Changes in Surface Air Temperature Caused by Desiccation of the Aral Sea

Abstract

A statistical method for establishing the cause?effect relationship between a land surface modification and some component of observed climatic change is presented. This method aids attribution in two ways. First, the climatic changes that are unique to the area influenced by some land surface modification are identified. This isolates changes caused by the spatially restricted forcing from changes caused by other factors. Second, most of the short-term climatic variability in the records from the affected area is removed based on information from the surrounding region. This makes it possible to identify smaller climatic changes. This method is used to identify the changes in surface air temperature that have resulted from desiccation of the Aral Sea (1960?97). Desiccation has weakened the ?lake effect? of the Aral Sea, so regional climatic changes are expected. Substantial temperature trends, unrelated to desiccation, are observed across a broad region of central Asia (?2000 km) between 1960 and 1997. These trends are similar in magnitude to the changes from desiccation. These trends are removed from the records from the Aral region because they would enhance or offset the local temperature changes caused by desiccation. There is also substantial year-to-year temperature variability that is spatially coherent across central Asia. The method used here removes ?80%?90% of this short-term variability in the observed temperature records from the Aral region. This lowers the climate change detection limit from ?3°?8°C to ?1°?2°C, which improves the identification of the spatial extent of the desiccation-induced changes. The climate records from around the Aral Sea show dramatic temperature changes between 1960 and 1997, once regionally coherent trends and variability are removed. Mean, maximum, and minimum temperature near the Aral Sea have changed by up to 6°C. Warming (cooling) is observed during spring and summer (autumn and winter), as expected to accompany the diminished lake effect caused by desiccation. The magnitude of temperature changes decreases with increasing distance from the 1960 shoreline, with changes extending up to ?200 km from the shoreline in the downwind direction. An increase in diurnal temperature range of 2°?3°C is observed in all months, demonstrating a weakening of the lake?s damping effect on the diurnal temperature cycle.