Droughts and climate change

With the past few posts looking at floods, let’s now move on to the next category of natural disasters – droughts. Droughts are generally defined as ‘a period of abnormally dry weather long enough to cause a serious hydrological imbalance’ (IPCC 2012). Droughts have been a relatively common phenomenon in several regions including Australia, Sahel, North America, and even the United Kingdom (though they are generally much less severe than those experienced in the other countries mentioned). In fact, since 2012, North America has been hit with one of its worst droughts, with 81% of the contiguous United States being covered with at least abnormally dry conditions (DO) at its peak on July 17 2012 (Fig.1). Currently, the situation has improved significantly although the western parts of the US are still affected by the droughts (Fig.2).

Figure 1 US drought monitor report for July 17 2012. 81% of the contiguous US is affected by at least abnormally dry conditions. Source: http://droughtmonitor.unl.edu/

Figure 2 US drought monitor report for December 10 2013. Most of the western part of the US is still affected by the drought. Source: http://droughtmonitor.unl.edu/

When discussing about droughts, it is important to note that there are different types of droughts – meteorological, hydrological and agricultural (Fig.3). The definitions for each category of droughts are taken from Mishra and Singh (2010):

• Meteorological – lack of precipitation over a region for a period of time.
• Hydrological – a period with inadequate surface and subsurface water resources for established water uses of a given water resources management system.
• Agricultural – a period with declining soil moisture and consequent crop failure. 

Figure 3 Links between the different types of droughts and the main drivers for the droughts. Source: IPCC SREX report (2012)

However, out of all the atmospheric hazards, droughts are at the moment the least well-understood and predictable (Mishra and Singh 2010). There are still large uncertainties involved regarding observed global-scale trends in droughts as well as how anthropogenic climate change will affect droughts in future (IPCC 2012).

Firstly, there has been a lack of consensus with regards to the observed trends in drought over the recent past. This is mainly due to the fact that there are few direct observations of drought-related variables available for global analysis and hence drought indices, which attempt to integrate precipitation, temperature and other variables, are used instead to infer the changes in drought conditions. One of the most prominent index used is the Palmer Drought Severity Index (PDSI) (Palmer 1965) that measures the cumulative departure in surface water balance. Based on this index, Dai et al. (2004) have suggested that globally, very dry areas have doubled in extent since 1970. However, such studies based on the PSDI have been criticised greatly due to the fact that the PSDI relies on a temperature-based method of calculation of potential evaporation (PE), known as the Thornthwaite equation. Sheffield et al. (2012) argued that temperature-based PE tend to overestimate the extent of drought as it does not factor in the effects of radiation, vapour-pressure deficit and wind speed. By using a physically-based estimate of PE based on the Penman-Monteith equation instead, Sheffield et al. (2012) showed that there has actually been little change in drought over the past 60 years (Fig.4). As such, due to the lack of direct observations of drought, it has been difficult to determine the global trends of droughts. The IPCC SREX report (2012) could at best conclude that there is only a medium confidence that some regions of the world have experienced more intense and longer droughts since 1950s.

Figure 4 A) PSDI calculated using the Thornthwaite PE method is shown in blue and using the Penman-Monteith method is shown in red. B) Area in drought (PSDI < -3). The shading shows the range derived from uncertainties in precipitation (both) and radiation (Penman-Montieth only). Source: Sheffield et al. (2012)

Secondly, there are also large uncertainties involved with our understanding of how climate change will affect droughts in the future. There have been preliminary studies based on model simulations that suggest that aridity increases and because severe by the 2060s over most of Africa, southern Europe, Middle East, most of Americas, Australia and Southeast Asia, while central and northern Eurasia, Alaska and northern Canada and India become increasingly wetter (Fig.5) (Dai 2011). Such results broadly correspond to the modelling results by Hirabayashi (2008) that suggest that drought frequency from 2070 to 2100 increases over North and South America, central and southern Africa, the Middle East, southern Asia and central and western Australia. Meanwhile, the eastern part of Russia shows significant decreases in drought days due to increases in precipitation and flood flows. 

Figure 5 Mean annual self-calibrated PDSI (sc-PDSI) using the Penman-Monteith PE method for years 2060-2069 and 2090-2099. These values were calculated using the 22-model ensemble-mean surface air temperature, precipitation, humidity, net radiation and wind speed from the IPCC AR4 using the SRES A1B 21st century simulations. Red to pink areas are extremely dry conditions while blue colors indicate wet areas relative to the 1950-1979 mean. Source: Dai 2011. 

However, these projections do not consider the impacts of changes in large-scale atmospheric and ocean circulations under climate change as there is insufficient knowledge of how these have an impact on droughts. Moreover, it is also not known how the behaviour of plant transpiration, growth and water use efficiencies will change under increasing CO₂ emissions, which would consequently affect soil moisture storages (IPCC 2012). Therefore, unfortunately, it is still hard to determine how trends in droughts will evolve in the future under climate change.