Interpreting IPCC AR6: future global climate based on projection under scenarios and on near-term information

doi:10.12006/j.issn.1673-1719.2021.239

Abstract

Abstract:

Based on the content of Chapter 4 from the Sixth Assessment Report (AR6) contributed by the IPCC Working Group I, we interpret the future projections of global climate change. The AR6 systematically assessed possible changes of global surface air temperature, precipitation, large-scale circulation and modes of variability, and changes in ocean and cryosphere, and further reasonably estimated the climate change beyond 2100. The assessments show that global mean surface air temperature would reach 1.5℃ or even beyond it. Mean-state and variability of precipitation would increase as well, but varying with seasons and regions. Large-scale circulation and modes of variability are more affected by internal variability rather than external forcing. By the end of the 21st century, ice-free period would be seen in the Arctic. Ocean acidification and increase of global mean sea level (GMSL) would continue at the century time scale with uncertain magnitudes depending on emission scenarios. The projected GMSL would go higher beyond 2100 under all the scenarios. Multiple constraining methods are introduced in this latest assessment, reducing the uncertainty range of future projection. By paying an additional attention to the low emission scenarios and low-likelihood high-impact storylines, the AR6 provided richer and more comprehensive information for addressing climate change. Integrating the assessment conclusions, we suggest that future studies need to reduce the projected uncertainties in regional climate change, especially in the monsoon regions, and that capability construction of climate projection in China need to be strengthened in both scientific research and model development.

Key words: IPCC AR6, Climate projection, SSP scenarios, Surface air temperature, Precipitation, Circulation, Sea ice, Sea level

ZHOU Tian-Jun, CHEN Zi-Ming, CHEN Xiao-Long, ZUO Meng, JIANG Jie, HU Shuai. Interpreting IPCC AR6: future global climate based on projection under scenarios and on near-term information[J]. Climate Change Research, 2021, 17(6): 652-663.

Figures/Tables 6

Fig. 1 Selected indicators of global climate change from CMIP6 historical and scenario simulations. (a) Global surface air temperature changes, (b) global land precipitation changes, (c) September Arctic sea-ice area, (d) global mean sea-level change (GMSL). (Source: IPCC AR6 Chapter 4 Fig. 4.2)

Fig. 2 Mid- and long-term change of annual mean surface temperature (Displayed are projected spatial patterns of multi-model mean change in annual mean near-surface air temperature in 2041-2060 and 2081-2100 relative to 1995-2014 for SSP1-2.6 and SSP3-7.0.The number of models used is indicated in the top right of the maps. No overlay indicates regions where the change is robust and significant. Hatching indicates regions with no change or no robust significant change. Cross-hatching indicates areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Source: IPCC AR6 Chapter 4 Fig. 4.19. For the definition of robustness and significant change, please see section 4.2.6 in the Chapter 4 of IPCC AR6)

Fig. 3 Long-term change of seasonal mean precipitation during 2081-2100 (Displayed are projected spatial patterns of multi-model mean change in winter and summer mean precipitation in 2081-2100 relative to 1995-2014, for SSP1-2.6 and SSP3-7.0. The number of models used is indicated in the top right of the maps. No map overlay indicates regions where the change is robust and significant. Hatching indicates regions with no change or no robust significant change. Cross-hatching indicates areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Source: IPCC AR6 Chapter 4 Fig. 4.24. For the definition of robustness and significant change, please see section 4.2.6 in the Chapter 4 of IPCC AR6)

Fig. 4 Multiple lines of evidence for GSAT changes for the long-term period 2081-2100, for all five priority scenarios, relative to the average over 1995-2014. (a) Future GSAT warming and uncertainty in the unconstrained projection, (b) future GSAT warming and uncertainty in the constrained projection, (c) the average of three constrained projection (grey) and the range estimated from emulator (green), (d) the average GSAT series of the constrained CMIP6 ranges and the emulator ranges. (The y-axes on the right-hand side are shifted upward by 0.85°C, the central estimate of the observed warming for 1995-2014, relative to 1850-1900. Source:IPCC AR6 Chapter 4 Fig. 4.11)

Table 1 Assessment results for 20-year averaged GSAT change, based on multiple lines of evidence

Table 2 CMIP6 annual mean surface air temperature anomalies

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