Guest essay by Eric Worrall

Lecturer in Physical Geography Michael Singer has noticed that climate hasn’t brought greater precipitation, as Trenberth predicted it would – but he still worries about the impact of climate change on watersheds.

How understanding regional rainstorms will help the world manage climate change

December 8, 2017 2.03am AEDT

Michael Singer

Lecturer in Physical Geography (Hydrology and Geomorphology), Cardiff University

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There is a theory in physics that tells us that a warmer atmosphere can hold more moisture (~7% more per 1°C increase), so we might expect that places with increasing temperatures will experience more water evaporation from the land, and also experience heavier rainfall. But we don’t have great evidence of more intense rainfall for many places across the world, even though the upward temperature trends are compelling for much of the globe.

It is actually very difficult to observe trends in rainfall, because we often rely on data and model outputs that are at the wrong scales. Global rainfall datasets and output from climate models are typically resolved on timescales of days or months and at spatial scales larger than most river basins.

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In general, scientists have had a poor understanding of how a warming climate will affect the magnitude, timing, and spatial patterns of rainfall. Yet these aspects of the climate system are fundamental to assess the sustainability of water resources and even flood risks, especially in drier parts of the globe.

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Several years ago, I came across a rich dataset on rainstorms for a place called Walnut Gulch, a watershed – an area of land that separates waters flowing into different rivers – near the city of Tombstone in south-eastern Arizona. The US Department of Agriculture has been collecting detailed information about every single storm that occurred from 1954 until the present day at 85 separate gauging locations. We already knew temperatures had been rising here, increasing by ~2°C in a matter of decades. And this trove of rainfall data enabled us to examine whether there were trends in rainstorms that corresponded to the rising temperatures.

We were surprised to find that even while total rainfall slightly increased over this period and more rainstorms occurred over time, each storm was less intense and lasted longer. This means that less rainwater has run off the landscape into rivers since the 1950s, so more of the water from the sky has returned to the atmosphere and less of it contributed to regional water resources.

In other words, the theory which predicts heavier (more intense) rainfall due to warming does not hold for this region. We believe it breaks down here and in other dry environments because there is not enough moisture in the landscape to evaporate and satisfy the higher demand of the atmosphere. Our findings also suggest that water resources in this desert region may become increasingly strained due to changes in the regional climate.

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