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Previous studies have suggested that CESM can reasonably reproduce the historical climatology and trend of mean near-surface air temperature on the TP (You et al., 2016; Lun et al., 2021), which provides credible evidence that CESM can be adopted for future climate projections. Monthly outputs of the historical simulation (1850–2005) and future projections (2006–2100) for two Representative Concentration Pathway (RCP) scenarios (i.e., RCP8.5 and RCP4.5, Kay et al., 2015; Sanderson et al., 2018) and low-warming scenarios (Sanderson et al., 2017) were obtained from the CESM experiments data archive (Table 1). Due to data availability limitations, the RCP4.5 simulation only lasts until 2080. We only used the first 11 members of all CESM simulations for consistency among scenarios. The topography of the TP used in CESM is displayed in Fig. 1.
Global warming Ensemble
members1.5°C–ref 2.0°C–ref 2.0°C–1.5°C Time period Radiative
forcing
(W m–2)Time period Radiative
forcing
(W m–2)Radiative
forcing
(W m–2)Transient scenarios RCP8.5 40* 2023−33 ~3.0 2035−45 ~3.7 ~0.7 RCP4.5 15* 2031−41 ~3.1 2050−60 ~3.6 ~0.5 Stabilized scenarios Low-warming 2.0°C 11 2090−2100 ~3.2 ~1.0 Low-warming 1.5°C 11 2090−2100 ~2.2 * Only the first 11 members of each simulation were adopted for consistency among scenarios. Table 1. Time periods during which global warming reaches 2.0°C/1.5°C relative to pre-industrial temperatures under different scenarios and their corresponding radiative forcing.
Figure 1. Topography of the Tibetan Plateau as represented in the CESM model. The elevation of the highest grid point in CESM is 5280 m.
In the following analysis, the transient climate response is derived from both RCP8.5 and RCP4.5. The timings of the 2.0°C and 1.5°C warming above pre-industrial (1850–1920) levels are determined using the 11-year running mean of the global mean near-surface air temperature calculated from the ensemble mean (Table 1, Fig. 2). The stabilized climate response is derived from low-warming simulations of CESM in which the global mean temperature reaches ~2.0°C/1.5°C by the end of the 21st century (Table 1, Fig. 2).
Figure 2. Time series of (a) global and (b) Tibetan Plateau averaged annual mean near-surface air temperature anomaly (relative to pre-industrial levels) in CESM simulations. Vertical color bars in red, blue, and orange indicate the time period in which global warming reaches the 2.0°C/1.5°C threshold in RCP8.5, RCP4.5, and low-warming simulations, respectively (see Table 1).
The Coupled Model Inter Comparison Project phase 6 (CMIP6), which includes various coupled general circulation models, is now available (Tebaldi et al., 2021). Although CESM is not the latest model compared to CMIP6, its low-warming simulations provide a rather complete description of the climate system at stabilized levels, which is not available in CMIP-type simulations. Note that "stabilized" in this study means a short-term stabilization response (late 21st-century output of simulations driven with emissions that stabilize mean global warming to 1.5°C/2.0°C by 2100), which is different from "equilibrium" or "control" simulations (where external radiative forcings are fixed) (Sanderson et al., 2017).
The surface energy budget equation proposed by Lu and Cai (2009) is adopted to explain the mechanisms of temperature changes on the TP. The surface temperature change (ΔTs) can be decomposed into seven terms (Eq. 1), including surface albedo feedback (SAF), changes in cloud radiative forcing (CRF), the non-SAF-induced change in clear-sky shortwave (SW) radiation, the change in downward clear-sky longwave (LW) radiation, the change in heat storage, combined changes in surface sensible/latent heat fluxes, and the residual of this decomposition (usually very small).
Here the overbar indicates the climatology for the reference period, and Δ means the difference between the climatologies of future scenarios and the reference period;
$ \sigma$ is the Boltzmann constant,$ \alpha$ is surface albedo, and clr indicates clear-sky conditions.
Global warming | Ensemble members | 1.5°C–ref | 2.0°C–ref | 2.0°C–1.5°C | |||||
Time period | Radiative forcing (W m–2) | Time period | Radiative forcing (W m–2) | Radiative forcing (W m–2) | |||||
Transient scenarios | RCP8.5 | 40* | 2023−33 | ~3.0 | 2035−45 | ~3.7 | ~0.7 | ||
RCP4.5 | 15* | 2031−41 | ~3.1 | 2050−60 | ~3.6 | ~0.5 | |||
Stabilized scenarios | Low-warming 2.0°C | 11 | 2090−2100 | ~3.2 | ~1.0 | ||||
Low-warming 1.5°C | 11 | 2090−2100 | ~2.2 | ||||||
* Only the first 11 members of each simulation were adopted for consistency among scenarios. |