A holistic approach for using global climate model (GCM) outputs in decision making

All human endeavours are affected by climate change and local weather, and 21st-century climate-change estimates will provide greater challenges. Therefore, numerical model-based climate projections (i.e., General Circulation Models - GCMs) are essential for making decisions about adaptation, mitigation, and resilience building to combat adverse climate impacts. However, these coarser resolution model projections have uncertainties resulting in significant doubts in decision-making, particularly in the local or regional domain. Thus, we suggest five principles for GCM use at local and regional scales to overcome uncertainties in decision-making. This study examines historical and projected precipitation from 44 GCMs, calculated under the Representative Concentration Pathways (RCP) 8.5 scenario considering regional, geographical, and temporal variability for establishing a decision-making support system based on the degree of confidence for nine diverse main river basins in tropical Sri Lanka as a case study. Each GCM confirmed the current climatic pattern in terms of wind vector and meridional wind patterns out of eight climate variables considered. GCM sensitivity varies geographically and temporally. An annual precipitation study is not enough to make climate change judgments because climate change signals change seasonally. Each basin's average annual precipitation was projected to likely increase over the near, middle, and far future (2025-2050, 2050-2075, 2075-2100, respectively); however, in the Mahaweli basin (MB), the largest basin neighboring most of the other basins, the annual precipitation was projected to extremely likely increase. Most of MB's surrounding basins show a more-likely-than-not decreasing precipitation tendency for inter-monsoon-1 in the future (2025-2100); however, MB shows a more-likely-than-not increasing trend. Our analysis shows that the southwest monsoon and inter-monsoon-2 are more likely than not or extremely likely to strengthen, whereas the northeast monsoon (NEM) and intermonsoon-1 are more likely than not to weaken, excluding NEM in the near future. Here, we introduced a simple color-coded climate change (C4) matrix for decision-making that provides spatial, temporal, and seasonal climate change projections likelihood trends with current observed seasonal trends. Future projections indicate a swapping in seasonal precipitation trends between IM-1 and SWM for almost basins compared to the past. NEM appears to have a weakening influence on the majority of northeast-facing basins during the middle and far future. A simple detailed climate analysis chart may be more effective in communicating scientific messages to the scientific community and the key public actors who make decisions.