Assessing Human Contribution of a Drought via CMIP6 Models: A Case Study of the 2021 Iran Drought

Abstract

Iran has 4.9 million people at medium and high risk of drought impacts over rural areas in 2021. The 2012 annual precipitation over Iran was lowest since 1979 while the level of public awareness of the ongoing drought was low, possibly due to the antecedent wet condition in 2020. The Arabian Subtropical High (ASH) expanded and was strong over 2020/21. The understanding of this event remains limited from the following perspectives: 1) How unusual the Iran drought in 2021 was? and 2) Was it attributable anthropogenic forcings? In this study, an extreme event is the unprecedented Iran drought in the calendar year 2021, defined by the 12-month standardized precipitation index (SPI12) calculated from the regional averages over Iran, excluding desert areas. After the predictive performance evaluation, the 13 CMIP6 models were used to compute the SPI12 values over 2002–2021 from individual experiment forcing ensemble runs (historical, greenhouse gas-only, aerosol-only, and natural-only). Using the SPI12 distributions, the attributable probability ratio of the 2021-like drought to anthropogenic forcings were computed in terms of drought intensity, persistence, and severity. Results show that the 2020/21-like severe and long-lasting Iran droughts have become more probable by at least 50% due mainly to anthropogenic greenhouse gas increases, which highlights the need for climate change adaptation for long-lasting Iran droughts in the future.