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Distinguishing anthropogenic warming from natural variability and reducing uncertainty in global-warming projections continue to present challenges. Here, we introduce a novel principle-based framework for predicting global warming from climate mean states that is based solely on carbon-dioxide-increasing scenarios without running climate models and relying on statistical trend analysis. By applying this framework to the climate mean state of 1980–2000, we accurately capture the subsequent global warming (0.403 K predicted versus 0.414 K observed) and polar warming amplification patterns. Our predictions from climate mean states of individual models not only exhibit a high map-correlation skill that is comparable to that of individual Coupled Model Intercomparison Project Phase 6 models for the observed warming, but also capture the temporal pace of their warming under the 1% annual CO2-increasing scenario. This work provides the first principle-based confirmation that anthropogenic greenhouse gases are the primary cause of the observed global warming from 1980–2000 to 2000–2020, independently of climate models and statistical analysis.

 

Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks’ relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet’s retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise.

 

Abstract. The mid-Pliocene Warm Period (mPWP, 3.3–3.0 Ma) was characterised by an atmospheric CO2 concentration exceeding 400 ppmv with minor changes in continental and orbital configurations. Simulations of this past climate state have improved with newer models but still show some substantial differences from proxy reconstructions. There is little information about atmospheric aerosol concentrations during the Pliocene, but previous work suggests that it could have been quite different from the modern period. Here we apply idealised aerosol scenario experiments to examine the importance of aerosol forcing on mPWP tropical precipitation and the possibility of aerosol uncertainty explaining the mismatch between reconstructions and simulations. The absence of industrial pollutants leads to further warming, especially in the Northern Hemisphere. The Intertropical Convergence Zone (ITCZ) becomes narrower and stronger and shifts northward after removal of anthropogenic aerosols. Though not affecting the location of monsoon domain boundary, removal of anthropogenic aerosol alters the amount of rainfall within the domain, increasing summer rain rate over eastern and southern Asia and western Africa. This work demonstrates that uncertainty in aerosol forcing could be the dominant driver in tropical precipitation changes during the mid-Pliocene: causing larger impacts than the changes in topography and greenhouse gases.

 

The Hadley circulation is the most prominent atmospheric meridional circulation, reducing the radiatively driven equator-to-pole temperature gradient. While the Hadley cell extent varies by several degrees from year to year, the detailed dynamical mechanisms behind such variations have not been well elucidated. During the expanded phase of the Hadley cell, many regions on the periphery of the subtropics experience unfavorable climatic conditions. In this study, using ERA5 reanalysis data, we examine the physical chain of events responsible for the interannual variation of the Hadley cell edge (HCE) latitude in the Northern Hemisphere. This variation is mainly caused by changing eddy activity and wave breaking from both stationary and transient waves. In particular, we show that transient waves cause the HCE to shift poleward by increasing the eddy momentum flux divergence (EMFD) and reducing the baroclinicity over 20°–40°N, shifting the region of peak baroclinicity poleward. El Niño/La Niña and the Arctic Oscillation (AO) account for a significant portion (60%) of the interannual fluctuation of the HCE latitude. Through the poleward displacement of eddy activity, La Niña and a positive AO state are associated with the poleward shift of the HCE. The analysis of 28 CMIP5 models reveals statistical relationships between EMFD, vertical shear, and HCE latitude similar to those observed.

 

Abstract Stratospheric aerosol geoengineering has been proposed as a potential solution to reduce climate change and its impacts. Here, we explore the responses of the Hadley circulation (HC) intensity and the intertropical convergence zone (ITCZ) using the strategic stratospheric aerosol geoengineering, in which sulfur dioxide was injected into the stratosphere at four different locations to maintain the global-mean surface temperature and the interhemispheric and equator-to-pole temperature gradients at present-day values (baseline). Simulations show that, relative to the baseline, strategic stratospheric aerosol geoengineering generally maintains northern winter December–January–February (DJF) HC intensity under RCP8.5, while it overcompensates for the greenhouse gas (GHG)-forced southern winter June–July–August (JJA) HC intensity increase, producing a 3.5 ± 0.4% weakening. The residual change of southern HC intensity in JJA is mainly associated with stratospheric heating and tropospheric temperature response due to enhanced stratospheric aerosol concentrations. Geoengineering overcompensates for the GHG-driven northward ITCZ shifts, producing 0.7° ± 0.1° and 0.2° ± 0.1° latitude southward migrations in JJA and DJF, respectively relative to the baseline. These migrations are affected by tropical interhemispheric temperature differences both at the surface and in the free troposphere. Further strategies for reducing the residual change of HC intensity and ITCZ shifts under stratospheric aerosol geoengineering could involve minimizing stratospheric heating and restoring and preserving the present-day tropical tropospheric interhemispheric temperature differences.