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Satellite observations indicate a substantial increase in Earth's top‐of‐atmosphere (the top of the atmosphere (TOA)) radiative imbalance since 2010. We estimate trends in effective radiative forcing (ERF) by separating TOA flux changes into forcing and response components, using feedback parameters derived from observed and simulated interannual variability and the CO2‐forced response. From 2010 to 2024, ERF trends are ∼1.0 W m−2 per decade for both net and shortwave fluxes, exceeding those for 2001–2024 and substantially larger than projections from state‐of‐the‐art models. This discrepancy persists across a wide range of climate sensitivities and forcing scenarios and shows limited sensitivity to feedback assumptions. The largest contribution arises from the shortwave component, with spatial patterns indicating particularly strong forcing increases over northern midlatitude oceans. These results suggest that the gap between observations and models is widening, although the contribution of internal variability cannot be entirely excluded.

The Arctic is notable as a region where the greatest rate of increase in precipitation associated with global warming is anticipated. The Arctic precipitation simulated by the Coupled Model Intercomparison Project Phase 6 models showed a strong increasing trend since the 1980s. We found that the forcing factor of the trend is a combination of the continued strengthening of greenhouse gas forcing and the leveling off of aerosol forcing dominated in earlier periods. From an energetic perspective, we found that the increased atmospheric radiative cooling and reduced sensible heat transport from lower latitudes contributed equally to the recent increase in Arctic precipitation. The combination of these two energetic factors suggests a doubling of the Arctic amplification factor for precipitation relative to that for temperature. Future Arctic precipitation will change in proportion to the temperature change, and the fractional contributions of the energetic factors will remain stable across various scenarios. Plain Language Summary The Arctic region is inherently a low‐precipitation area. However, because of global warming, precipitation is expected to increase substantially in the Arctic region compared with the global average when viewed as a percentage change from the original precipitation. This severely affects climate change in the Arctic environment. The latest climate model simulations show that there has been a rapid increase in precipitation in the Arctic region in recent decades. The driving factors behind the rapid increase are the effects of the accelerating growth of greenhouse gas concentrations, which were previously suppressed by the increasing anthropogenic aerosol emissions before the 1980s. Based on the heat budget of the atmosphere, we identified important factors contributing to these precipitation changes. These include enhanced radiative cooling (responding locally to increased air temperature) and reduced heat transport from lower latitudes due to greater temperature increases at higher latitudes. Future precipitation will change in proportion to the temperature change while maintaining consistent fractional contributions across different scenarios. Key Points Trends in Arctic precipitation in the recent and future decades are examined from multimodel simulations The recent rapid increase is driven by accelerating greenhouse gas concentrations and plateauing growth in anthropogenic aerosol emissions Increased radiative cooling and reduced poleward sensible heat transport equally contributed to the Arctic precipitation changes

Atmospheric black carbon (BC) absorbs solar radiation, and exacerbates global warming through exerting positive radiative forcing (RF). However, the contribution of BC to ongoing changes in global climate is under debate. Anthropogenic BC emissions, and the resulting distribution of BC concentration, are highly uncertain. In particular, long-range transport and processes affecting BC atmospheric lifetime are poorly understood. Here we discuss whether recent assessments may have overestimated present-day BC radiative forcing in remote regions. We compare vertical profiles of BC concentration from four recent aircraft measurement campaigns to simulations by 13 aerosol models participating in the AeroCom Phase II intercomparison. An atmospheric lifetime of BC of less than 5 days is shown to be essential for reproducing observations in remote ocean regions, in line with other recent studies. Adjusting model results to measurements in remote regions, and at high altitudes, leads to a 25% reduction in AeroCom Phase II median direct BC forcing, from fossil fuel and biofuel burning, over the industrial era. The sensitivity of modelled forcing to BC vertical profile and lifetime highlights an urgent need for further flight campaigns, close to sources and in remote regions, to provide improved quantification of BC effects for use in climate policy.

The Aerosol Radiative Forcing in East Asia (A‐FORCE) aircraft campaign was conducted over East Asia in March–April 2009. During the A‐FORCE campaign, 120 vertical profiles of black carbon (BC) and carbon monoxide (CO) were obtained in the planetary boundary layer (PBL) and the free troposphere. This study examines the wet removal of BC in Asian outflow using the A‐FORCE data. The concentrations of BC and CO were greatly enhanced in air parcels sampled at 3–6 km in altitude over the Yellow Sea on 30 March 2009, associated with upward transport due to a cyclone with modest amounts of precipitation over northern China. In contrast, high CO concentrations without substantial enhancements of BC concentrations were observed in air parcels sampled at 5–6 km over the East China Sea on 23 April 2009, caused by uplifting due to cumulus convection with large amounts of precipitation over central China. The transport efficiency of BC (TEBC, namely the fraction of BC particles not removed during transport) in air parcels sampled above 2 km during the entire A‐FORCE period decreased primarily with the increase in the precipitation amount that air parcels experienced during vertical transport, although their correlation was modest (r2 = 0.43). TEBC also depended on the altitude to which air parcels were transported from the PBL and the latitude where they were uplifted locally over source regions. The median values of TEBC for air parcels originating from northern China (north of 33°N) and sampled at 2–4 km and 4–9 km levels were 86% and 49%, respectively, during the A‐FORCE period. These median values were systematically greater than the corresponding median values (69% and 32%, respectively) for air parcels originating from southern China (south of 33°N). Use of the A‐FORCE data set will contribute to the reduction of large uncertainties in wet removal process of BC in global‐ and regional‐scale models. Key Points Aircraft obtained 120 vertical profiles of BC and CO over East Asia Wet removal of BC was greater for air originating from south than north China Removal of BC depended on amount of precipitation experienced during transport

The largest uncertainty in the historical radiative forcing of climate is caused by changes in aerosol particles due to anthropogenic activity. Sophisticated aerosol microphysics processes have been included in many climate models in an effort to reduce the uncertainty. However, the models are very challenging to evaluate and constrain because they require extensive in situ measurements of the particle size distribution, number concentration, and chemical composition that are not available from global satellite observations. The Global Aerosol Synthesis and Science Project (GASSP) aims to improve the robustness of global aerosol models by combining new methodologies for quantifying model uncertainty, to create an extensive global dataset of aerosol in situ microphysical and chemical measurements, and to develop new ways to assess the uncertainty associated with comparing sparse point measurements with low-resolution models. GASSP has assembled over 45,000 hours of measurements from ships and aircraft as well as data from over 350 ground stations. The measurements have been harmonized into a standardized format that is easily used by modelers and nonspecialist users. Available measurements are extensive, but they are biased to polluted regions of the Northern Hemisphere, leaving large pristine regions and many continental areas poorly sampled. The aerosol radiative forcing uncertainty can be reduced using a rigorous model–data synthesis approach. Nevertheless, our research highlights significant remaining challenges because of the difficulty of constraining many interwoven model uncertainties simultaneously. Although the physical realism of global aerosol models still needs to be improved, the uncertainty in aerosol radiative forcing will be reduced most effectively by systematically and rigorously constraining the models using extensive syntheses of measurements.

Size distributions of black carbon (BC) measured by aircraft over East Asia in spring 2009 were highly correlated with BC transport efficiency in air parcels uplifted from the planetary boundary layer to the free troposphere. The average single‐particle BC mass decreased with decreasing transport efficiency, which suggests that aerosols containing larger BC mass were removed more efficiently. This is the first successful observation of the size‐dependent wet removal of aerosols, qualitatively consistent with the Köhler theory. The size distribution of BC uplifted to the free troposphere with high efficiency was similar to the size distribution of BC in the planetary boundary layer. Conversely, the size distribution of BC uplifted with low efficiency was similar to that of background air in the free troposphere. We conclude that wet removal during upward transport is important in controlling the size distribution of BC in the free troposphere. Key Points Wet removal efficiency of aerosols tightly correlates their particle size Wet removal process is a regulator of the size of tropospheric aerosols We use chemically inert black carbon aerosols to show above propositions

When laser radiation is skilfully applied, atoms and molecules can be cooled 1 – 3 , allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy 4 – 7 , ultracold gases with quantum statistical properties 8 – 10 and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium 11 , 12 , an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen 13 , a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation 14 – 18 in this matter–antimatter system. The one-dimensional laser cooling of positronium enables testing of quantum electrodynamics and could realize Bose–Einstein condensation in positronium.

Concentrations of elemental carbon (EC), carbon monoxide (CO), and carbon dioxide (CO2) were measured in Beijing between 2005 and 2006. EC was measured every hour with a semicontinuous thermal optical analyzer. The observed concentrations were rather uniform over a distance of about 50 km from the observation site. The annual average concentrations of EC and CO were 6.9 μgC m−3 and 1120 parts per billion by volume, respectively. The concentrations of these species increased with decreasing near‐surface wind speed (WS). The slopes of the CO‐CO2, EC‐CO2, and EC‐CO correlations are used to estimate major EC and CO sources. In the weak wind regime (WS ≤ 2.0 m s−1), the median EC, ΔEC/ΔCO2, and ΔEC/ΔCO (except for winter) increased in the late evening and remained high until early morning. The traffic of heavy duty diesel trucks during nighttime was about 20 times higher than that during daytime. These results indicate a dominant contribution of exhaust from diesel vehicles to the nighttime EC. In winter, the nighttime CO and ΔCO/ΔCO2 ratio were largely higher than those in the other seasons. The most likely cause is the increase in the CO emissions from the exhaust of gasoline vehicles at low temperature. The ΔEC/ΔCO2 ratio in winter was lower than that in fall, indicating no significant additional EC emissions. The diurnal variations of EC, CO, CO2, and ΔEC/ΔCO were similar between weekdays and weekends. The slopes of the CO‐CO2‐EC correlations are compared with the CO‐CO2‐EC ratios derived from a published emission inventory in the Beijing area.

East Asia, including China, is the largest source of anthropogenic black carbon (BC). In estimating the BC emissions from this region, it is advantageous to use BC mass concentrations measured at remote locations on the ocean appropriately distant from the large sources because of spatially uniform distributions through mixing during transport. We made continuous measurements of the BC mass concentration with an accuracy of about 10% at Cape Hedo on Okinawa Island, Japan, in the East China Sea, from February 2008 to May 2009, simultaneously with carbon monoxide (CO). The seasonal median BC concentrations at Hedo were highest (0.23–0.31 μg m−3 at standard temperature and pressure) in winter and spring when plumes from China, predominantly northern China north of 33°N, were often transported to the site. A three‐dimensional chemical transport model is used to calculate the mass concentration of BC using the annual mean emission inventory of Zhang et al. (2009) for the base year 2006. The model results and the observed BC‐CO correlation are used to exclude the BC data substantially influenced by wet deposition. The calculated BC mass concentrations agree with those observed to within about 30% in air strongly affected by emissions in China for winter and spring on average. We estimate the annually averaged BC emission flux over the whole of China to be 1.92 Tg yr−1 with an uncertainty of about 40%. This value is very close to the value of 1.81 Tg yr−1 estimated by Zhang et al. (2009). The overall uncertainty of 40% of the present estimate is a substantial improvement in the uncertainty (208%) of the bottom‐up inventory. Key Points CMAQ model reproduced the temporal variations of BC in the Asian outflows Our estimate of BC emissions from China is close to that of Zhang et al. [2009] The uncertainty of the estimated BC emissions is 54%, a great improvement

The Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution with resolution of a mixing state of black carbon (BC) (referred to as MADRID‐BC hereinafter) has recently been developed to accurately simulate the time evolution of the entire BC mixing state. In this study, we apply MADRID‐BC to evaluate the influence of changes in BC mixing state on aerosol optical properties and cloud condensation nuclei (CCN) activities in air parcels horizontally transported out from an urban area in Japan within the planetary boundary layer (PBL) over the ocean. The evaluation shows that the coatings on BC particles enhance light absorption at a wavelength of 550 nm by 38% in air leaving the source region and by 59% after transport over the ocean for half a day. When the model treats aerosols using the conventional size‐resolved sectional representation that does not resolve BC mixing states, the simulated absorption coefficients and single scattering albedos are greater by 35–44% and smaller by 7–13%, respectively, than those from a simulation that resolves the BC mixing state. These results indicate that it is essential to take into account BC‐free particles in atmospheric models for accurate prediction of aerosol optical properties, because the conventional representation cannot separately treat BC‐containing and BC‐free particles in each size section. The evaluation also shows that BC‐containing particles having 55% and 83% of the BC mass can act as CCN at a supersaturation of 0.05% when they leave the source region and after transport for half a day, respectively. These results suggest the importance of the uplifting of BC particles from the PBL near source regions for their efficient long‐range transport in the free troposphere. Results from comparisons with aerosol optical measurements conducted during various campaigns, such as the Asian Aerosol Characterization Experiment (ACE Asia) and the Indian Ocean Experiment (INDOEX), suggest that MADRID‐BC simulations can capture general features of aerosol optical properties in outflow from anthropogenic sources.