The agreement between modelled and observed values and trends for the period 1980–2018 is good enough to support use of the future projections. Agreement is better for physical variables than for biogeochemical, as is common in this type of modelling, but spatial and temporal trends are captured in all cases.
The projections show a sea that is, on average, warming by 1.1–2.9°C through the 21st century, with surface pH falling by up to 0.02 and dissolved oxygen decreasing by 5 to 13 mmol m− 3. The changes reach all parts of the water column and the bottom levels, in particular, are projected to experience conditions well outside the range seen at the start of the century. There are considerable local variations (Fig. 5), emphasising the value of using a regional rather than global model.
Warming seas mean that some parts of the region will experience temperatures not seen in the region at present (Fig. 6). As a response, some species population may be able to move with the present-day temperature contours, seeking to maintain optimum conditions for their growth, reproduction and survival (Pinsky et al. 2013; Poloczanska et al. 2013). However, this adaptation strategy has limited success near warmer regions of species distributions, and does not apply to all types of species. This is the case for corals, whose algal symbionts are tightly dependant on light near the sea surface, and where range expansion to deeper water is therefore limited.. There are clear consequences for fishing, in a region already experiencing significant challenges due to overcapacity and declining catches, exacerbated by poorly regulated fisheries (Pomeroy 2012; Teh et al. 2017). As populations re-distribute in response to changes in habitat conditions, fishers may need to travel further to find their target species, shift to catching different species, perhaps requiring an investment in new gear, or in the worst cases be faced with declining catches and no incoming new species. This is especially a concern in tropical regions, such as Southeast Asia, where declining local diversity resulting from poleward retreat of distribution leading edges is not necessarily compensated by new species arrivals. Therefore, adapting to climate change will be a significant challenge, especially for the many small-scale fishers in the region, and good management will be essential for protecting livelihoods (Lam et al. 2020).
Increased temperature and reduced salinity, as seen in these projections, may result in increased incidence of harmful algal blooms (HABs). The consequences of such HAB events include reduced water quality and toxin build-up in fish and shellfish, with potential subsequent impacts on human health (GEOHAB 2010; Young et al. 2020). Rising temperatures can also increase the risk of disease in cultured fish (Reverter et al. 2020), shellfish (Allison et al. 2011) and seaweed (Largo et al. 2017) and can lead to a rising number of jellyfish blooms or invasions that may affect aquaculture (Xu et al. 2013; Bosch-Belmar et al. 2020). Climate change driven changes in the timing of seasons is also likely to affect aquaculture production cycles, where activities are tightly timed to sharp climatic variations between monsoon/inter-monsoon periods. Such changes can be seen in the projected changes for coastal regions, notably Palawan and Taka Bonerate-Kepulauan Selayar (Fig. 7). A reduction in the predictability of seasonal cycles often leads to reduced harvests (Handisyde et al. 2006; Hamdan et al. 2015).
All the analysed coastal regions had projected end-century temperature increases close to 1.5°C under RCP4.5, enough to cause significant thermal stress leading to coral bleaching, while the 2.7°C increase seen under RCP8.5 would cause widespread loss of coral (Lough et al. 2018). The smallest temperature increases were seen at Cu Lao Cham, but at 1.5°C (RCP4.5) or 2.5°C (RCP8.5) these are still too high to prevent coral damage. Ocean acidification and the overall alteration of the ocean carbonate system resulting from rising atmospheric CO2 levels provides further stress to coral reefs by affecting the ability of reef organisms to maintain sufficient calcification rates in the face of increased dissolution rates and, in extreme cases, prevent the deposition of carbonate minerals needed for skeleton construction through the occurrence of insufficient saturation levels (Eyre et al. 2018). Our projections show surface pH decreasing, though not beyond the range currently experienced; however any acidification will act as an additional stressor on coral reefs and make it more difficult for them to recover from bleaching events. The conditions under which coral reefs are able to recover can be complex (Graham et al. 2015) and this has not been considered in the current study.
The effect of climate change on typhoons is a key concern for coastal communities in Southeast Asia, however changes in the frequency and intensity of storms are one of the least certain climate features reported by the IPCC (IPCC 2013). Regional models can simulate stronger storms than global models, because they have higher resolution, but the uncertainty in the projections remains high. We investigated the surface wind speeds in the regional atmospheric model HadGEM2-ES-RCA4, which was used as input to the marine model reported here, for any changes in the strength, frequency or timing of storms. There was some indication of an increase in the number of days per year with strong winds, but not in maximum wind strength or in the pattern across the year. By contrast, Herrmann et al. (2020), based on a much more thorough analysis of outputs from a similar regional model (CNRM-CM5_RegCM4), found a decrease in projected wind speeds, in most of the region and all seasons, except for some increase in average speeds for December to February in the north of the region. The number of tropical cyclones also decreased in all seasons. There is a clear need for more investigation of changes in storminess in this region.
This study, using a single regional-scale model driven by a single global climate model, provides useful, novel information about the potential scale of climate change effects that may occur in the marine environment at different locations across Southeast Asia. However, it does not provide any indication of the certainty in these changes, which is of key importance for decision-making. Further projections are needed, using a range of models with different climate sensitives and alternative global emissions scenarios, to estimate the uncertainty and give confidence levels in the change projected for different variables and different locations. In a region of exceptionally high marine biodiversity, and where the sea supports the livelihoods of millions of people, such projections are a key tool for communities planning how they will adapt to the challenge of climate change.