Wee Ho Lim

Global floods and water availability driven by atmospheric rivers

Whilst emerging regional evidence shows that atmospheric rivers (ARs) can exert strong impacts on local water availability and flooding, their role in shaping global hydrological extremes has not yet been investigated. Here we quantify the relative contribution of ARs variability to both flood hazard and water availability. We find that globally, precipitation from ARs contributes 22% of total global runoff, with a number of regions reaching 50% or more. In areas where their influence is strongest, ARs may increase the occurrence of floods by 80%, whilst absence of ARs may increase the occurrence of hydrological droughts events by up to 90%. We also find that ~300 million people are exposed to additional floods and droughts due the occurrence of ARs. ARs provide a source of hydro-climatic variability whose beneficial or damaging effects depend on the capacity of water resources managers to predict and adapt to them.

The Predictability of Annual Evapotranspiration and Runoff in Humid and Nonhumid Catchments over China: Comparison and Quantification

AbstractClimate change and its potential threats on water security call for reliable predictions of evapotranspiration (ET) and runoff Q at different time scales, but current knowledge of the differences in their predictability between humid and nonhumid regions is limited. Based on spatially distributed catchments in China, the authors characterized their predictability and provided plausible explanations. Using the Budyko framework, it was confirmed that annual ET is predictable in nonhumid regions but less predictable in humid regions, and annual Q is predictable in humid regions but less reliable in nonhumid regions. The main cause of the varied predictability lies in the variation of water storage change ΔS in the water balance equation. It affects both the estimation and the variability of Q in nonhumid catchments more than that in humid catchments, which increases the challenge of predicting annual Q in nonhumid regions, while the opposite effect occurs in annual ET prediction between humid and non...

Global Freshwater Availability Below Normal Conditions and Population Impact Under 1.5 and 2 °C Stabilization Scenarios

Based on the large ensembles of the half a degree additional warming, prognosis, and projected impacts historical, +1.5 and +2 °C experiments, we quantify changes in the magnitude of water availability (i.e., precipitation minus actual evapotranspiration; a function of monthly precipitation flux, latent heat flux, and surface air temperature) below normal conditions (less than median, e.g., 20th percentile water availability). We found that, relative to the historical experiment, water availability below normal conditions of the +1.5 and +2 °C experiments would decrease in the midlatitudes and the tropics, indicating that hydrological drought is likely to increase in warmer worlds. These cause more (less) people in East Asia, Central Europe, South Asia, and Southeast Asia (West Africa and Alaska/Northwest Canada) to be exposed to water shortage. Stabilizing warming at 1.5 °C instead of 2 °C would limit population impact in most of the regions, less effective in Alaska/Northwest Canada, Southeast Asia, and Amazon. Globally, this reduced population impact is ~117 million people. Plain Language Summary This study emerges from the lack of scientific investigations to inform climate policy about differences between two global warming targets (i.e., 1.5 and 2 °C) for the “Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5°C.” We seek to understand the following: How would water availability below normal conditions (the drier end of hydrological extremes) change at these targets? Howwould they affect the water shortage of human society? Could we limit the impact by stabilizing the global warming at 1.5 °C instead of 2 °C? To address these questions, we employ the HAPPI (half a degree additional warming, prognosis, and projected impacts) experiments, explicitly designed to differentiate impacts between these targets. Relative to the historical period, future water availability below normal conditions (less than median, e.g., 20th percentile or lower) would decrease in the midlatitudes and the tropics; the globe and most of the regions would endure water shortages. Relative to the 2 °C warming target, stabilizing temperature increase at 1.5 °C would constrain adverse impact on people suffering water shortages in most of the regions (particularly Central Europe, East Africa, East Asia, South Asia, and West Africa) but ineffective in Alaska/Northwest Canada, Southeast Asia, and Amazon. A global sum of this reduced risk is ~117 million people.

Long‐Term Changes in Global Socioeconomic Benefits of Flood Defenses and Residual Risk Based on CMIP5 Climate Models

A warmer climate is expected to accelerate the global hydrological cycle, causing more intense precipitation and floods. Despite recent progress in global flood risk assessment, the socioeconomic benefits of flood defenses (i.e., reduction in population/economic exposure) and the residual risk (i.e., residual population/economic exposure) are poorly understood globally and regionally. To address these knowledge gaps, we use the runoff data from a baseline and 11 Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models to drive the Catchment-based Macro-scale Floodplain model incorporating the latest satellite river width information. From the simulated annual maxima, we use a Gumbel distribution to estimate the river water depth-flood return period relationship. We independently evaluate flood impacts on population and economy (i.e., gross domestic product) for a range of flood return periods. We estimate the socioeconomic benefits and the corresponding residual risk for the globe and 26 subcontinental regions. The global population (gross domestic product) exposed to flooding is ∼8% (∼7%) per year lower when implementing existing flood protection infrastructure extracted from the FLOod PROtection Standards database. If the current flood defenses were to be unchanged in the future (Representative Concentration Pathway 4.5 [RCP4.5] and RCP8.5, i.e., ∼2 to ∼4.3∘C above the preindustrial levels), the globe and most of the regions (particularly where developing countries are concentrated) would experience an increase in residual risk. This increase is especially obvious when the gap of climate forcing between RCP8.5 and RCP4.5 widens by the end of the 21st century. We finally evaluate the impact of changed flood defense levels on the socioeconomic benefits and the corresponding residual risk.