Reto Knutti ETH @Knutti_ETH Passionate about climate, how the world and society work. Professor for climate physics and assoc VP for sustainability @ETH_en Zurich, president @proclimCH May. 23, 2018 1 min read

Experienced "Applied climatology" today...
2pm: writing about increased risk from extreme events for new Swiss climate change scenarios
7pm: fighting water in my basement after heavy thunderstorm

Bad luck? Unlikely. Heavy rainfall intensity and frequency has increased at >90% of all stations in Switzerland. Almost all other data sets worldwide agree.

Models robustly predict an increase in heavy rainfall frequency and intensity, even in regions that overall get drier. Models in fact agree better on heavy rainfall.

Why? Basic physics. Warmer air can hold more water. All other things equal, the same thunderstorm will bring more water in a warmer world. Argues Clausius in 1834, Clapeyron in 1850.

Early models showed this many decades ago. 
Of course not all other things are equal, but to first order the argument still holds.

Bottom line 1: mean precipitation is difficult because it's a mix of thermodynamics and dynamics. But precipitation variability robustly increases in most places, from seasonal to daily timescales 

Bottom line 2: Heavy precipitation intensity and frequency increases in most places, and has already been linked to human-induced warming. Damage is typically nonlinear, so every °C disproportionately increases the potential risk. 

You can follow @Knutti_ETH.


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