In response to anthropogenic forcing, the Earth's surface generally warms as greenhouse gases trap outgoing longwave radiation. Counterintuitively, however, some regions exhibit surface cooling against this global warming background-a phenomenon known as a warming hole. Beyond the well-documented warming holes over the North Atlantic and southeastern United States, here we show that increasing atmospheric CO2 concentrations can also induce summertime cooling over India. Due to the direct radiative effect of CO2, warming of the Eurasian continent relative to surrounding oceans, low-level moisture transport and vertical motion are enhanced over India. Combined with abundant summer-monsoon moisture and the topographic blocking effects of the Himalayas and Hindu Kush Mountains, these circulation changes increase cloud cover. The resulting cloud enhancement reduces incoming solar radiation at the surface, producing the observed regional cooling. These results reveal a previously underappreciated mechanism whereby greenhouse gas forcing can paradoxically induce regional cooling through atmospheric dynamical pathways.

Abstract Heat extremes have become a major health hazard around the world. Understanding their mechanisms remains a major challenge because the physical drivers interact in a nonlinear way. Here we introduce a globally perturbed reforecast framework driven by the Neural general circulation model (NeuralGCM). Sensitivity reforecast experiments that independently remove initial condition anomalies over spatially distinct patches identified the high impact regions (HIRs) for the record‐breaking August 2022 South China heatwave (SCH22) in Europe and North America (NA) through changes in forecast skill, which are further confirmed by dynamic diagnostics. Forecasts initialized using anomalies only from HIRs covering just 25% of the global domain successfully reproduce the evolution and spatial pattern of SCH22. These findings can also generalize to another AI‐based weather model FuXi. Our proposed framework helps to improve accessibility to global‐scale diagnostic for extreme events with robust results.

The surface mixed layer depth (MLD) of the tropical Indian Ocean is projected to shoal significantly under increased atmospheric CO2, but its further response to subsequent CO2 removal remains unclear. This work investigates this issue utilizing climate models’ simulations under an idealized scenario with symmetric increase and decrease in atmospheric CO2. The results show that the increased CO2-induced basin-wide MLD shoaling recovers rapidly when CO2 decreases. However, the MLD changes display large spatial variations and leave a prominent overall deepening trend in the North Indian Ocean (NIO) but an overall shoaling trend south of 10°S when CO2 returns to its initial level. The former comprises an overall deepened winter deep MLD but shoaled summer MLD, amplifying the seasonal MLD contrast north of 10°N. The overall winter deepening is dominated by a prominent Newtonian cooling over large residual surface warming as the overall winter monsoon changes are weak when CO2 level is restored. While the overall summer shoaling primarily results from the prominent monsoon weakening, the shoaling effect from reduced wind overwhelm the Newtonian cooling by reducing wind stirring, suppressing latent heat loss and increasing cloud-related radiative flux. In contrast, the overall MLD shoaling south of 10°S displays minor seasonal differences due to persistent weakening in the trade winds year-round, resulting from an enhanced Southern Ocean warming. Despite complicated ocean–atmosphere coupling processes in the overall Indian Ocean MLD trend, the residual sea surface warming and distinct winter and summer changes are essential. The results highlight the compound and coupled effects of different surface forcing on MLD changes under external forcing and imply that while CO2 removal actions can largely recover the Indian Ocean MLD shoaling and seasonal cycle changes induced by anthropogenic warming, nonlinear atmospheric response may leave asymmetric changes in oceanic conditions and hence climatic and biological systems.

In the present study, a multivariate probabilistic framework is used to identify the meridional positions of regional tropical edges (RTEs), which are based on two variables: sea level pressure and precipitation minus evaporation. This new defined metric effectively captures inter-annual variability and long-term trend of the commonly adopted zonal mean tropical edge based on meridional mass stream function and near-surface winds. Besides, pronounced RTE trends are primarily located over the oceanic regions, and the terrestrial areas exhibit substantial inter-annual variability. These results are consistent among three modern reanalysis datasets. Moreover, the impacts of climate modes on RTE are investigated. The El Nino-Southern Oscillation, the Atlantic multi-decadal oscillation, and the Southern Annular Mode are important both on the inter-annual variations and long-term trends of RTE. The Pacific Decadal Oscillation is more inclined to affect long-term contribution rather than inter-annual relationship, and the Pacific-North American teleconnection, the North Atlantic Oscillation, and the Arctic oscillation highlight the inter-annual relationship with RTE in the specific regions, such as North Pacific, North Atlantic, and North Africa, respectively.

Victoria mode (VM), the second dominant mode of North Pacific sea surface temperature variability, has been identified as one of the important factors influencing the Indian summer monsoon (ISM) onset. The positive phase of the May VM delays the ISM onset by both tropical and extratropical pathways. Here, we found a significant interdecadal enhancement of their relationship since the early 1990s, which is mainly attributed to the structure changes and increased variance of the VM. In recent decades, the VM has shown more significant warm SST anomalies in the tropical central Pacific, which drive the large-scale divergent circulation more effectively. This enhanced divergent circulation leads to low-level divergence and reduced rainfall in the tropical Asian summer monsoon region. The reduced rainfall excites equatorial Rossby wave response and anomalous easterly winds in the northern Indian Ocean, delaying the ISM onset. Besides, the increased variance of the VM after 1992/1993 stimulates a stronger extratropical Rossby wave train. This stationary Rossby wave train induces a stronger cooling to the northwest of India, which weakens the land-sea thermal contrast and leads to the delayed ISM onset. This finding should be taken into account to improve short-term predictions of the monsoon onset.

Analysis of a suite of global climate model projections under symmetric CO2 ramp-up and ramp-down (RD) scenarios, our results demonstrate a progressive strengthening of the western North Pacific anticyclone (WNPAC) with rising CO2 concentrations, a trend that persists as CO2 declines, followed by gradual recovery without fully returning to its initial state when CO2 concentrations restore. The overshoot of the WNPAC in the CO2 RD phase is highly correlated with the enhanced anomalous Maritime Continent (MC) convection, which influences WNPAC through reinforced Kelvin wave response or local Hadley circulation adjustment. This enhanced convection is attributed to increased Indo-Pacific zonal SST gradient associated with strengthened MC warming and accelerated decay of El Ni & ntilde;o in the Central Pacific, ultimately linked to climatological equatorial Pacific El Ni & ntilde;o-like warming pattern-related air-sea processes. The overshoot of the WNPAC during the CO2 RD phase may exacerbate flood and high temperature risks in densely populated East Asia.

A notable anomalous lower-tropospheric anticyclone appears over the western North Pacific (WNP) during post-El Niño summer, exerting a profound influence on the East Asia summertime climate. Here, we employ a suite of global climate model projections under symmetric CO2 ramp-up (RU) and ramp-down (RD) scenarios to reveal the asymmetric response of the WNP anticyclone (WNPAC). Our results demonstrate a progressive strengthening of the WNPAC with rising CO2 concentrations, a trend that persists as CO2 declines, followed by gradual recovery, and the anomalous anticyclone fails to return to its initial state when CO2 concentrations return to pre-industrial levels, attributed to Indo-Pacific zonal SST gradient variations. In contrast to the CO2 RU phase, the increased zonal SST gradient is witnessed in the CO2 RD phase, favoring anomalous moisture convergence over the Maritime Continent (MC). This strengthens the WNPAC through Kelvin wave-induced Ekman divergence or local Hadley circulation adjustment. The increased zonal SST gradient is associated with strengthened SST warming in the MC and accelerated decay of El Niño-related positive SST anomalies in the equatorial Central Pacific, ultimately attributed to a climatological equatorial Pacific El Niño-like warming pattern driving intricate air-sea interactions and processes. Our findings indicate that the overshoot of the WNPAC during the CO2 RD phase may exacerbate the flood and high temperature risks in densely populated East Asia.

AbstractUnderstanding the response of the Indian summer monsoon (ISM) onset to CO2 forcing is of utmost importance for rain‐fed agriculture and water management. In this study, we utilized an idealized symmetric CO2 removal scenario from the sixth phase of the Coupled Model Intercomparison Project to analyze the reversibility of monsoon onset. The results show that ISM onset is reversible but exhibits strong asymmetry: it undergoes minimal changes during the ramp‐up phase, but experiences rapid postponement as the CO2 begins to decline; Eventually, it is delayed more than 1 week when the CO2 concentration is restored to the initial level. To investigate the possible underlying mechanisms, we decomposed the climate response to CO2 forcing into the fast and slow processes. Notably, it is the enhanced slow response, which is driven by long‐term sea surface temperature (SST) changes, that dominates the asymmetric response of ISM onset. This slow response delays the ISM onset by strengthening near‐surface poleward land‐sea moist static energy contrast, thereby weakening the lower‐tropospheric monsoonal circulation. Based on the atmospheric component model simulations, we found that both the uniform SST change and patterned SST changes in the slow response contribute to the delay of ISM onset, but the latter plays a dominant role. Our results emphasize the importance of thoroughly assessing regional hydrological cycle features when designing the CO2 removal pathways.

Over the Amazon region, rainfall-induced changes to CO2 pathways significantly impact humans and multiple ecosystems. Its resilience is of vital importance, and idealized CO2 removal experiments indicate that declining trends in rainfall amounts are irreversible and exhibit a deficiency when the CO2 concentration returns to the pre-industrial level. The irreversible decline in Amazon rainfall is mainly due to the weakened ascent, further led by two main causes. (1) Enhanced tropospheric warming and a wetter atmospheric boundary layer over the tropics during CO2 removal generate a strong meridional gradient of temperature and specific humidity; driven by prevailing northeasterly winds, negative moist enthalpy advection occurs, which in turn weakens the ascent over the Amazon and results in anomalous drought. (2) The enhanced radiative cooling of atmospheric column. Driven by the negative lapse-rate feedback, the outgoing longwave radiative flux increases in the clear-sky atmosphere. As a result, the anomalous diabatic descent generates to maintain the energy balance of the atmospheric column. This result implies that the symmetric removal of CO2 does not guarantee full recovery of regional precipitation.

The Amazon basin plays a crucial role in biodiversity and carbon storage, but its local rainfall is anticipated to decrease under global warming. Carbon dioxide removal (CDR) is being considered as a method to mitigate the impact of global warming. However, the specific effects of CDR on Amazon rainfall have not been well understood. Here, an idealized CDR experiment reveals that the reduced rainfall over the Amazon basin does not recover. Significantly weaker rainfall is found during the ramp-down period compared to the ramp-up period at the same CO2 concentration. This response is associated with the enhanced El Niño-like warming in the tropical Pacific Ocean during the CDR period. This warming pattern has dual effects: weakening the zonal circulation and causing anomalous descent directly over the Amazon basin, while also triggering a stationary Rossby wave train that propagated downstream and generated anomalous ascent over the Sargasso Sea. This anomalous ascent induces anomalous descent and weakens moisture transport over the Amazon basin by the local meridional circulation. Consequently, precipitation is reduced over the Amazon basin in response to the weakened zonal and meridional circulation. Our findings indicate that even if the atmospheric CO2 concentration is lowered, the Amazon basin will remain susceptible to drought. Effective local climate adaptation strategies are urgently needed to address the vulnerability of this critical ecosystem.

During the boreal winter, the El Ni & ntilde;o-Southern Oscillation (ENSO) influences the East Asia-western North Pacific (WNP) climate by triggering an anomalous WNP anticyclone (WNPAC). Analysis of a suite of coupled model projections under symmetric CO2 ramp-up (RU) and ramp-down (RD) scenarios, the results reveal that WNPAC strengthens with increasing CO2 concentrations, peaks early in the CO2 RD phase, and then gradually weakens without fully returning to its initial state when CO2 concentrations restore. The irreversible recovery of WNPAC is related to enhanced negative precipitation anomalies in the tropical WNP and positive precipitation anomalies in the equatorial central and eastern Pacific. These changed precipitation anomalies are primarily driven by the climatological equatorial Pacific El Ni & ntilde;o-like warming pattern due to various external and internal feedback processes. Our findings indicate that the irreversible change of WNPAC to CO2 forcing may hinder the winter monsoon and exacerbate climate risks in the East Asia-WNP region.

The reversibility of South Asian summer monsoon (SASM) precipitation under the CO2 removal scenario is critical for climate mitigation and adaptation. In the idealized CO2 ramp-up (from 284.7 to 1138.8 ppm) and symmetric ramp-down experiments, SASM precipitation is largely reversible while exhibiting strong asymmetry: it may overshoot the unperturbed level when CO2 recovers. Such asymmetric response is mainly due to the enhanced El Niño-like and Indian Ocean dipole-like warming during the ramp-down period. The uneven sea surface warming weakens Walker circulation, with anomalous sinking over the SASM region. Meanwhile, the warming also affects the rainfall over the Maritime Continent and tropical western Indian Ocean. The suppressed rainfall over the Maritime Continent triggers the equatorial Rossby wave, which weakens the ascent over the SASM region; the increased rainfall over the tropical western Indian Ocean excites the equatorial Kelvin wave, which reduces moisture transport. Additionally, tropic-wide warming reduces the land-sea thermal contrast and weakens monsoonal circulation. Consequently, the combined effects of the weakened ascent and moisture transport lead to the overshooting of SASM rainfall. Our results suggest that symmetric CO2 removal, although unlikely in the foreseeable future, may result in a risk of local drought over the SASM region.

Abstract Carbon dioxide removal (CDR) is a crucial approach in achieving the goals set by the Paris Climate Agreement. However, understanding the cooling effect (CE) of CDR and its primary controlling factors has been challenging due to the limited number of models conducting the CDR simulation. To address this, we employed an energy balance model (EBM), which effectively captures or reproduces the global mean surface air temperature change to CO2 forcing. The outputs of the EBM revealed that even with the same CDR rate, the CE diverse significantly ( $$\sigma =0.7{K}$$ σ = 0.7 K ). Two main factors significantly affect the CE of CDR. The primary factor is the coefficient of the vertical heat exchange in the ocean [contributes to $$\sigma \left({CE}\right)=0.7{K}$$ σ CE = 0.7 K ], which governs the heat uptake and release of the deep ocean. It directly impacts the heat absorbed by the Earth’s surface and influences the magnitudes of the transient climate responses. The second factor is the estimation of effective radiative forcing (ERF) resulting from changes in CO2 concentration [contributes to $$\sigma \left({CE}\right)=0.6{K}$$ σ CE = 0.6 K ]. This estimation is directly associated with the amplitude of the CO2 radiative forcing and the responses of relevant energy processes, thereby influencing the temperature change. Regarding the timing of the CE emergence, the influences of processes within the EBM are quite small. Therefore, an accurate estimation of the vertical heat exchange in the ocean and ERF may favor designation of a better pathway to achieve the temperature goals outlined in the Paris Climate Agreement.

The South Asian high (SAH), a large-scale anticyclone at 100 and 200 hPa on the Asian continent, is driven by South Asian summer monsoon rainfall and heating over the Tibetan Plateau. As one member of the Asian summer monsoon system, the changes of its location and intensity may cause other climate responses. Rainfall-induced latent heating can affect the SAH change. Previous study revealed that rainfall responses in Indian Ocean are opposite in periods of increased and stabilizing radiative forcing (RF) due to the deep ocean warming over the Southern Ocean. SAH responses during RF increase and stabilization are studied with 13 models from the Extended Representative Concentration Pathway scenario 4.5 (ECP4.5) experiment. At 100 hPa, the SAH intensifies and moves equatorward in increased RF scenario; when RF stabilizing, SAH still shifts southward but with little change in intensity. At 200 hPa, the SAH changes little in both RF increase and stabilization. These opposite responses at different altitudes may be due to the maximum potential temperature at 150-200 hPa, leading to the opposite changes in vertical motion. Results of the linear baroclinic model indicate that (a) during RF increase, diabatic he....ating (Q* (1)) contributes to SAH strengthening,bothQ(*) (1) and the mean advection of stratification change (MASC) lead to SAH southward movement; (b) during RF stabilization,Q(*) (1) and MASC, contribute to the SAH equatorward displacement. Two components ofQ(*) (1,) the latent heating and residual heating, are canceled out and lead to little change in SAH intensity.

AbstractBased on the multimodel ensemble of 22 models in Coupled Model Intercomparison Project Phase 6 (CMIP6), the present manuscript found that in response to increasing CO2, the South Asian high (SAH) displays the opposite response over its southern region in the upper troposphere: an anticyclonic response at 100 hPa and a weak (insignificant) cyclonic response at 200 hPa. This opposite response is a product of tropospheric warming. In response to increasing CO2, the troposphere warms, with the potential temperatures peaking at 150–200 hPa over the northern Indian Ocean (IO). With transportation by local vertical motion (ascendance over the northeastern IO and descendance over the northwestern IO), various changes in vertical temperature advection with height form at 100 and 200 hPa. Finally, the changes contribute to a decrease in the ω response with pressures at 100 hPa but an increase in the ω response with pressures at 200 hPa over the northwestern IO. Over the northwestern IO, ω change is inversely related to pressure. At 100 hPa, the sign of ω changes with pressure, which yields distinct vorticity forcing over the northeastern and northwestern IO. This causes an anticyclonic response, which may generate zonal vorticity advection and balance the vorticity forcing. At 200 hPa, the contribution is roughly opposite to that at 100 hPa. In addition, although diabatic heating contributes to the vertical profile of ω, it yields the same‐sign vorticity response at 100 and 200 hPa.

In 2020, China announced the "emission peak, carbon neutrality" policy, that is, China aims to have CO2 emissions peak before 2030 and achieve carbon neutrality before 2060. The scenario of carbon neutrality will be significantly distinguished from the scenario we experienced since the industrial revolution. However, instrumental data are unavailable in the future carbon-neutral scenario. Earth system models and climate dynamics theory are needed to comprehend and project the climate change. In this paper, we illustrate our perspective of the issues related to "emission peak, carbon neutrality", including climate dynamics, climate-carbon feedback, interaction between China and global climate and carbon emissions and solutions, etc. We highlight that climate change has profoundly affected human production and life. The frequent occurrence of extreme weather disasters in recent years, together with the impact of epidemics, make the future "carbon peak & carbon-neutral" scenario more complex. There is whopping uncertainty but also a massive challenge to the scientific community. Thus, carbon neutrality is closely related to domestic production and lives, and there is little time left for planning. We believe that we will make a breakthrough in climate dynamics in the context of carbon neutrality with our joint efforts, which will serve our country's carbon emission policy at different stages.

We evaluated the dependence of the sensible heat flux trend over the Tibetan Plateau on elevation by comparing the 29 climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) with ground observations in the time period 1980-2005. The sensible heat flux trend over the Tibetan Plateau shows an elevation-dependent variation in both the observations and reanalysis datasets, with a larger negative trend at higher altitudes. Most of the models analysed in this study performed poorly in simulating the linear trend of the sensible heat flux, although two models (HadGEM2-CC and HadGEM2-ES) reasonably captured the elevation range and seasons with a prominent decreasing trend in the sensible heat flux over the Tibetan Plateau. These two models possess good skills in depicting both the sensible heat flux trend and the terrain of the plateau in every 1,000 m wide altitudinal band. The coherence of the elevation-dependent variation in the sensible heat flux trend between the observations and models is therefore not fortuitous. The sensible heat flux trend in most models of CMIP5 is sensitive to variations in the surface wind speed and the difference in temperature between the ground surface and the air, although these two factors show large biases deviating from the reanalysis product in almost all models in this study. In the HadGEM2-CC and HadGEM2-ES models, which showed a good performance in capturing the elevation-dependent sensible heat flux trend, the leaf area index was shown to be the predominant factor affecting the variation in the sensible heat flux trend with elevation. That maybe link with the dynamic vegetation scheme in these two models.

The rainfall responses over the Indian Ocean (IO) are investigated based on the Representative Concentration Pathways 4.5 (RCP4.5) experiments of 13 models, in which the experiments are extended to the year 2300, from the Coupled Model Intercomparison Project Phase 5 (CMIP5). During the radiative forcing (RF) increase, the rainfall pattern displays northwest-southeast dipole asymmetry. After RF stabilization, rainfall increases over the southern IO and decreases over the northern IO where is a wet region in climatology. Diagnostic analysis demonstrates that both the changes in atmospheric circulation (dynamic component) and the moisture increase (thermodynamic component) play a key role in determining this rainfall dipole during RF increase, but the effect of the latter is reduced after RF stabilization. The responses of rainfall, sea surface temperature (SST) and atmospheric circulation are well coupled during the two periods: (a) the anomalous circulation affects the rainfall change by transporting abundant moisture to maintain the energy balance, with easterlies (northerlies) in RF increase (stabilization); and (b) in turn, heat released by the SST warming further induces the circulation change. Furthermore, during RF increase, the attribute of SST pattern is mainly led by ocean dynamics, especially heat transport due to ocean current changes, while after RF stabilization, it is mainly due to ocean heat transport leading by temperature changes.

Based on the 1%CO2 experiment of CMIP6, in response to increasing CO2, the summer-mean radiative heating (RH) over the global monsoon area (MA) generally features an increasing response in the mid-troposphere and a decreasing response in the lower and upper troposphere. The pressure level of the maximum RH increase over the Asian MA is the highest and largest in range (500-775 hPa); the maximum increases over the North African, South American, and Australian MA are at 550-600 hPa; throughout the North American MA, the maximum heating increase is at 600 hPa; and the levels of the maximum over South Africa are 600 and 775 hPa. For most of the global MA, the maximum enhancement of RH is at 500, 550, and 600 hPa. It is mainly led by the increase in cloud water at and above the maximum level and the decrease in cloud water below, which leads to similar changes in total cloud mass. Because of the longwave heating (cooling) effect at the cloud base (top), the RH enhancements peak at those levels. For the northeast part of the Asian MA and southeast part of the South African MA, RH enhancement peaks at 700 and 775 hPa, mainly attributable to the cloud water reduction below. The reduction leads to similar changes in total cloud. Due to the longwave cooling effect at the cloud top, the reduction contributes to the RH enhancement at the corresponding maximum levels.

碳中和作为 21世纪最大规模的有序人类活动，亟待科学应对。文章从地球系统科学角度，讨论了支撑“碳达峰、碳中和”目标的大气、陆地和海洋相关的地球系统科学中的若干科学和技术问题及现存的知识不足。从地球系统模式、气候监测指标、温室气体监测技术、碳源/汇核算方法体系等方面，阐述了支撑碳中和的关键技术手段及现存的问题。基于目前存在的挑战和不足，建议深入理解气候系统多圈层相互作用过程和机制，完善地球系统理论与模式，从多圈层角度加强“碳达峰、碳中和”目标和气候变化理论基础；自主构建气候变化监测指标系统，研发温室气体监测与核查手段和平台，为碳中和目标提供先进的技术手段支撑。