Historical LSAT trend: observation and CMIP6 simulations
Figure 1a shows the 50-year running mean LSAT trend from 1950 to 2014 in the CRU dataset. Almost the entire global land surface has experienced significant surface warming. The warming trend is remarkable over the mid- and high- latitudes of the Northern Hemispheric continent and northeast Africa, weakest over the Tibetan Plateau and its downstream area, and the Andes Mountains along the western edge of South America. The global warming features can be generally well captured by the ensemble mean of the 35 CMIP6 models (MME, Fig. 1b). The spatial correlation coefficient between MME and GISTEMP is 0.57, higher than the correlations for 31 of the 35 models. However, the LSAT trend in MME is spatially much smoother than in CRU. As shown in Fig. 1d, regions with LSAT trend underestimated by more than 0.5 oC/100years in MME are where the observed warming rate higher than 3oC/100years, e.g., over Northern Asia and Alaska. And the overestimation in MME is where observed warming rate lower than 1oC/100years, e.g., over south America and the Continental United States. Amplitude of the underestimation over East Asia (EAS) and the overestimation over the U.S. are about 1oC/100years, comparable with the warming rate in the observation. That is, there is a big challenge for recent models to reasonably reproduce the LSAT trend in EAS and U.S.. The longitudinal gradient between Europe and northern Asia shown in CRU is also missing in MME.
General features of LSAT trend in CMME (Fig.1c), constrained by the observed LSAT trend, are similar to those in MME. The blank areas over land, such as the Andes Mountains, are where no model can capture the LSAT trend reasonably. The improvement from MME to CMME is evident all over the world, although the global mean LSAT trend is barely changed (1.96 oC/100years in MME versus 1.95 oC/100years in CMME). The spatial correlation coefficient with GISTEMP increases to 0.94 in CMME, 0.37 higher than in MME. As expected, the differences between CMME and MME (Fig.1e) are similar to the MME biases but in opposite sign over 94.4% of the total area. Regions with the largest differences between CMME and MME are the areas where the MME shows the largest biases, i.e., most parts of Asia, the United States, Western South America, and surrounding the Black Sea. We conclude that, although the geographic distribution of recent warming in MME is better than most individual models, the warming amplitude may be largely biased at regional scale. The CMME result further improves the geographic distribution of LSAT trend in MME and its amplitude at regional scale, because by construction the CMME is closer to the observed trends.
The 35 models’ mean capabilities in reproducing regional LSAT trend are further quantified in 44 different landscape regions (Fig. 1f). Model capability is defined as the percentage of area where model results are chosen in CMME. Region definitions follow the IPCC AR6 Working Group 1 (WG1) reference regions over land (Iturbide et al., 2020). Region names and their acronym are introduced in Table 2. Model grids with multi-model mean model capability higher than 50% covers over 77.4% of land area north of 60oS. Model capability is relatively high over the Central Eurasian continent (WCE, EEU, WSB, ESB, WCA, ECA, TIB, ARP, and SAS), Africa (SAM, WAF, SEAF, and ESAF), and Central Australia (CAU), but relatively less good over the American continents, Northern Europe, East Asia, and Australasia.
Future LSAT projections in the raw and constrained ensembles
Here we examine the projected future LSAT trend in the 21st century based on CMIP6 MME projections under the high scenario SSP5-8.5 and assess the adjustments in CMME. The LSAT trends in four 20-year periods are examined (Figs.2a-2d): 2021~2040, 2041~2060, 2061~2080, and 2081~2100. The LSAT changes in MME features global warming in all four periods. The spatial patterns of the warming are also similar to those featured in the 20th century. Globally, the LSAT trend in 2021~2040 is about 4.86oC/100yrs, and the warming rate accelerates gradually in time: 6.81 oC/100yrs in 2041~2060, 7.57 oC/100years in 2061~2080, and 7.90 oC/100years in 2081~2100.
Consistent with what we observed in the historical period, the global mean LSAT trend projections in CMME are also close to those in MME (Figs. 2e-2h). However, the regional differences are pronounced (Figs. 2i-2l). Left out low model capability over the Andes Mountains, constraint of LSAT projection in all the four projection periods generally resemble the effect of constraint ensemble in historical period (Fig. 1e) over more than 2/3 of land surface: regions that show exaggerate historical warming trend are also the regions where CMME tends to reduce the projected warming rate, and vice versa. According to CMME adjustments, warming projection over the Eurasian continent may be more intense than the raw ensemble (MME), whereas the warming risk over North America may be lower than expected.
Regions with the most considerable adjustments in future projection
The regional LSAT projections in the raw and constrained ensembles are further detailed over the 44 landscape reference regions. As shown in Fig.3a, LSAT increases monotonically over all regions but the warming rates are quite different. The LSAT trend during the last 20 years (2080-2100) are listed in Table 2. There are five regions with increasing rate higher than 10oC/100years in MME (RAR, NEN, RFE, WSB, and ESB), four of which locate at high latitude of Asia. Most of the least warming regions with LSAT trend lower than 6oC/100years are in South America and Australasia.
The CMME adjustments (Fig. 3b) vary from -1.01 oC/100year in Central North America (CNA) to 0.49 oC/100years in East Siberia (ESB). Most of the regions show a cooling adjustment in CMME. Accounting for the adjustment larger than 0.2 oC/100years, warming trend are intensified only over 5 regions but suppressed over 17 regions. The amplification of warming is evident in Asia, and suppression of warming is mostly located over American continents, Europe and North Africa. Relative to the regional LSAT trend in MME, the warming adjustment is generally weaker and most significant over ESB (4.7%); the cooling adjustment prevails over NEN (-5.3%), ENA (-8.4%), and CNA (-13.2%) in North America, NWS (-5.4%), SES (-5.6%), and NSA (-6.0%) in South America, and NEU (-10.0%) in Europe. The adjustment may be partly attributed to forcings other than GHG. Previous observational and model-based studies have found a cooling trend over the southeast and central United States, i.e., the United States ‘warming hole’. This is suggested to be attributed to anthropogenic aerosol forcing or internal climate variability (Mascioli et al., 2017) with dominant variation by season, region, and time period.
Generally, as suggested by CMME, the warming challenge may be more intense over Asia and less intense over American continents, Europe and North Africa than suggested by the raw CMIP6 MME projections. The physical mechanism of the CMME adjustment is investigated and discussed in the next subsection.
LSAT-response to CO2-forcingand its impact on the LSAT trend reproduction
Global warming in the 20th century is mainly attributable to the increase in well-mixed greenhouse gases (WMGHGs), mainly the CO2. The 1pctCO2 experiment is a transient climate simulation, in which the CO2 is the only anthropogenic external forcing. It is an idealized CO2-forced experiment that resembles the CO2 forcing during the Industrial Era. Here we compare the LSAT-response to CO2-forcing in MME and CMME in 1pctCO2 to quantify the effect of our bias-correction procedure in eliminating CO2-forcing response biases in climate models.
Figures 4a and 4b show the LSAT linear trend from 1850 to 1910 in the 1pctCO2 experiments for MME and CMME. The great similarity between the LSAT-response to CO2-forcing (Figs. 4a) and the LSAT trend with all forcings (Figs. 1b) with correlation coefficient of 0.91, confirms the dominant role of CO2 on global warming. Warming trend in CMME generally resemble the MME results, but with different amplitude of regional warming (Fig. 4b). Fig. 4c shows the differences between MME and CMME in LSAT-response to CO2-forcing in the 1pctCO2 experiment, which allows us to compare the CO2-response in MME to what the response would be if the historical LSAT biases was smaller. Interestingly, their differences closely resemble the historical trend bias in the MME (Fig. 1d). Over more than 15,000 model grids, the corresponding spatial correlation coefficient is 0.72, significant at 99% confidence level using student's t-test. That is, the historical LSAT trend biases in MME are mainly due to model capability in simulating CO2-forcing response. Future constraint by CMME is therefore physically relative to its more reasonable CO2-forcing response, since CO2 is also the dominant forcing in future emission scenario.