In IPCC literature, anomalies—departures from a baseline—are generally calculated versus the 1850-1900 average value, known as the “pre-industrial” era. Global precipitation is harder, however, because there’s no proxy data for rainfall over the ocean. The best we can do is use proxy data from land and extrapolate over the ocean using atmospheric models. Today’s precipitation anomaly plot shows the 1980-2019 average relative to the 1981-2010 average.
Precipitation is a function of many factors, but mostly water vapor content in the atmosphere. Air’s carrying capacity for water vapor increases with temperature. With increased global temperatures, water vapor content is likely to increase, and therefore precipitation, but it isn’t always that simple.
Where heavy precipitation dries out the air, warmer
temperatures can result in extremely dry air following major precipitation
events. Moisture-laden air moving toward mountains is forced upward, cooling
adiabatically. The vapor which condenses into droplets falls as rain or snow,
leaving the air drier, so when it continues to move past the mountains and
return to lower elevations, the air is now too dry for condensation to occur.
This effect is called the rain shadow, and is reality on the lee (downwind)
side of almost every mountain range on earth.
As mentioned in yesterday’s post on salinity, increased
rainfall over mountainous and continental areas leads, without an influx of
fresh moisture, to significant rain shadows. The intensity of rainfalls, and
the severity of the subsequent desiccation of the air, are increasing. This is
a factor in the phenomenon called “global weirding”, where established weather
patterns are changing, and extreme events are becoming more common. So the
added moisture in the atmosphere is not necessarily leading to increased gentle
rainfall, but rather, increasingly intense, localized, frequently destructive
downpours, and extended dry zones.
Tomorrow: overview of COP 26.
Be brave, and be well.
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