4. Conclusions

judgement of 22 glaciological and modelling experts [ Bamber et al., 2019 ], there might even be a 5% probability of reaching extreme

This chapter presented some updated assessments on climate change in Viet Nam. Observed daily data for the period 1981–2018 were used to assess past climate trends. The dynamical downscaling and BCSD statistical downscaling methods were applied to forecast climate change in Viet Nam in the XXIst century.

The key results presented in this chapter are as follows:

uAnnual temperatures increased over the whole country, with a nationwide average increase of 0.89°C (~0.146°C/decade) for the period 1958–2018. Rates of increase vary between regions and seasons. The rates of increase were 0.205°C and 0.231°C per decade for the periods 1981–2018 and 1986–2018, respectively. This indicates that temperature is gradually rising over time, and was highest in the last decade. The numbers of hot days and very hot days increased; by contrast, the numbers of cold days and very cold days in the North decreased;

uAnnual rainfall over the whole country slightly increased by 5.5% for the period 1981–2018, but with contrasting trends depending on the region under examination. Rainfall decreased at several stations in the North of Viet Nam, and increased at many stations in the North East, North Delta, South Central and Central Highlands regions. Rainfall extremes tended to vary throughout the climatic sub-regions: they declined in stations in the coastal areas in the North Central region, and increased in many stations in the North East, North West and South Central regions;

values above 60 cm and 2 m in 2050 and 2100 respectively, under a high global warming scenario (5°C in 2100).

uBoth station and satellite altimetry data show an increasing sea level trend in the coastal areas of Viet Nam. The trend was about 3.6 mm/year for the period 1993–2018, with the highest rate of 4.2–5.8 mm/year from Quang Ngai to Binh Thuan, and a lower rate of 2.2–2.5 mm/year in the Southern region;

uThe average temperature in Viet Nam in the middle of the XXIst century was projected to increase from 1.13±0.87°C under RCP2.6 to 1.9±0.81°C under RCP8.5, relative to the baseline period 1986–2005. At the end of the century temperature was projected to increase from 1.34 ±1.14°C under RCP2.6 to 4.18±1.57°C under RCP8.5. Temperature is expected to increase faster in the North than in the South;

uIn the results of the dynamical downscaling experiments provided by IMHEN, annual rainfall was projected to increase in most parts of Viet Nam in the future. Under RCP4.5, projected annual rainfall increases throughout Viet Nam by 10–15% in the mid-century, and by 10–20% at the end of the century. Under RCP8.5, projected annual rainfall also increases by 10–15% over almost all Viet Nam in the middle of the XXIst century;

uTemperature and rainfall changes in Viet Nam corresponding to the times when the global warming levels (GWL) of 1.5°C, 2°C, 3°C, 4°C are reached, have been calculated using both the dynamical downscaling and the BCSD experiments. With all the different GWLs, temperature in Viet Nam was projected to increase, with the highest rate in the North and the lowest one in the coastal Southern Central region and Southern Viet Nam. Annual rainfall was projected to generally increase with the GWLs, especially in Central Viet Nam and the Central Highlands;

uThe sea level rise in coastal areas of Viet Nam is projected to increase from 0.24 (0.13÷0.32) under RCP2.6 to 0.27 m (0.19÷0.36) under RCP8.5 in the mid-century period. At the end of the century, the projected increase is 0.44 m (0.27÷0.66) and 0.73 m (0.49÷1.03) under RCP2.6 and RCP8.5, respectively. It is worth noting that, due to large uncertainties in the future behaviour of polar ice sheets, much higher values — up to over 2 m in 2100 — cannot be ruled out.

One of the highlights of this Chapter is the successful application of the BCSD statistical downscaling method to construct the high-resolution (~10km) climate scenario set for Viet Nam with the input of 31 CMIP5 GCMs. With the BCSD method, future scenarios for Viet Nam are not only limited to RCP4.5 and RCP8.5 but also available for RCP2.6 and RCP6.0, which are the scenarios little mentioned in previous studies of Viet Nam.