Bias adjustment of the NEX-GDDP-CMIP6 climate data and predicted change in the future climate of Iceland

Climate change adaptation depends on reliable, high-resolution climate data that meet the needs of decision-makers and society. While global climate models (GCMs) provide essential information, they contain systematic biases that must be corrected before use at local scales. This study evaluates statistical bias-adjustment methods applied to temperature and precipitation from the NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP-CMIP6) and assesses their suitability for deriving hydro-agro-climatic indicators for Iceland. 

The analysis uses data from 12–14 climate models covering a historical period (1950–2014) and a future period (2015–2100) under three Shared Socioeconomic Pathways (SSP2-4.5, SSP3-7.0, SSP5-8.5). The high-resolution (2.5 km) CARRA reanalysis dataset serves as the reference for bias adjustment. For near-surface air temperature, six methods were evaluated: linear scaling; empirical quantile mapping (EQM) with constant, monthly-varying, and 31-day moving-window adjustment factors; and two trend-preserving approaches, detrended quantile mapping (DQM) and quantile delta mapping (QDM). For precipitation, EQM with a 31-day moving window and frequency adaptation, as well as QDM, were assessed. 

Results show that trend-preserving methods perform best for bias-adjusting temperature, as they maintain long-term climate signals while reducing systematic errors. For daily precipitation, EQM outperforms QDM, particularly in correcting high-intensity events, reducing biases at the upper 95th percentile more effectively. QDM was less successful in reducing precipitation biases and did not substantially improve trends. 

Bias-adjusted projections indicate that Iceland will experience a temperature increase of approximately 2.4–3.1 °C [0.6–4.8 °C] by the end of the century (2071–2100) relative to 1981–2010, depending on the emissions scenario, with the strongest warming in northern regions. Precipitation is projected to increase by more than 2% per degree of warming, with larger increases in autumn than in winter. Annual maximum 24-hour precipitation is expected to rise by 6–7% by mid-century and by 6–14% by late century, corresponding to increases of 3–7 mm per day, with the largest changes under the high-emissions scenario (SSP5-8.5). Extreme precipitation events will become more frequent, with 100-year events potentially occurring three to four times more often under high emissions. 

Regionally, southern and southeastern Iceland are projected to become drier, while northern Iceland becomes wetter. This work, conducted as part of the Climate Atlas of Iceland, provides high-resolution, bias-adjusted climate data for national-level impact and adaptation studies and highlights the strengths and limitations of commonly used bias-adjustment methods.