Our Planet Climate and Geopolitics
On Wednesday more than seven inches of rain fell in Bermuda. On Tuesday up to 15 inches were forecast for parts of Cuba. Last month at least 16 people died in floods in Poland, Austria, Hungary, Romania and the Czech Republic. Last year was the seventh wettest on record for the UK, where five of the ten wettest years since 1836 have been recorded in this century. Across the northern hemisphere, sodden citizens are wondering why there's so much rain. They are entitled to complain, but not to be surprised. Since at least the 1830s it has been known that hot air can hold more moisture than cold. In the mid-19th century, as diligent Sensemaker readers know, the precise relationship between water surface temperature and the water content of the air above it was established by Émile Clapeyron (a French railway engineer) and Rudolf Clausius (a German physicist). Since then, industrialisation has fuelled global warming and raised sea surface temperatures (SSTs) and atmospheric water content to their current prodigious levels. There's more water in the air, and what goes up tends to come down in the end. Hence all this rain.
The standard meteorological application of the Clausius-Clapeyron equation states that for every 1 degree Celsius increase in average SSTs the moisture content in the atmosphere goes up exponentially, by 7 per cent.
US satellite measurements point to an overall SST increase since the 1970s of about 0.6 degrees, which according to Dr Kevin Trenberth of the US National Center for Atmospheric Research has put at least an extra 500 cubic kilometres of water in the air. That, he says, is one Lake Erie or more than three Dead Seas. But he's being conservative; alternative SST data series suggest an overall increase of 0.9 degrees since pre-industrial times, equivalent to an extra 750 cubic kilometres of water – five Lake Tahoes, 20 Loch Nesses or 2,380 Lake Windermeres.
The 2021 science update to the UN-backed IPCC report on climate change mentioned the Clausius-Clapeyron equation 36 times and forecast 1 to 3 per cent more global precipitation as a result of a one-degree atmospheric temperature increase. That corresponds with eerie accuracy to the extra Lake Windermeres. The moisture hangs around – literally – as clouds, which reflect some solar radiation back into space but also contribute to global warming. The result is a positive feedback loop in a negative sense: hotter oceans, warmer air, more water vapour – and a lot more rain.
