Spanish and Portuguese researchers have produced some of the strongest evidence yet that warming climate is making droughts more severe. Sergio Vicente-Serrano from the Pyrenean Institute of Ecology (IPE) in Zaragoza and his colleagues have used detailed data from their countries to overcome uncertainties seen in worldwide studies. They have shown that a local warming of 1.5°C from 1961-2011, and 2.1°C in summer months, and rainfall that has decreased by around a sixth increased drought severity in the region. “Future scenarios in the Iberian Peninsula and southern Europe indicate an increase of temperature even more than 3°C for the 21st century,” Sergio told me. “If we have already observed an important decrease of water resources, you can imagine that in the future water resources in these regions will be at higher risk.”
Air holds and ‘demands’ more water as it gets warmer, which is a fundamental reason for why we might expect both worse droughts and heavier rainfall with climate change. Scientists have already used real-world measurements to look at global changes in drought severity. However, they have disagreed on whether things really have got worse in recent years or not. Sergio stressed that such worldwide research faces important limitations. He emphasised that evapotranspiration – the water released by Earth’s surface and by plants breathing – is important in drought studies. But it has to be worked out from a combination of direct measurements, and the records needed are patchy in areas like Africa or South America.
“I’m very critical of the conclusions of these kinds of global studies, not about the methodology, but the input data,” Sergio said. “The problem is the use of highly uncertain variables. There are problems with precipitation data sets in terms of density of observations. The problems for precipitation are much higher for variables that are necessary to estimate the water demand of the atmosphere. Estimating these kinds of variables with confidence is really difficult. Also, there’s no validation in terms of impact on crop production, stream flows, reservoirs, soil moisture, this information is not available. That’s really the approach that must be followed to determine if drought is increasing in severity and impact.”
Not just less rain in Spain
The Iberian Peninsula has high-quality weather stations, but their data hadn’t been combined in the best way to assess drought severity until Sergio’s team looked at it. Global studies had used the kind of standard climate data weather agencies normally supply, which is averaged across square areas, dividing the world into a grid. In this new study, published online in Environmental Research Letters last week, the scientists delved into the raw data from the Spanish and Portuguese weather agencies. From those figures they could accurately calculate evapotranspiration. Sergio’s team also collected evaporation measurements from instruments called Piché and pan evaporimeters and river and stream flow data from the various water agencies in the two countries. “To obtain this information from different sources and put it together was really difficult,” he stressed. “For some variables the processing is really crazy.”
The researchers combined their data into two different measures of drought, the standardized precipitation evapotranspiration index (SPEI), which included evapotranspiration, and the standardized precipitation index (SPI), which did not. Both showed droughts in Portugal and Spain had got more severe from 1961-2011 and that they were affecting a greater area. Droughts looked worse in the SPEI, which Sergio’s team put down to the atmosphere demanding more water, something also shown by evaporimeter measurements. Their data showed stream flow fell more than rainfall – another sign of greater evaporation and matched the SPEI better than SPI. The atmosphere’s demand for water increased by around a tenth from 1961–2011, and the scientists concluded that this had been a key factor in making drought severity worse.
A climate to envy?
“These kinds of studies can’t be done at the global scale because the information is not available,” Sergio admitted. “But for this case, for this region, we have high confidence in the conclusion that temperature rise is driving higher aridity and consequently a higher severity of drought. I’d like to stress the priority of making these kinds of regional studies, considering careful control of the quality of the data. If we have a mosaic of regional studies using this information and all of them follow a common approach, then we will have better information about the important issue of drought under global warming conditions.”
Sergio’s team has more findings set to be published looking in more detail at how and why the air is sucking up more water. “A brief summary is that warming is increasing the water demand of the atmosphere, not only because temperature is related to the capacity of the air to hold water, but also it is related to the availability of moisture in the soil,” he explained. And while still stressing that these are just local results, he added that they do have global relevance.
“In a region in which we have already observed 2°C of temperature increase and a decrease in precipitation that models predict for other regions of the world for the 21st century, we observe that drought severity has increased. If this phenomenon is recorded in other regions under these conditions, probably we can give some support to the prophecies about warming’s impact on drought severity.”
Vicente-Serrano, S., Lopez-Moreno, J., Beguería, S., Lorenzo-Lacruz, J., Sanchez-Lorenzo, A., García-Ruiz, J., Azorin-Molina, C., Morán-Tejeda, E., Revuelto, J., Trigo, R., Coelho, F., & Espejo, F. (2014). Evidence of increasing drought severity caused by temperature rise in southern Europe Environmental Research Letters, 9 (4) DOI: 10.1088/1748-9326/9/4/044001