Abstract
The effects of the emissions of anthropogenic greenhouse gases (GHGs), aerosols, and natural forcing on the summer-mean surface air temperature (TAS) in the East Asia (EA) land surface in the 20th century are analyzed using six-member coupled model inter-comparison project 5 (CMIP5) general circulation model (GCM) ensembles from five single-forcing simulations. The simulation with the observed GHG concentrations and aerosol emissions reproduces well the land-mean EA TAS trend characterized by warming periods in the early (1911–1940; P1) and late (1971–2000; P3) 20th century separated by a cooling period (1941–1970; P2). The warming in P1 is mainly due to the natural variability related to GHG increases and the long-term recovery from volcanic activities in late-19th/early-20th century. The cooling in P2 occurs as the combined cooling by anthropogenic aerosols and increased volcanic eruptions in the 1960s exceeds the warming by the GHG increases and the nonlinear interaction term. In P3, the combined warming by GHGs and the interaction term exceeds the cooling by anthropogenic aerosols to result in the warming. The SW forcing is not driving the TAS increase in P1/P3 as the shortwave (SW) forcing is heavily affected by the increased cloudiness and the longwave (LW) forcing dominates the SW forcing. The LW forcing to TAS cannot be separated from the LW response to TAS, preventing further analyses. The interaction among these forcing affects TAS via largely modifying the atmospheric water cycle, especially in P2 and P3. Key forcing terms on TAS such as the temperature advection related to large-scale circulation changes cannot be analyzed due to the lack of model data.
Highlights
Intense research in the recent decades confirmed that the increase in the global-mean surface air temperature (TAS), most notably since early 20th century, has been caused by the emissions of anthropogenic greenhouse gases (GHGs) and aerosols/aerosol precursors from industrial sources with minimal contributions from the natural variability [1,2,3,4,5,6]
historical simulation (HST) is run with the observed emissions of well-mixed GHGs, anthropogenic aerosols, and natural agent change during the simulation period; is expected to generate the most realistic TAS among the five simulations
Unlike most previous studies that explore these effects for an equilibrium state [23,34], we attempt to understand the relationship between the historical TAS variation and these external forcing more realistically from transient simulations
Summary
Intense research in the recent decades confirmed that the increase in the global-mean surface air temperature (TAS), most notably since early 20th century, has been caused by the emissions of anthropogenic greenhouse gases (GHGs) and aerosols/aerosol precursors from industrial sources with minimal contributions from the natural variability [1,2,3,4,5,6]. Geographical variations in the temperature trend are an important concern for regional policymakers as the timing and magnitude of temperature changes are crucial in shaping the climate change impacts on specific sectors of regional interests [8]. The time series of the 20th century land-mean temperature from the observation-based records show smooth increasing trends for the entire globe (black line with open circle in Figure 1a) disrupted by a slight cooling episode between the mid-1940s and the early 1970s. Similar variations in the land-mean TAS during the 20th century occur over East Asia (EA; black line with open circle in Figure 1b); the magnitude of the overall warming trend and the mid-20th century disruption of the warming over EA are much more pronounced than for the entire globe. The impacts of GHGs and aerosols on TAS vary widely according to the radiative properties of each species and associated feedback within the climate system
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