Growing up in the south-west of the USA, I lived through the severe drought of the late 1970s. Then, drinking water at restaurants was given only on demand, and my eyes would burn from the smoke of regional forest fires. The nights were balmy, even in November.
At that stage I did not understand that these weather conditions were unusual – and perhaps a harbinger of what might become a new normal in a changing climate. Neither did I have any inclination that I’d grow up to study climate but it is these early experiences that have driven me towards my current research – figuring out how climate change affects relatively small areas.
We often hear in the news about increasing CO₂ levels and higher land and sea surface temperatures. But what most of us want to know – or need to know, in order to understand climate change within our own personal context – is how it will affect the weather, particularly rainfall, where we live. Will my region get wetter or drier? Will rainstorms become more or less intense?
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.
Extreme rainfall can occur in a matter of minutes and spread over several square kilometres. But though we have some information on large rainstorms from dense gauging networks and local radar installations, data on these heavy rains is not captured across most of the world, so it is hard to know whether they are becoming more or less frequent.
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.
So, with all this in mind, I set out to figure out how climate change would affect the weather of one drought-prone region.
Walnut Gulch
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.
We need to start framing climate change discussions in regional contexts, rather than limiting our perspective to global warming and CO₂ targets. Scientists and policy makers need to think carefully about the regional expression of rainfall, its impact on the water cycle (including implications for human society and ecology), and how precipitation will continue to evolve to the changing global climate system.
Punishing drought conditions have already occurred in recent decades across south-western USA, in Australia, and East Africa due to failed rains. Droughts make forests more vulnerable to pestilence and forest fires. They increase the price of food and threaten livestock too.
Those balmy November nights of my youth were caused by drought. My hope is that with new understanding of climate change in regional contexts, we can provide improved tools for people, organisations and governments to better manage water for human and ecological needs during increasingly frequent and prolonged droughts.