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Summary
Much of the world's population lives in monsoon Asia and depends on monsoon rainfall for water and agricultural fertility. The monsoon also affects climate in other parts of the world (1). It results from an interplay between the ocean, atmosphere, and land surface (see the figure). Many factors thus affect its strength, including sea surface temperatures (SSTs) in the Indian and Pacific Oceans; variations in solar output; land snow cover and soil moisture over the Asian continent; and the position and strength of prevailing winds (1). The links between these factors and the monsoon appear to wax and wane over time, and the observational record is too short to explain this longer-term variability (2). This lack of information makes it difficult to forecast and plan for anomalous monsoon activity, and to predict how the Asian monsoon may be affected by global climate change. This situation is now changing: On page 486 of this issue, Cook et al. (3) report a Monsoon Asia Drought Atlas (MADA) that contains reconstructions of summer dryness and wetness for the region since 1300 C.E., based on tree-ring data.
