365 Days of Climate Awareness 121 – Extreme Weather Events: Droughts

 

In chapter 11 of the Physical Science Basis report from IPCC’s Sixth Assessment (AR6-WG1), drought is defined as a “period of time with substantially below-average moisture conditions, usually covering large areas, during which limitations in water availability result in negative impacts for various components of natural systems and economic sectors.”

It further describes four types:

1)      Meteorological: precipitation deficit;

2)      Agricultural: crop yield reduction or failure, often related to soil moisture deficit;

3)      Ecological: plant water stress leading to mortality;

4)      Hydrological: water shortages in streams or storages such as lakes, reservoirs, lagoons and groundwater.

Factors contributing to drought, especially on a regional scale, are heat and moisture exchanges, and plant cover and physiology. The main driver, however, is lack of precipitation. Second behind that is evapotranspiration—the movement of liquid water in the ground and plants into the air as vapor. These can be difficult to quantify, however, and efforts to predict droughts have not met with much success.

They occur on the timescale of seasons to years, and on a spatial scale of hundreds to thousands of miles. Precipitation deficits have been recorded all over the world in recent years, but long-term drying trends in precipitation and soil moisture and in atmospheric capacity to absorb water vapor, though present in Africa and Asia, show very strong regional and seasonal variability.

It's difficult to attribute atmospheric droughts to human agency, but those related to land use can be.

Extreme events are one of the thorniest areas of climate change research: long-term trends, on a global or even regional scale, to singular events (even multi-year events such as droughts can be). Atmospheric and ground-based evidence from the past several decades shows little evidence for human causes for meteorological (loss of rain) droughts, but medium confidence in agricultural and hydrological droughts, where farming and groundwater consumption have depleted groundwater and fostered evaporative loss.

Tomorrow: extreme heat events.

Be brave, and be well.