## 3 Results  
### Climate Anomalies  
We firstly evaluated the anomalies in surface air temperature and precipitation of the abrupt-4 \(\times\) CO\({}_{2}\) simulation, relative to the pre-industrial simulation. Globally, all of the models projected a warmer and wetter climate in the abrupt-4 \(\times\) CO\({}_{2}\) than in piControl (Figure 1). The CAS-ESM2 projected an average of 7.11 K warming, slightly higher than the multi-model ensemble mean (hereafter MME; 6.82 K). Of the 22 CMIP6 models, 12 projected less warming than the MME, with INM-CM5-0 showing the weakest amplitude, and the remaining 10 projected more warming than the MME, with CanESM5 showing the strongest amplitude. As for precipitation, the CAS-ESM2 projected a positive anomaly with a value of 0.17 mm day\({}^{-1}\), which was comparable to the MME (0.17 mm day\({}^{-1}\)). For the 22 CMIP6 models, their one standard deviation of the wetter anomalies was 0.07 mm day\({}^{-1}\), 41.2% that of the MME, indicating very diverse anomalies in precipitation.  
Figure 2 shows the spatial pattern of anomalies in surface air temperature and precipitation for the CAS-ESM2 and MME, and compares their differences. For both the CAS-ESM2 and MME, warm anomalies were seen over all land grids, with larger values over northern high latitudes. Their spatial differences showed that the overall stronger warm anomaly of the CAS-ESM2 relative to the MME was mainly contributed by northern Europe and the Amazon (Figure 2c), while northeast Canada and northern Russia were characterized with a weaker warm anomaly for the CAS-ESM2, compared to that for the MME. The spatial pattern of precipitation anomalies showed large regional differences. For the CAS-ESM2 and MME, wetter anomalies occurred over 81.2% and 79.5% of land grids, respectively, while the remaining land grids showed drier anomalies. Most of the wetter anomalies were seen over northern mid-high latitudes, but central America and the Amazon region showed the largest drier anomalies. In comparison to the MME, the CAS-ESM2 projected stronger wetter anomalies or weaker drier anomalies over 51.9% of land grids, including western Eurasia, northern Africa, and southern Africa; meanwhile, stronger drier anomalies or weaker wetter anomalies were seen over the remaining land grids, including the Amazon, central Africa, and tropical Asia (Figure 2f).  
These differences in climate anomalies between the CAS-ESM2 and MME, as well as among the 22 CMIP6 models, illustrate their diverse climatic response to quadrupled CO\({}_{2}\) forcing. These diversities, together with individual model biological processes, contributed to different model behaviors in their NPP responses.  
Figure 1: Global averaged anomalies in (**a**) surface air temperature (TAS, units: K) and (**b**) precipitation (Pre, units: mm day\({}^{-1}\)) for the CAS-ESM2 and the 22 CMIP6 models. The dashed lines represent the multi-model ensemble values (MME).  
Figure 2: Spatial distribution of anomalies in surface air temperature (TAS, units: K) for (**a**) CAS-ESM2, (**b**) multi-model ensemble mean, and (**c**) their differences. (**d–f**) are the same as (**a–c**) but for precipitation (Pre, units: mm day\({}^{-1}\)).