Wilder rainfall swings make record storms more likely
Future record-shattering daily downpours become much more probable because extreme-rain variability grows, not only because the mean inches up.
Source
Increasing extreme precipitation variability plays a key role in future record-shattering event probability
Study at a glance
- Design
- Computational / modelling — 100-member CESM2-LE analysis of record-shattering daily extremes under SSP3-7.0
- N
- N=100 · 100-member CESM2 large ensemble (other CMIP6 ensembles as checks)
- Population
- Land daily precipitation extremes in CESM2 large-ensemble climate projections
- Outcome
- Record-shattering extreme-rain probability driven by growing variability vs mean shifts
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Wilder rainfall swings make record storms more likely
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A record-shattering margin is at least one pre-industrial standard deviation from 1850–1949.
In this work, record-shattering events are defined as events that break, indeed shatter, the historical maximum value (record) by a margin of at least one pre-industrial standard deviation, defined as the mean standard deviation of the period 1850–1949.
A GEV fit to the prior 71 years shows the event exceeded the 1000-year return level.
This event is heavier than any other observation by a large margin, and a generalised extreme value (GEV) distribution fitted to the region-averaged data in the 71 years preceding, but not including, the event shows that the event exceeded the 1000-year return level.
What they did
Using the 100-member CESM2 large ensemble under SSP3-7.0, the authors tracked 5-year block-maximum daily precipitation (5yRx1d) that beat each member’s own record by at least one 1850–1949 standard deviation, compared that with a stationary climate, and split mean versus variability contributions; other CMIP6 ensembles provided a check.
What they found
Record-shattering 5yRx1d probability rises over most land. In some tropics the late-21st-century probability ratio reaches 15. Variability (σ) changes dominate CESM2-LE in most regions, though other CMIP6 models have more balanced mean and variability terms. A CESM2 event physically resembles the 2021 western European floods, which exceeded a 1000-year return level in observations.
The limits
What it doesn't show
CESM2 has high climate sensitivity and SSP3-7.0 is a high-warming path, so ratios may be upper-end. Extreme-precipitation statistics remain uncertain, observations are short, and tropical mean and variability are still underestimated in models.
Key terms
- Record-shattering event
- A new extreme that exceeds the previous record by at least one pre-industrial standard deviation (1850–1949).
- 5yRx1d
- The maximum 1-day precipitation in each 5-year block, used to sample rare storms.
- Probability ratio
- Record-shattering probability under climate change divided by that in a stationary pre-industrial-like climate.
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Record-shattering here means beating the old record by at least:
Common questions
Why isn’t a rising mean enough to explain more shattered records?
Decomposition shows σ (variability) contributions exceed μ (mean) in most CESM2-LE regions; wider tails produce larger record jumps.
Did the 2021 European flood look like a model record-shatterer?
Yes: E-OBS Rx1d beat a 1000-year GEV level, and a CESM2-LE analogue had similar moisture-transport and cyclone structure.
Are other CMIP6 models consistent?
Zonal-mean probability-ratio correlations with CESM2-LE are positive but moderate (0.44–0.76 in summer); CESM2’s σ term is unusually strong.
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