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
Lecturer in Physical Geography (Hydrology and Geomorphology), Cardiff University
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.
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.
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.
The rainfall study quoted by Michael Singer is a 2011 Trenberth study;
Changes in precipitation with climate change
Kevin E. Trenberth*
National Center for Atmospheric Research, Box 3000, Boulder, Colorado 80307, USA
ABSTRACT: There is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere. Hence, storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones, supplied with increased moisture, produce more intense precipitation events. Such events are observed to be widely occurring, even where total precipitation is decreasing: ‘it never rains but it pours!’ This increases the risk of flooding. The atmospheric and surface energy budget plays a critical role in the hydrological cycle, and also in the slower rate of change that occurs in total precipitation than total column water vapor. With modest changes in winds, patterns of precipitation do not change much, but result in dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in the mid- to high latitudes: the ‘rich get richer and the poor get poorer’. This pattern is simulated by climate models and is projected to continue into the future. Because, with warming, more precipitation occurs as rain instead of snow and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in summer, especially over continental areas. However, with more precipitation per unit of upward motion in the atmosphere, i.e. ‘more bang for the buck’, atmospheric circulation weakens, causing monsoons to falter. In the tropics and subtropics, precipitation patterns are dominated by shifts as sea surface temperatures change, with El Niño a good example. The volcanic eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and to widespread drought, as precipitation shifted from land to oceans and evaporation faltered, providing lessons for possible geoengineering. Most models simulate precipitation that occurs prematurely and too often, and with insufficient intensity, resulting in recycling that is too large and a lifetime of moisture in the atmosphere that is too short, which affects runoff and soil moisture.
Michael Singer worries that less intense but more frequent rain will reduce available runoff. But the effect of more frequent rainfall on arid regions in a lot of cases is likely to be profound. Intense one off rain events mostly drain away – parched soil often can’t absorb water until it has been soaked repeatedly by rain. But more frequent longer lasting rainfall, in many regions, should make it possible for grasslands and even trees to establish in formerly extremely arid regions. Vegetation tends to establish and protect its own soil moisture – providing there is some moisture available to protect.