Southeast Asia features a complex range of terrains and land–sea contrasts. Across the region, temperature has been increasing at a rate of 0.14°C to 0.20°C per decade since the 1960s (Tangang et al., 2007), coupled with a rising number of hot days and warm nights, and a decline in cooler weather (Manton et al., 2001; Caesar et al., 2011). A positive trend in the occurrence of heavy (top 10% by rain amount) and light (bottom 5%) rain events and a negative trend in moderate (25 to 75%) rain events has been observed (Lau and Wu, 2007). Annual total wet-day rainfall has increased by 22 mm per decade, while rainfall from extreme rain days has increased by 10 mm per decade (Alexander et al., 2006; Caesar et al., 2011).

Several large-scale phenomena influence the climate of this region. While ENSO (Section 14.4) influence is predominant in East Malaysia and areas east of it, Maritime continent monsoon (Section 14.2.3) influences the climate in Peninsular Malaya. The impact of the IOD (Section 14.3.3) is more prominent in eastern Indonesia. Thus climate variability and trends differ vastly across the region and between seasons. Between 1955 and 2005 the ratio of rainfall in the wet to the dry seasons increased (Aldrian and Djamil, 2008). This appears to be at least in part consistent with an upward trend of the IOD. While an increasing frequency of extreme events has been reported in the northern parts of South East Asia, decreasing trends in such events are reported in Myanmar (Chang, 2011); see also Figure 14.25.

For a given region, strong seasonality in change is observed. In Peninsular Malaya during the southwest monsoon season, total rainfall and the frequency of wet days decreased, but rainfall intensity increased in much of the region (Deni et al., 2010). During the northeast monsoon, total rainfall, the frequency of extreme rainfall events, and rainfall intensity all increased over the peninsula (Suhaila et al., 2010). High-resolution model simulations are necessary to resolve complex terrain such as in Southeast Asia (Nguyen et al., 2012; Section In a RCM downscaling simulation using the A1B emission scenario (Chotamonsak et al., 2011), regional average rainfall was projected to increase, consistent with a combination of the ‘warmer getting wetter’ mechanism (Section 14.3.1), an increase in summer monsoon, though there is a lack of consensus on future ENSO changes. The spatial pattern of change is similar to that projected in the AR4 (Christensen et al., 2007, Section 11.4).

The median increase in temperature over land ranges from 0.8°C in RCP2.6 to 3.2°C in RCP8.5 by the end of this century (2081–2100). A moderate increase in precipitation is projected for the region: 1% in RCP2.6 increasing to 8% in RCP8.5 by 2100 (Table 14.1, Supplementary Material Table 14.SM.1a to 14.SM.1c, Figures 14.27 and AI.64 to AI.65). On islands neighbouring the southeast tropical Indian Ocean, rainfall is projected to decrease during July to November (the IOD prevalent season), consistent with a slower oceanic warming in the east than in the west tropical Indian Ocean, despite little change projected in the IOD (Section 14.3.3).

In summary, warming is very likely to continue with substantial sub-regional variations. There is medium confidence in a moderate increase in rainfall, except on Indonesian islands neighbouring the southeast Indian Ocean. Strong regional variations are expected because of terrain.