Changes in water availability point to humans as the driver of global climate change

The word “drought” conjures images of farmers praying for rain, of brown, lifeless suburban lawns, or urban residents rationing water. Recent years have seen an intensification of dry spells, and climate change is often cited as the cause. Teasing apart changes in global moisture patterns due to long-term climate variability from human driven climate change is the task of Dr. Ryan Padrón and a team of global climate scientists.  

Throughout Earth’s long history, droughts have been a normal part of climate variation. They are generally caused by slight changes in the atmosphere or oceans that would tip global moisture balance briefly towards drier conditions for a few weeks to months. However, scientists in the twenty-first century are witnessing new causes of droughts, such as seasonal changes in snowpack, human over-use, or mega-droughts that can last decades. Explaining variation in global moisture balance will shed light onto how and why droughts have been increasing in frequency and severity. This understanding will be important for advancing climate mitigation policies that address the increasingly common, negative drought-caused impacts on communities worldwide. 

In a study recently published in Nature Geoscience, Dr. Padrón and his team investigated the drivers in the global moisture balance through evaluation of changes in precipitation (P) and evapotranspiration (ET). These two processes transfer water between the atmosphere and land. Precipitation brings water to Earth’s surface in the form of rain and snow. Evapotranspiration, the process of evaporation combined with plants losing water from their tissues and leaves, cycles moisture back to the atmosphere. The resulting transaction reads like any other budget. “Land available water” is the amount left after ET is subtracted from precipitation (i.e. P – ET).  

In their study, Padrón and colleagues use historical records and models to determine the causes of changes to land available water over the past hundred years. Using these historically based models, the research team compared the reference period of 1902 to 1950 with the period of 1985 to 2014 to determine the actual, observed changes in available water. The initial model showed a decrease of water availability across 57 percent of global land area, supporting the theory that dry seasons have been getting drier throughout the century. With these estimates of actual moisture change from the past century, the researchers used climate models to investigate if human-influence is a potential driver for these changes in water availability. By comparing models that include and exclude human-influences, the research team found that the models that included human-caused climate change most closely matched the observed changes in water availability seen across the world. In contrast, the models that did not include human influence did not match the historic observational records. 

These findings are significant because they show that current patterns in water availability are driven by humans – not just by natural variability in the climate system. This study supports previous research that indicates human actions over the past century have pushed the climate system beyond natural variability. With the growing threat of droughts, it will be important to redesign current water management techniques to address this exacerbated human influence on water scarcity and subsequent negative impacts on communities. Both in the United States, and worldwide, this reframing of climate impacts in terms of water resources strengthens the case for policy makers to implement swift and comprehensive climate measures.