wet gets wetter, dry gets drier" (WWDD) paradigm.

The "wet gets wetter, dry gets drier" (WWDD) paradigm describes an expected pattern under global warming: regions that are already relatively wet are projected to become wetter, while dry regions are expected to become drier. This concept, introduced by Held and Soden (2006), is grounded in the idea that rising atmospheric temperatures increase the capacity of air to hold moisture, intensifying the global hydrological cycle123.

Scientific Basis

• Mechanism: As the atmosphere warms, it can hold more water vapor, leading to increased precipitation where atmospheric moisture converges (typically wet, tropical regions) and increased evaporation where it diverges (typically dry, subtropical regions)23.

• Ocean Salinity Evidence: Changes in ocean salinity over the past 60 years support this paradigm—regions with more rainfall and river outflow have become less salty, while regions with more evaporation have become saltier4.

Observational and Model Evidence

• Global Trends: Observations and models generally show that wet regions (e.g., Northern Europe, tropics) are getting wetter and dry regions (e.g., subtropical drylands) are getting drier, though the rate is sometimes slower than initially predicted54.

• Magnitude: Over the last 60 years, wet and dry regions have each changed by about 2%. If global warming exceeds 3°C, wet regions could get more than 10% wetter and dry regions more than 10% drier4.

• Land vs. Ocean: The WWDD pattern is most robust over oceans and at broad latitudinal scales. Over land, especially at regional or local scales, the pattern is less consistent—only about 10–15% of global land areas strictly follow the WWDD trend, with significant portions showing the opposite or no clear trend123.

• Regional Variability: While wet areas often see increased extreme rainfall and flooding, dry areas face more frequent and intense droughts5.

Limitations and Challenges

• Data and Definitions: The proportion of the planet that follows the WWDD paradigm depends heavily on the datasets and definitions used; studies report that only 11–41% of global land confirms the pattern, while 10–35% shows the opposite12.

• Breakdown at Regional Scales: The WWDD mechanism tends to break down at smaller, regional scales due to complex atmospheric dynamics and land-surface feedbacks3.

• Future Projections: The percentage of land area supporting the WWDD paradigm may decrease under higher emission scenarios, with both WWDD-confirming and -opposing regions expanding1.

"As the Earth warms up, the water cycle has begun to intensify in a 'wet-gets-wetter-dry-gets-drier' pattern. This means more and more freshwater is leaving dry regions of the planet and ending up in wet regions... This shift is already happening." 5

In summary: The "wet gets wetter, dry gets drier" paradigm is a useful generalization for understanding large-scale changes in the global water cycle due to climate change, especially over oceans and broad regions. However, its application to land areas is more limited and complex, with substantial regional variability and exceptions123.

1. https://hess.copernicus.org/articles/26/6457/2022/
2. https://www.proquest.com/docview/2586475765
3. https://dspace.mit.edu/bitstream/handle/1721.1/138942/weidman-sweidman-sb-eaps-2021-thesis.pdf?sequence=1&isAllowed=y
4. https://www.business-standard.com/article/news-ians/climate-change-widening-world-s-wet-dry-region-divide-116121300373_1.html
5. https://www.weforum.org/stories/2022/03/climate-change-fresh-water-cycle-global-warming-oceans/
6. https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/joc.5863
7. https://www.extension.iastate.edu/agdm/articles/hof/HofFeb23.html
8. https://ui.adsabs.harvard.edu/abs/2014EGUGA..16.3914G/abstract
9. https://www.nature.com/articles/srep18018
10. https://www.pure.ed.ac.uk/ws/portalfiles/portal/30234127/30234079._PFV._Polson._Hegerl..pdf