Picture (above): Desert road. Credit: Fares Nimri (Unsplash)

The world is widely expected to become more arid as temperatures increase with climate change. This prediction is largely based on a single metric, the Aridity Index, which is commonly used to understand future changes in aridity. The Aridity Index describes the balance of how much water is gained from rainfall and lost via evaporation.

As temperatures increase, the atmosphere can hold more moisture and this can, in theory, lead to higher evaporation as the atmospheric “demand” for water increases. The Aridity Index predicts that water losses from evaporation will outweigh increases in rainfall, leading to increasing aridity and expansion of deserts worldwide.

Yet, over past decades satellites have observed greening of the Earth’s vegetation while precipitation and river flows increase in many regions. CLEX researchers and colleagues set out to understand this apparent contradiction, also known as the “aridity paradox”.

Aridity describes the amount of moisture available to sustain and promote life but, in practice, it can be defined in many ways. The Aridity Index only considers one aspect of aridity – how dry the atmosphere is, and predicts drier atmospheres worldwide. However, many of us understand aridity more widely as the abundance of water and the life it supports on the land.

When the researchers analysed climate model predictions of rainfall, runoff and plant productivity that more directly reflect landscape dryness, a different picture emerged. Globally climate models predict higher rainfall, runoff and plant photosynthesis globally, suggesting a wetter future. Regionally, both increases and decreases are predicted. For Australia, climate models suggest possible increases in runoff and plant productivity despite declining rainfall.

These findings highlight the much more nuanced nature of future aridity changes than the widespread drying predicted by the Aridity Index. The researchers concluded that the Aridity Index was too simplistic to capture the many aspects that define landscape aridity, including the amount of rainfall, water resources and vegetation productivity, and is a poor indicator of future aridity changes.