Achievement
Atmosphere

Atmospheric Environment and Climate Forcings in the Arctic

Research Background

The main cause of Arctic warming, which is progressing more than three times faster than the global average, is thought to be the global increase in atmospheric CO₂ concentrations and various feedbacks (warming amplification) operating in the Arctic. At the same time, however, short-lived climate forcers (SLCFs), i.e., short-lived greenhouse gases such as CH₄ and black carbon (BC) aerosols (fine particles suspended in the atmosphere) that absorb solar radiation, are thought to play an important role. Along with the atmospheric heating effect, BC may also promote ice albedo feedback by reducing its albedo (reflectance) through deposition on snow/ice surfaces. It has been suggested that these SLCFs may be a means of slowing Arctic warming because their emissions can reduce atmospheric concentrations with a shorter time constant than CO₂. For this reason, the SLCF Expert Group of the Arctic Monitoring and Assessment Programme (AMAP), which is one of the Arctic Council's (AC) working groups, published an SLCF Assessment Report in 2021 (AMAP, 2021^*), reiterating to the world the importance of obtaining scientific knowledge on the subject.

However, there are significant uncertainties in estimating the radiative impact and abatement effects of the SLCF. First, BC concentrations in the Arctic atmosphere have been observed for many years at multiple stations, mainly by Western research institutes, but the accuracy of the measurements has not always been sufficiently verified, and systematic intercomparisons have not been made. Therefore, there are uncertainties in the atmospheric BC concentrations that form the basis of BC impact assessment. Second, although transport from mid-latitudes and wildfires at high latitudes with large emissions are considered to be important sources of BC and other aerosols in the Arctic, there are uncertainties in their contribution and impact assessment due to a lack of observational data and uncertainties in numerical model calculations. Third, there are various sources of emissions (e.g., wetlands, fossil fuels, wildfires, ruminants, etc.) in CH₄, but it is not easy to determine their broad distribution and changes, so there are large uncertainties in assessing their source and sink budgets. In addition, permafrost thawing is a potential source of CO₂ and CH₄ and has been noted as a possible positive feedback in Arctic warming, but the observations necessary for quantitative discussion are extremely limited.

Purpose of the Study and Summary of Research Results

The purpose of this research is to evaluate and elucidate the dynamics, sources and sinks, sources of variability, and cloud and radiation effects of atmospheric substances such as greenhouse gases and aerosols that affect the Arctic climate through advanced observations and numerical modeling studies, which is one of Japan's strengths. To this end, in greenhouse gas research, atmospheric concentrations of CO₂, CH₄, N₂O and their isotopic ratios (δ¹³C-CO₂, δ¹³C-CH₄) and O₂/N₂ ratios were measured with high precision from the ground, ship (Research Vessel Mirai) and commercial aircraft. Using these observations and numerical modeling, the project estimated the average uptake rate of CO₂ by the ocean and terrestrial biosphere. We also showed that the rapid increase in CH₄ concentrations since around 2018 can be attributed to elevated microbial CH₄ emissions.

Aerosol studies on BC include reliable estimation of its refractive index, presentation of the scientific basis for building a unified BC dataset for the Arctic, estimation of the contribution to Arctic BC from anthropogenic and biomass burning sources in various regions, and evaluation of the influence of meteorological fields such as the Arctic Oscillation on BC transport to the Arctic. In the non-BC Arctic aerosol study, we proposed the summer increase in ice nucleating particles (INPs) with high ice crystal formation ability, the possibility that these INPs are mineral dust and/or bioaerosols, and evaluated the impact of these INPs on clouds using numerical models that incorporate these observational findings. During the R/V Mirai Arctic cruises, we conducted the integrated aerosol observations including BC and INP in the Pacific Arctic Ocean almost every year, and showed that marine organisms contribute the source of bioaerosols, cloud condensation nuclei, and INP over the Arctic Ocean during periods of high biological activity. Using numerical models validated and refined by these various observations, the effective radiative forcing of greenhouse gases and aerosols in the Arctic was evaluated. In addition, the role of greenhouse gases and aerosols in Arctic warming and cooling in the first half and middle of the 20th century, respectively, was also evaluated.

Some of the observations made in this project have been continued before this project, and these high-precision observations are being used in greenhouse gas and aerosol studies around the world. The results of this study were also reflected in AMAP's SLCF assessment report (AMAP, 2021*). These scientific findings contributed not only to Strategic Goal 1 of the project, but also to Strategic Goals 2, 3, and 4.

*AMAP, 2021. AMAP Assessment 2021: Impacts of Short-lived Climate Forcers on Arctic Climate, Air Quality, and Human Health. Arctic Monitoring and Assessment Programme (AMAP), Tromso, Norway. x + 375pp

Research Background and Overview

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List of the Research Achievements

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Obtained Data

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