CO2-induced Warming Research Articles

The Attribution of February Extremes over North America: A Forecast-Based Storyline Study

Abstract The importance of extreme event attribution rises as climate change causes severe damage to populations resulting from unprecedented events. In February 2019, a planetary wave shifted along the U.S.–Canadian border, simultaneously leading to troughing with anomalous cold events and ridging over Alaska and northern Canada with abnormal warm events. Also, a dry-stabilized anticyclonic circulation over low latitudes induced warm extreme events over Mexico and Florida. Most attribution studies compare the climate model simulations under natural or actual forcing conditions and assess probabilistically from a climatological point of view. However, in this study, we use multiple ensembles from an operational forecast model, promising statistical as well as dynamically constrained attribution assessment, often referred to as the storyline approach to extreme event attribution. In the globally averaged results, increasing CO2 concentrations lead to distinct warming signals at the surface, resulting mainly from diabatic heating. Our study finds that CO2-induced warming eventually affects the possibility of extreme events in North America, quantifying the impact of anthropogenic forcing over less than a week’s forecast simulation. Our study assesses the validity of the storyline approach conditional on the forecast lead times, which is hindered by rising noise in CO2 signals and the declining performance of the forecast model. The forecast-based storyline approach is valid for at least half of the land area within a 6-day lead time before the target extreme occurrence. Our attribution results highlight the importance of achieving net-zero emissions ahead of schedule to reduce the occurrence of severe heatwaves.

Efficient Lead Removal by Assembly of Bio-Derived Ellagate Framework, Which Enables Electrocatalytic Reduction of CO2 to Formate.

Lead (Pb) poisoning and CO2-induced global warming represent two exemplary environmental and energy issues threatening humanity. Various biomass-derived materials are reported to take up Pb and convert CO2 electrochemically into low-valent carbon species, but these works address the problems separately rather than settle the issues simultaneously. In this work, cheap, natural ellagic acid (EA) extracted from common plants is adopted to assemble a stable metal-organic framework (MOF), EA-Pb, by effective capture of Pb2+ ions in an aqueous medium (removal rate close to 99%). EA-Pb represents the first structurally well-defined Pb-based MOF showing selective electrocatalytic CO2-to-HCOO- conversion with Faradaic efficiency (FE) of 95.37% at -1.08V versus RHE. The catalytic mechanism is studied by 13CO2 labeling, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and theoretical calculation. The use of EA-Pb as an electrocatalyst for CO2 reduction represents a 2-in-1 solution of converting detrimental wastes (Pb2+) as well as natural resources (EA) into wealth (electrocatalytic EA-Pb) for addressing the global warming issue.

Seasonal Variations in Anthropogenic and Natural Particles Induced by Rising CO2 Levels

Using an aerosol–climate coupled model, this paper has investigated the changes in distributions of anthropogenic and natural particles due to 4 × CO2-induced global warming, under the low emission scenario of Representative Concentration Pathway 4.5 (RCP4.5). Special attention is paid to the seasonal variations of aerosol size modes. With rising CO2 levels, surface warming, and changes in atmospheric circulations and hydrologic cycles are found during both summer (JJA) and winter (DJF). For anthropogenic particles, changes in fine anthropogenic particulate matter (PM2.5, particles with diameters smaller than 2.5 μm) decrease over high-latitude regions and increase over the tropics in both DJF and JJA. Global mean column concentrations of PM2.5 decrease by approximately 0.19 mg m−2, and concentrations of coarse anthropogenic particles (CPM, particles with diameters larger than 2.5 μm) increase by 0.005 mg m−2 in JJA. Changes in anthropogenic particles in DJF are similar to those in JJA, but the magnitudes of maximum regional changes are much smaller than those in JJA. The coarse anthropogenic particles (CPM, particles with diameters larger than 2.5 μm) increase over northern Africa and the Arabian Peninsula during JJA, whereas changes in anthropogenic CPM during DJF are minimal. During both JJA and DJF, changes in anthropogenic CPM are about two orders of magnitude smaller than those of anthropogenic PM2.5. Enhanced wet deposition by large-scale precipitation under rising CO2-induced surface warming is the critical factor affecting changes in anthropogenic particles. For natural particles, the distribution of change in the natural PM2.5 burden is similar to that of natural CPM, but much larger than natural CPM during each season. Both natural PM2.5 and CPM burdens increase over northern Africa and the Arabian Peninsula during JJA, but decrease over most of the continental regions during DJF. Changes in surface wind speed, divergence/convergence of surface wind, and precipitation are primary reasons for the variation of natural particles.

Robust changes in global subtropical circulation under greenhouse warming

The lower tropospheric subtropical circulation (SC) is characterized by monsoons and subtropical highs, playing an important role in global teleconnections and climate variability. The SC changes in a warmer climate are influenced by complex and region-specific mechanisms, resulting in uneven projections worldwide. Here, we present a method to quantify the overall intensity change in global SC, revealing a robust weakening across CMIP6 models. The weakening is primarily caused by global-mean surface warming, and partly counteracted by the direct CO2 effect. The direct CO2 effect is apparent in the transient response but is eventually dominated by the surface warming effect in a slow response. The distinct response timescales to global-mean warming and direct CO2 radiative forcing can well explain the time-varying SC changes in other CO2 emission scenarios. The declined SC implies a contracted monsoon range and drying at its boundary with arid regions under CO2-induced global warming.

Commentary: climate policy in Aotearoa New Zealand – a backwards flip?

In 1975, Chris Nobbs, a young New Zealander trained in chemistry and economics and working at the Environment Directorate of the OECD in Paris, wrote a paper titled ‘The economics of stock pollutants: the example of cadmium’. Together with David Pearce, later an eminent environmental economist, he noted that ‘pollution taxes will not secure the optimal level of pollution … since such taxes are relevant only for flow pollutant situations’ (Nobbs and Pearce, 1975). It was not until a generation later that it became widely accepted that carbon dioxide itself is a stock pollutant and that only ‘net zero’ CO2 emissions will halt CO2-induced global warming (Fankhauser et al., 2022). This has led to nearly 100 states and 1,000 large companies establishing net zero goals (Hale et al., 2022). One such is New Zealand’s Climate Change Response (Zero Carbon) Amendment Act 2019, which requires the government to set and meet carbon budgets to reach net zero long-lived greenhouse gas emissions by, and beyond, 2050.

High-resolution (1 km) Köppen-Geiger maps for 1901–2099 based on constrained CMIP6 projections

We introduce Version 2 of our widely used 1-km Köppen-Geiger climate classification maps for historical and future climate conditions. The historical maps (encompassing 1901–1930, 1931–1960, 1961–1990, and 1991–2020) are based on high-resolution, observation-based climatologies, while the future maps (encompassing 2041–2070 and 2071–2099) are based on downscaled and bias-corrected climate projections for seven shared socio-economic pathways (SSPs). We evaluated 67 climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) and kept a subset of 42 with the most plausible CO2-induced warming rates. We estimate that from 1901–1930 to 1991–2020, approximately 5% of the global land surface (excluding Antarctica) transitioned to a different major Köppen-Geiger class. Furthermore, we project that from 1991–2020 to 2071–2099, 5% of the land surface will transition to a different major class under the low-emissions SSP1-2.6 scenario, 8% under the middle-of-the-road SSP2-4.5 scenario, and 13% under the high-emissions SSP5-8.5 scenario. The Köppen-Geiger maps, along with associated confidence estimates, underlying monthly air temperature and precipitation data, and sensitivity metrics for the CMIP6 models, can be accessed at www.gloh2o.org/koppen.

Recent advancement in CO2 capturing: An electrochemical approach

Various methods have been suggested to address the problem of elevated levels of carbon dioxide (CO2) in the atmosphere. A potential technique among these strategies is the electrochemical reduction of CO2 (ERC), which can simultaneously solve issues like CO2-induced global warming and enable sustainable energy storage. The ERC procedure takes place within an electrochemical cell at the interface between an ionic conductor (the electrolyte) and an electron conductor (the cathode). In this process, the anode facilitates the oxidation of water, while the cathode enables CO2 reduction. However, a significant challenge in the ERC process lies in selecting an appropriate catalyst for the cathode material. In addition to explore the new transition metals, researchers have also investigated metal complex catalysts and nanostructures to enhance catalyst activity and product selectivity. Non-aqueous electrolytes have been employed to avoid the hydrogen evolution reaction. Ionic liquids have been proposed as a solution to address the challenge of low CO2 solubility in the reaction medium, offering significant potential for enhancing conversion rates to resolve the limitations faced by aqueous and non-aqueous systems solid polymer electrolytes are being considered. The cell configuration has been continuously improved to improve the mass transfer effects and to better separate the reaction products. This review paper provides a preface to the ERC process and an inclusive review of several decades of research work on ERC process by analyzing the adopted various and novel cathode materials, electrolytes and cell configuration.

Combining local model calibration with the emergent constraint approach to reduce uncertainty in the tropical land carbon cycle feedback

Abstract. The role of the land carbon cycle in climate change remains highly uncertain. A key source of the projection spread is related to the assumed response of photosynthesis to warming, especially in the tropics. The optimum temperature for photosynthesis determines whether warming positively or negatively impacts photosynthesis, thereby amplifying or suppressing CO2 fertilisation of photosynthesis under CO2-induced global warming. Land carbon cycle models have been extensively calibrated against local eddy flux measurements, but this has not previously been clearly translated into a reduced uncertainty in terms of how the tropical land carbon sink will respond to warming. Using a previous parameter perturbation ensemble carried out with version 3 of the Hadley Centre coupled climate–carbon cycle model (HadCM3C), we identify an emergent relationship between the optimal temperature for photosynthesis, which is especially relevant in tropical forests, and the projected amount of atmospheric CO2 at the end of the century. We combine this with a constraint on the optimum temperature for photosynthesis, derived from eddy covariance measurements using the adjoint of the Joint UK Land Environment Simulator (JULES) land surface model. Taken together, the emergent relationship from the coupled model and the constraint on the optimum temperature for photosynthesis define an emergent constraint on future atmospheric CO2 in the HadCM3C coupled climate–carbon cycle under a common emissions scenario (A1B). The emergent constraint sharpens the probability density of simulated CO2 change (2100–1900) and moves its peak to a lower value of 497 ± 91 compared to 607 ± 128 ppmv (parts per million by volume) when using the equal-weight prior. Although this result is likely to be model and scenario dependent, it demonstrates the potential of combining the large-scale emergent constraint approach with a parameter estimation using detailed local measurements.

Feedbacks of CaCO3 dissolution effect on ocean carbon sink and seawater acidification: a model study

The oceanic absorption of atmospheric CO2 acidifies seawater, which accelerates CaCO3 dissolution of calcifying organisms, a process termed dissolution effect. Promoted CaCO3 dissolution increases seawater ALK (alkalinity), enhancing ocean carbon sink and mitigating ocean acidification. We incorporate different parameterizations of the link between CaCO3 dissolution and ocean acidification into an Earth System Model, to quantify the feedback of the dissolution effect on the global carbon cycle. Under SRES A2 CO2 emission scenario and its extension with emissions of 5,000 PgC in ∼400 years, in the absence of the dissolution effect, accumulated ocean CO2 uptake between year 1800 and 3500 is 2,041 PgC. The consideration of the dissolution effect increases ocean carbon sink by 195–858 PgC (10%–42%), and mitigates the decrease in surface pH by 0.04–0.17 (a decrease of 10%–48% in [H+] (hydrogen ion concentration)), depending on the prescribed parameterization scheme. In the epipelagic zone, relative to the Arc-Atlantic Ocean, the Pacific-Indian Ocean experiences greater acidification, leading to greater dissolution effects and the resultant stronger feedbacks on ocean carbon sink and acidification in the Pacific-Indian Ocean. Noteworthy, the feedback of dissolution effect on ocean carbon sink can be comparable with or stronger than the feedback from CO2-induced radiative warming. Our study highlights the potentially critical role played by CaCO3 dissolution effect in the ocean carbon sink, global carbon cycle and climate system.

The responses of SST annual cycle in the eastern equatorial Pacific to global warming

The eastern equatorial Pacific exhibits a pronounced westward propagating sea surface temperature annual cycle (SSTAC). The responses of the equatorial Pacific SSTAC to CO2-induced global warming are examined using 15 Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. The annual cycle patterns of global-warming simulations over 2006-2100 are compared with that of present-day simulations over 1850-2005. We see no statistically significant changes in SSTAC amplitude in the future. A coupled dynamical diagnostic framework is adopted to assess four factors, including the damping rate, phase speed and strength of the annual and semi-annual harmonic forcing of SSTAC. Under global warming, changes relative to the present-day simulations in these four diagnostic factors have a clear multi-model trend. Most coupled models exhibit relatively weaker (an average of 18%) propagation speed, and stronger annual (18%) and semi-annual (39%) external forcing. Half of the models show a relatively stronger (about one time) damping rate, while the rest show a weaker (30%) damping rate. When these four diagnostic factors are further condensed into a dynamical response factor and a forcing factor, it is revealed that the same annual cycle amplitudes with respect to the present-day simulations may result from the compensations in terms of bias in the dynamical response factor and forcing factor under increased CO2-induced warm climate.

Synergistic effect of hydrogen and nanoscale zero-valent iron on ex-situ biogas upgrading and acetate recovery

Hydrogen (H2) assisted ex-situ biogas upgrading and liquid chemicals production can augment the fossil fuel-dominated energy market, and alleviate CO2-induced global warming. Recent investigations confirmed that nanoscale zero-valent iron (nZVI) enabled the enhancement of anaerobic digestion for biogas production. However, little is known about the effect of nZVI on the downstream ex-situ biogas upgrading. Herein, different levels (0 mg L−1, 100 mg L−1, 200 mg L−1, 500 mg L−1, 1000 mg L−1, 2000 mg L−1) of nZVI were added for H2-assisted ex-situ biogas upgrading, to study whether nZVI could impact the biomethane purity and acetate yield for the first time. Results showed that all tested nZVI levels were favorable for biogas upgrading in the presence of H2, the highest biomethane content (94.1 %, v/v), the CO2 utilization ratio (95.9 %), and acetate yield (19.4 mmol L−1) were achieved at 500 mg L−1 nZVI, respectively. Further analysis indicated that increased biogas upgrading efficiency was related to an increase in extracellular polymeric substances, which ensures the microbial activity and stability of the ex-situ biogas upgrading. Microbial community characterization showed that the Petrimonas, Romboutsia, Acidaminococcus, and Clostridium predominated the microbiome during biogas upgrading at 500 mg L−1 nZVI with H2 supply. These results suggested that nZVI and H2 contributed jointly to promoting the bioconversion of CO2 in biogas to acetate. The findings could be helpful for paving a new way for efficient simultaneous ex-situ biogas upgrading and liquid chemicals recovery.

The East Asian Summer Monsoon Response to Global Warming in a High Resolution Coupled Model: Mean and Extremes

Current climate models still have considerable biases in the simulation of the East Asian summer monsoon (EASM), which in turn reduces their reliability of monsoon projections under global warming. We hypothesize that a higher-resolution coupled climate model with atmospheric and oceanic components at horizontal resolutions of 0.25° and 0.1°, respectively, will better capture regional details and extremes of the EASM. Present-day (PD), 2 × CO2 and 4 × CO2 simulations are conducted with the Community Earth System Model (CESM1.2.2) to evaluate PD simulation performance and quantify future changes. Indeed, our PD simulation well reproduces the climatological seasonal mean and intra-seasonal northward advancement of the monsoon rainband, as well as climate extremes. Compared with the PD simulation, the perturbed CO2 experiments show an intensified EASM response to CO2-induced warming. We find that the precipitation increases of the Meiyu-Baiu-Changma band are caused by comparable contributions from the dynamical and thermodynamical components in 2 × CO2, while they are more driven by the thermodynamical component in 4 × CO2 due to stronger upper atmospheric stability. The regional changes in the probability distribution of the temperature show that extreme temperatures warm faster than the most often temperatures, increasing the skewness. Fitting extreme precipitation values with a generalized Pareto distribution model reveals that they increase significantly in 4 × CO2. Changes of temperature extremes scale with the CO2 concentrations over the monsoon domain but not for precipitation extreme changes. The 99th percentile of precipitation over the monsoon region increases at a super Clausius-Clapeyron rate, ~ 8% K–1, which is mainly caused by increased moisture transport through anomalous southerly winds.

Global Warming Pattern Formation: The Role of Ocean Heat Uptake

Abstract This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2 with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQnet). We demonstrate that the transient patterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by the equilibrium solutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQnet distribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO2-induced thermodynamic warming effect and the ΔQnet (ocean heat uptake)–induced cooling effect. As ΔQnet is largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO2 warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQnet-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO2 warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQnet, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQnet effect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern.

Minimal CMIP Emulator (MCE v1.2): a new simplified method for probabilistic climate projections

Abstract. Climate model emulators have a crucial role in assessing warming levels of many emission scenarios from probabilistic climate projections based on new insights into Earth system response to CO2 and other forcing factors. This article describes one such tool, MCE, from model formulation to application examples associated with a recent model intercomparison study. The MCE is based on impulse response functions and parameterized physics of effective radiative forcing and carbon uptake over ocean and land. Perturbed model parameters for probabilistic projections are generated from statistical models and constrained with a Metropolis–Hastings independence sampler. Some of the model parameters associated with CO2-induced warming have a covariance structure, as diagnosed from complex climate models of the Coupled Model Intercomparison Project (CMIP). Perturbed ensembles can cover the diversity of CMIP models effectively, and they can be constrained to agree with several climate indicators such as historical warming. The model's simplicity and resulting successful calibration imply that a method with less complicated structures and fewer control parameters offers advantages when building reasonable perturbed ensembles in a transparent way. Experimental results for future scenarios show distinct differences between CMIP-consistent and observation-consistent ensembles, suggesting that perturbed ensembles for scenario assessment need to be properly constrained with new insights into forced response over historical periods.

The influence of direct radiative forcing versus indirect sea surface temperature warming on southern hemisphere subtropical anticyclones under global warming

Southern hemisphere subtropical anticyclones are projected to change in a warmer climate during both austral summer and winter. A recent study of CMIP 5 & 6 projections found a combination of local diabatic heating changes and static-stability-induced changes in baroclinic eddy growth as the dominant drivers. Yet the underlying mechanisms forcing these changes still remain uninvestigated. This study aims to enhance our mechanistic understanding of what drives these Southern Hemisphere anticyclones changes during both seasons. Using an AGCM, we decompose the response to CO2-induced warming into two components: (1) the fast atmospheric response to direct CO2 radiative forcing, and (2) the slow atmospheric response due to indirect sea surface temperature warming. Additionally, we isolate the influence of tropical diabatic heating with AGCM added heating experiments. As a complement to our numerical AGCM experiments, we analyze the Atmospheric and Cloud Feedback Model Intercomparison Project experiments. Results from sensitivity experiments show that slow subtropical sea surface temperature warming primarily forces the projected changes in subtropical anticyclones through baroclinicity change. Fast CO2 atmospheric radiative forcing on the other hand plays a secondary role, with the most notable exception being the South Atlantic subtropical anticyclone in austral winter, where it opposes the forcing by sea surface temperature changes resulting in a muted net response. Lastly, we find that tropical diabatic heating changes only significantly influence Southern Hemisphere subtropical anticyclone changes through tropospheric wind shear changes during austral winter.

Performance Analysis of Blended Membranes of Cellulose Acetate with Variable Degree of Acetylation for CO2/CH4 Separation.

The separation and capture of CO2 have become an urgent and important agenda because of the CO2-induced global warming and the requirement of industrial products. Membrane-based technologies have proven to be a promising alternative for CO2 separations. To make the gas-separation membrane process more competitive, productive membrane with high gas permeability and high selectivity is crucial. Herein, we developed new cellulose triacetate (CTA) and cellulose diacetate (CDA) blended membranes for CO2 separations. The CTA and CDA blends were chosen because they have similar chemical structures, good separation performance, and its economical and green nature. The best position in Robeson’s upper bound curve at 5 bar was obtained with the membrane containing 80 wt.% CTA and 20 wt.% CDA, which shows the CO2 permeability of 17.32 barrer and CO2/CH4 selectivity of 18.55. The membrane exhibits 98% enhancement in CO2/CH4 selectivity compared to neat membrane with only a slight reduction in CO2 permeability. The optimal membrane displays a plasticization pressure of 10.48 bar. The newly developed blended membranes show great potential for CO2 separations in the natural gas industry.

Multidecadal trends in Ikogosi Temperature and Rainfall to Climate change

Ikogosi is located in a humid rainforest typical in its virtually uniform temperature through the year averaging 21°C to 28°C. Thirty-eight years temperature and precipitation data on Ikogosi, reveals a deviation in this erstwhile norm and portends a trend that on short-term and regional variations predict a more extreme warm climate. In contrast to previous ones, the fifth IPCC report emphasizes on the socio-economic facets of climate change. It also discusses on its dual role in sustainability and risk management, it dwells on reducing greenhouse gases and adaptation approaches to climate change. In view of this impending change, precipitation and temperature are vital climatic constraints which should be investigated for deviations that could adversely affect agriculture, architecture, societal and urban planning. Also, since a warming atmosphere is allied with substantial precipitation, all human based activities either directly or indirectly trigger greenhouse gas emissions and high precipitation. This explores the most prominent of these greenhouse gases, CO2 which warms the atmosphere as its volume increases in the atmosphere. In effect, the degree of future global warming will be a function of how much more volume of CO2 humankind emits. Although current CO2-induced warming of Earth is essentially irreversible on human timescales, weather researches to obtain scientific information, is very relevant. This is a dynamic prerequisite for society to make informed decisions on mitigation, adaptation, and other ways to tackle climate change. Thus, this study seeks to analyse the measure of observed climatic changes in Ikogosi. This is in order to promote the application of this study model to similar geographical locations. The potential energy available for certain extreme weather events has been analysed with existing theoretical explanations, ORIGIN software program and statistical regression analysis.

Advanced multiple-layer composite CTA/CDA hollow fiber membranes for CO2 separations

CO2 separation has become an important global agenda because of the requirements of industrial products and CO2-induced global warming. Membrane-based technologies have provided an alternative for CO2 separation. To make membrane-based separation more competitive, membranes must have a high permeance and high selectivity. Herein, we have developed a new high-performance multiple-layer hollow fiber membrane of cellulose triacetate (CTA) and cellulose diacetate (CDA) blends for CO2 separation. CTA and CDA blends were chosen because they have similar chemical structures, good separation performance, economical and green nature. Both single-layer and dual-layer hollow fibers were fabricated by various spinning strategies. The optimized dual-layer membrane spun at outer dope flow rate of 1 mL/min exhibits a CO2 permeance of 45 GPU and an ideal CO2/CH4 selectivity of 30.3 at 2 bar. Comparing with the optimized single-layer hollow fiber spun at 5 cm air gap distance and 15 m/min take-up speed, the former has a 100% higher CO2 permeance without compromising the selectivity. The as-produced hollow fibers display a plasticization pressure of about 10 bar. In addition, they have impressive mixed gas CO2/CH4 selectivity of >40 at 2 bar. Therefore, the newly developed dual-layer hollow fiber membranes may have great potential for CO2/CH4 separation.

Response of summer extreme precipitation over East Asia during the mid-Holocene versus future global warming

Changes in extreme precipitation have significant impacts on society and the environment. In recent years, the response of extreme precipitation over East Asia to global warming has received considerable attention; however, few studies have focused on the extreme precipitation in geological warm periods, especially the mid-Holocene—the most recent warm period on the orbital timescale. In this study, the outputs of mid-Holocene, pre-industrial and CO2-induced warming experiments using a coupled climate model (MPI-ESM-P) were employed to examine the changes in extreme precipitation over East Asia in the mid-Holocene and under future warming scenario, respectively. The results show that, during the mid-Holocene, summer extreme precipitation increases over the east of China but decreases over Japan and the surrounding oceans. However, under CO2-induced warming, summer extreme precipitation increases over the southeast of China, Japan, and the surrounding oceans, but decreases over the north of East Asia. Moisture budget analysis shows that these distinct responses come from different changes in moisture and circulation during the two periods. During the mid-Holocene, higher insolation over the Northern Hemisphere leads to a larger ocean–land thermal contrast and a stronger East Asian summer monsoon (EASM). The strengthened EASM brings more moisture to continental East Asia, resulting in an increase in both mean and extreme precipitation. In contrast, the CO2-induced warming facilitates an anomalous heating center in the upper troposphere over the tropical part of Northwestern Pacific ocean, which weakens the EASM. However, enhanced atmospheric moisture counterbalances the effect of the weakened EASM, resulting in more extreme precipitation over the south of East Asia.

Uncertainty in carbon budget estimates due to internal climate variability

Remaining carbon budget specifies the cap on global cumulative CO2 emissions from the present-day onwards that would be in line with limiting global warming to a specific maximum level. In the context of the Paris Agreement, global warming is usually interpreted as the externally-forced response to anthropogenic activities and emissions, but it excludes the natural fluctuations of the climate system known as internal variability. A remaining carbon budget can be calculated from an estimate of the anthropogenic warming to date, and either (i) the ratio of CO2-induced warming to cumulative emissions, known as the Transient Climate Response to Emissions (TCRE), in addition to information on the temperature response to the future evolution of non-CO2 emissions; or (ii) climate model scenario simulations that reach a given temperature threshold. Here we quantify the impact of internal variability on the carbon budgets consistent with the Paris Agreement derived using either approach, and on the TCRE diagnosed from individual models. Our results show that internal variability contributes approximately ±0.09 °C to the overall uncertainty range of the human-induced warming to-date, leading to a spread in the remaining carbon budgets as large as ±50 PgC, when using approach (i). Differences in diagnosed TCRE due to internal variability in individual models can be as large as ±0.1 °C/1000 PgC (5%–95% range). Alternatively, spread in the remaining carbon budgets calculated from (ii) using future concentration-driven simulations of large ensembles of CMIP6 and CMIP5 models is estimated at ±30 PgC and ±40 PgC (5%–95% range). These results are important for model evaluation and imply that caution is needed when interpreting small remaining budgets in policy discussions. We do not question the validity of a carbon budget approach in determining mitigation requirements. However, due to intrinsic uncertainty arising from internal variability, it may only be possible to determine the exact year when a budget is exceeded in hindsight, highlighting the importance of a precautionary approach.