Extreme Weather Events

Floods and droughts

Floods and droughts are commonly-occurring weather hazards around the world. Amid the global climate change, there is an urgent need on behalf of the decision makers to know the risks and frequencies of such events in the future to better prepare adaptation measures. To this end, we analyzed extreme compound events in Iran to quantify frequency of dry (for ≥120 days, rainfall<2mm, Tmax≥30oC) and wet periods (for ≤3 days, total rainfall≥110 mm) using past (1980-2004) and future (2025-2049) climate data; and for some flood-prone regions.

Compound Extreme events

The Global Risks Landscape identified extreme weather events as the most likely and highest impacting risk of 2018 . Zscheischler et al. (2018), defined compound weather/climate events as the combination of multiple drivers and/or hazards that contributes to societal or environmental risk. The simultaneous occurrence of multiple dependent hazards (e.g. drought, heat, fire and air pollution, or storms and flood) causes devastating impacts in many regions, where the magnitude of that is more than a univariate hazard. 

Extreme events in Iran

Extreme meteorological and hydrological events, especially floods in Iran, have not been thoroughly studied, even though their impacts have been seriously increasing. Recent studies of extreme events have mostly focused on single-driver climate indices, such as annual precipitation, maximum one-day precipitation, number of days above a certain threshold, precipitation exceeding the 99th percentile threshold, annual frequency of warm days and nights, etc., in different parts of the country25-29. However, there are disagreements on the value and frequency of these indices across the country attesting to the large uncertainties in the past and future climate data and the period of study. 

Although the risk of extreme rainfall or temperature events may extend over a large geographic area, the vulnerability to flooding or drought events is a highly local phenomenon. Hence, while commonly used indices are useful in predicting the extreme trends, they are of little use for assessing local floods and droughts. In this article, we look at compound extremes of rainfall and temperature and also identify past floods and the associated climate conditions (in terms of duration, intensity, and extent of rainfall) in different locations across Iran. Based on the learned knowledge of each location, we can predict the frequency of future floods by searching for similar patterns in future data. If no changes were made in the hydro-morphological regime of the region, we could expect a similar or worse weather condition would lead to similar or worse floods.

The future of climate in Iran

Flood projection in Iran

Our results show that average maximum temperature may increase by 1.1 to 2.75 C across the country, while rainfall may see an annual decrease of up to 150 mm in the western parts of the country and an increase of up to 150 mm in the north and south-central regions. Except for the north and north-west, the frequency of dry periods will significantly increase in most of the country, and for some regions in the north and particularly in the west frequency of wet conditions will substantially increase. Finally, future flooding frequencies at several investigated sites showed dramatic increases of up to 27 times in the western parts of the country, while the north sees small increases. The increase in the frequency of floods and wet periods in the west, while rainfall sees a decrease, is indicative of higher frequency of convective rains of shorter durations and higher intensities in most of the country