365 Days of Climate Awareness 124 – Extreme Events: Floods

Floods are linked closely to rainfall, so increased precipitation generally implies increased flooding activity. Extreme rain events overwhelm natural and manmade drainage systems and result in flash floods (including a variety known as “urban” floods, where buildings and impermeable street surfaces become riverbeds when sewers are at capacity). The changing global climate has also changed river flow patterns, with increase in some regions of the world and decrease in others, and a trend toward earlier peak runoff from spring snow melt.

There are several varieties of flood, including:

·           --Pluvial flood (rain event)

·           --Flash flood (caused usually by rain; a rapid overwhelming of local drainage)

·           --Groundwater flood;

·           --Surge flood (storm surge)

·           --Coastal flood (tidal and wind-driven)

Elements of a coastal flood.

With rising sea levels, ocean-related (surge and coastal) floods have becoming increasingly frequent and hazardous since the 1950’s. As with droughts, regional dynamics play a significant role in generating flood events. Furthermore, plant use of water also becomes more efficient at higher CO₂ levels, reducing evapotranspiration and serving as a negative feedback on precipitation and flooding. For these and other reasons, modeling regional flood trends into the future can’t be done with great confidence.

The same applies to pluvial and fluvial floods. Regional weather patterns influence amounts and severity of snow and rainfall, and reduce our predictive ability on the regional scale. The frequency of torrential downpours and floods have increased over the past several decades. NOAA reported 25 separate 500-year floods from 2010 to 2017, and in recent years, a growing number have occurred well away from coasts and rivers, as rainfall increasingly overwhelms natural and manmade drainage systems. Given the current warming rate of 0.32°C per decade, this trend is expected to increase in years to come.  

Precipitation trends, 1950-2008. Tropics are drying and temperate zones are growing wetter.

Floods are yet another type of extreme event which cannot be reliably correlated, on an individual basis, with the overall trend of greenhouse gas gains and global warming, though the broad correlation is very well established.

Tomorrow: cyclones.

Be brave, and be well.