regions.html). Box-whisker plots show the area averaged (land only) difference between 2 °C and 1.5 °C climates (2 °C minus 1.5 °C) for the total precipitation (dP), frequency term (Frq), and intensity term (Int) during wet spells in mm per decade (in the middle panel) globally and over the East North America (ENA), Amazon (AMZ), South Europe/Mediterranean (MED), and East Asia (EAS) regions. The bottom panel is the same as the middle panel but for dry spells where dD is the total number of dry days and the unit is dry days per decade. Box-whisker plots indicate the 10th, 25th, 50th, 75th, and 90th percentiles, and the other points are outliers.

Figure 2

Global spatial map and zonal mean of the 2 °C minus 1.5 °C E2E multi-model ensemble 99th percentile (P99) of the HAPPI data (95% significant level is stippled). Black boxes represent the IPCC AR5 reference regions (http://www.ipcc-data.org/guidelines/pages/ar5_regions.html). Histograms and kernel density estimations (KDE; thick line) are of the area averaged (land only) E2E P99 distribution in each region for the historical (ALL) period and 1.5 °C (15) and 2 °C (20) future scenarios globally and over the East North America (ENA), Amazon (AMZ), South Europe/Mediterranean (MED), and East Asia (EAS) regions. The x-axis shows the E2E P99 values, and the y-axis shows the probability. ALL, 1.5 °C, and 2 °C scenarios are shown by green, blue, and red colors, respectively. The bin width of the P99 histograms was set to 0.1.