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Tundra and boreal ecosystems encompass the northern circumpolar permafrost region and are experiencing rapid environmental change with important implications for the global carbon (C) budget. We analysed multi-decadal time series containing 302 annual estimates of carbon dioxide (CO2) flux across 70 permafrost and non-permafrost ecosystems, and 672 estimates of summer CO2flux across 181 ecosystems. We find an increase in the annual CO2sink across non-permafrost ecosystems but not permafrost ecosystems, despite similar increases in summer uptake. Thus, recent non-growing-season CO2losses have substantially impacted the CO2balance of permafrost ecosystems. Furthermore, analysis of interannual variability reveals warmer summers amplify the C cycle (increase productivity and respiration) at putatively nitrogen-limited sites and at sites less reliant on summer precipitation for water use. Our findings suggest that water and nutrient availability will be important predictors of the C-cycle response of these ecosystems to future warming.

Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO2and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO2sink with lower net CO2uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO2sink was located in western Canada (median: −52 g C m−2y−1) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m−2y−1). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH4m−2y−1). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO2and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects. Climate change and the consequent thawing of permafrost threatens to transform the permafrost region from a carbon sink into a carbon source, posing a challenge to global climate goals. Numerous studies over the past decades have identified important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Overall, studies show high wetland methane emissions and a small net carbon dioxide sink strength over the terrestrial permafrost region but results differ among modeling and upscaling approaches. Continued and coordinated efforts among field, modeling, and remote sensing communities are needed to integrate new knowledge from observations to modeling and predictions and finally to policy. Rapid warming of northern permafrost region threatens ecosystems, soil carbon stocks, and global climate targetsLong‐term observations show importance of disturbance and cold season periods but are unable to detect spatiotemporal trends in C fluxCombined modeling and syntheses show the permafrost region is a small terrestrial CO2sink with large spatial variability and net CH4source Rapid warming of northern permafrost region threatens ecosystems, soil carbon stocks, and global climate targets Long‐term observations show importance of disturbance and cold season periods but are unable to detect spatiotemporal trends in C flux Combined modeling and syntheses show the permafrost region is a small terrestrial CO2sink with large spatial variability and net CH4source

Recent warming in the Arctic, which has been amplified during the winter1–3, greatly enhances microbial decomposition of soil organic matter and subsequent release of carbon dioxide (CO2)4. However, the amount of CO2released in winter is not known and has not been well represented by ecosystem models or empirically based estimates5,6. Here we synthesize regional in situ observations of CO2flux from Arctic and boreal soils to assess current and future winter carbon losses from the northern permafrost domain. We estimate a contemporary loss of 1,662?TgC per year from the permafrost region during the winter season (October–April). This loss is greater than the average growing season carbon uptake for this region estimated from process models (-1,032?TgC per year). Extending model predictions to warmer conditions up to 2100 indicates that winter CO2emissions will increase 17% under a moderate mitigation scenario—Representative Concentration Pathway 4.5—and 41% under business-as-usual emissions scenario—Representative Concentration Pathway 8.5. Our results provide a baseline for winter CO2emissions from northern terrestrial regions and indicate that enhanced soil CO2loss due to winter warming may offset growing season carbon uptake under future climatic conditions.

Boreal‐Arctic regions are key stores of organic carbon (C) and play a major role in the greenhouse gas balance of high‐latitude ecosystems. The carbon‐climate (C‐climate) feedback potential of northern high‐latitude ecosystems remains poorly understood due to uncertainty in temperature and precipitation controls on carbon dioxide (CO2) uptake and the decomposition of soil C into CO2and methane (CH4) fluxes. While CH4fluxes account for a smaller component of the C balance, the climatic impact of CH4outweighs CO2(28–34 times larger global warming potential on a 100‐year scale), highlighting the need to jointly resolve the climatic sensitivities of both CO2and CH4. Here, we jointly constrain a terrestrial biosphere model with in situ CO2and CH4flux observations at seven eddy covariance sites using a data‐model integration approach to resolve the integrated environmental controls on land‐atmosphere CO2and CH4exchanges in Alaska. Based on the combined CO2and CH4flux responses to climate variables, we find that 1970‐present climate trends will induce positive C‐climate feedback at all tundra sites, and negative C‐climate feedback at the boreal and shrub fen sites. The positive C‐climate feedback at the tundra sites is predominantly driven by increased CH4emissions while the negative C‐climate feedback at the boreal site is predominantly driven by increased CO2uptake (80% from decreased heterotrophic respiration, and 20% from increased photosynthesis). Our study demonstrates the need for joint observational constraints on CO2and CH4biogeochemical processes—and their associated climatic sensitivities—for resolving the sign and magnitude of high‐latitude ecosystem C‐climate feedback in the coming decades. We use in situ measurements to constrain the modeled joint climatic sensitivity of land‐atmosphere CH4and CO2exchangesA continued 1970‐present climate trend leads to positive C‐climate feedback in wet tundra sites but negative feedback in boreal and shrub fen sitesCH4respiration dominates the positive tundra site feedback, CO2respiration dominates the negative boreal and shrub fen sites feedback We use in situ measurements to constrain the modeled joint climatic sensitivity of land‐atmosphere CH4and CO2exchanges A continued 1970‐present climate trend leads to positive C‐climate feedback in wet tundra sites but negative feedback in boreal and shrub fen sites CH4respiration dominates the positive tundra site feedback, CO2respiration dominates the negative boreal and shrub fen sites feedback

Small water bodies (i.e., ponds; <0.01 km2) play an important role in Earth System processes, including carbon cycling and emissions of methane. Detection and monitoring of ponds using satellite imagery has been extremely difficult and many water maps are biased toward lakes (>0.01 km2). We leverage high‐resolution (3 m) optical satellite imagery from Planet Labs and deep learning methods to map seasonal changes in pond and lake areal extent across four regions in Alaska. Our water maps indicate that changes in open water extent over the snow‐free season are especially pronounced in ponds. To investigate potential impacts of seasonal changes in pond area on carbon emissions, we provide a case study of open water methane emission budgets using the new water maps. Our approach has widespread applications for water resources, habitat and land cover change assessments, wildlife management, risk assessments, and other biogeochemical modeling efforts. Small water bodies (<0.01 km2) are an important driver of many Earth system processes. Despite their importance, many existing water mapping products have difficulty detecting these small water features and their seasonal changes in surface area. We used deep learning and high‐resolution (3 m) satellite imagery to map and monitor seasonal changes in the areal extent of lakes and small ponds across four regions in Alaska. The resulting water maps accounted for considerably more water coverage than existing products. The maps also effectively tracked widespread seasonal changes in pond and lake area that were not previously identified. This demonstrates the importance of monitoring surface water at high spatial resolutions and across seasons. Deep learning and 3 m resolution satellite imagery from Planet Labs can detect and track ponds and lakes >0.0001 km2Total surface area for ponds (<0.01 km2) in boreal forest and tundra environments can vary by 20%–40% throughout an individual seasonPonds can contribute to a broad range (8%–37%) of total methane emissions from lakes and ponds in northern boreal forest and tundra Deep learning and 3 m resolution satellite imagery from Planet Labs can detect and track ponds and lakes >0.0001 km2 Total surface area for ponds (<0.01 km2) in boreal forest and tundra environments can vary by 20%–40% throughout an individual season Ponds can contribute to a broad range (8%–37%) of total methane emissions from lakes and ponds in northern boreal forest and tundra

The increase in wildfire occurrence and severity seen over the past decades in the boreal and Arctic biomes is expected to continue in the future in response to rapid climate change in this region. Recent studies documented positive trends in gross primary productivity (GPP) for Arctic boreal biomes driven by warming, but it is unclear how GPP trends are affected by wildfires. Here, we used satellite vegetation observations and environmental data with a diagnostic GPP model to analyze recovery from large fires in Alaska over the period 2000–2019. We confirmed earlier findings that warmer‐than‐average years provide favorable climate conditions for vegetation growth, leading to a GPP increase of 1 Tg C yr−1, contributed mainly from enhanced productivity in the early growing season. However, higher temperatures increase the risk of wildfire occurrence leading to direct carbon loss over a period of 1–3 years. While mortality related to severe wildfires reduce ecosystem productivity, post‐fire productivity in moderately burned areas shows a significant positive trend. The rapid GPP recovery following fires reported here might be favorable for maintaining the region's net carbon sink, but wildfires can indirectly promote the release of long‐term stored carbon in the permafrost. With the projected increase in severity and frequency of wildfires in the future, we expect a reduction of GPP and therefore amplification of climate warming in this region. We analyzed the indirect impact of wildfires in Alaska on the amount of carbon fixed during photosynthesis, which is known as ecosystem gross primary productivity (GPP). Satellite observations and our remote sensing GPP model confirmed earlier findings of increasing vegetation growth in warmer‐than‐average years. On the other hand, risk of wildfire occurrence is higher during the warmer years, which leads to direct vegetation loss, and as result reduction in ecosystem carbon uptake over a period of 1–3 years. We also observed rapid GPP recovery following fires, but severe wildfires are expected to reduce the GPP and therefore amplification of climate warming in this region due to the release of long‐term stored carbon in the permafrost. Gross primary productivity (GPP) data in Alaska show increasing trends in the last two decades due to increased carbon uptake in the early growing seasonTemperature enhances plant productivity in the early growing season and increases the risk of wildfire occurrencesEcosystem GPP is negatively affected by wildfires, but shows fast recovery following less severe wildfires Gross primary productivity (GPP) data in Alaska show increasing trends in the last two decades due to increased carbon uptake in the early growing season Temperature enhances plant productivity in the early growing season and increases the risk of wildfire occurrences Ecosystem GPP is negatively affected by wildfires, but shows fast recovery following less severe wildfires

Soil organic carbon (SOC) is an important metric of soil health and the terrestrial carbon balance. Short‐term climate variations affect SOC through changes in temperature and moisture, which control vegetation growth and soil decomposition. We evaluated a satellite data‐driven carbon model, operating under the NASA Soil Moisture Active‐Passive (SMAP) mission, as a means of monitoring global surface SOC dynamics. The SMAP Level 4 Carbon (L4C) product estimates a daily global carbon budget including surface (0‐ to 5‐cm depth) SOC. We found that the L4C mean latitudinal SOC distribution is generally consistent with alternative assessments from static soil inventory records and dynamic global vegetation models (r≥ 0.89). Within forest systems, based on inventory data, L4C SOC is most similar in magnitude to litterfall but is correlated with coarse woody debris ( r=0.86) and total SOC ( r=0.81). L4C SOC is sensitive to seasonal and annual climate variability, with mean residence times that range from 1.5 years in the wet tropics to 17 years in the cold tundra. Incorporating soil moisture retrievals from the SMAP L‐band (1.4 GHz) microwave radiometer within the L4C algorithm provides enhanced soil moisture sensitivity under low‐to‐moderate vegetation cover (<5 kg m−2vegetation water content). The L‐band soil moisture had the greatest impact on the L4C carbon budget in semiarid regions, which span almost 60% of the globe and account for substantial variability in the terrestrial carbon sink. The L4C operational product enables prognostic investigations into effects of recent climate trends and anomalies (e.g., droughts and pluvials) on shallow soil carbon dynamics. Healthy soils are important for agriculture, climate‐change adaptation, and biodiversity. The most common way that scientists describe soil health is by taking a sample and measuring the amount of soil organic carbon, which is carbon stored in living things like microbes and fungi or that came from once‐living things like trees and other plants and is now in the soil. Collecting these samples requires a lot of time and work, which makes it very difficult for scientists to describe the soil health of large areas like countries or continents. Earth‐orbiting satellites that measure surface conditions like the temperature and amount of moisture in the soil have been used to inform computer models of how plants and soil change in response to a changing climate. Because satellites can see the entire globe, the computer models they inform have global coverage. We used one such computer model to estimate the amount of soil organic carbon stored in the world's soils. We compared our estimates of soil organic carbon to estimates from other methods and found that they agree very well once we accounted for the different soil depths used. What is particularly new and exciting about using this computer model for this purpose is that we can estimate seasonal and annual changes in soil organic carbon. This allows us to estimate how much soil health is impacted by short‐term natural events like droughts or floods. SMAP L4C provides an estimated daily, global terrestrial carbon budget including surface (0‐ to 5‐cm depth) soil organic carbon (SOC)L4C SOC is consistent with other global benchmarks including soil inventory records and dynamic global vegetation models (r> 0.89)SMAP L‐band soil moisture has greatest influence on SOC variability in semiarid lands, which encompass about 60% of the global domain

Observations taken over the last few decades indicate that dramatic changes are occurring in the Arctic‐Boreal Zone (ABZ), which are having significant impacts on ABZ inhabitants, infrastructure, flora and fauna, and economies. While suitable for detecting overall change, the current capability is inadequate for systematic monitoring and for improving process‐based and large‐scale understanding of the integrated components of the ABZ, which includes the cryosphere, biosphere, hydrosphere, and atmosphere. Such knowledge will lead to improvements in Earth system models, enabling more accurate prediction of future changes and development of informed adaptation and mitigation strategies. In this article, we review the strengths and limitations of current space‐based observational capabilities for several important ABZ components and make recommendations for improving upon these current capabilities. We recommend an interdisciplinary and stepwise approach to develop a comprehensive ABZ Observing Network (ABZ‐ON), beginning with an initial focus on observing networks designed to gain process‐based understanding for individual ABZ components and systems that can then serve as the building blocks for a comprehensive ABZ‐ON. While numerous scientific datasets of the Arctic Boreal Zone (ABZ) confirm that this region is rapidly changing, the current observational suite is insufficient to understand many of the complex interactions between components of the ABZ, which includes the cryosphere, biosphere, hydrosphere, and atmosphere. Such a process‐based understanding is necessary for the development of informed mitigation and adaptation response strategies and the prediction of future change. We review the strengths and limitations of the current suite of observations from satellites, which have the unique advantage of spatial coverage as compared to observations collected from near‐surface instruments. We make recommendations for improving satellite observations of individual components of the ABZ and recommend an interdisciplinary and stepwise approach to develop a comprehensive ABZ Observing Network (ABZ‐ON). We review the strengths and limitations of space‐based observational capabilities for several important Arctic‐Boreal Zone componentsWe make recommendations for improving the current Arctic‐Boreal Zone observing network and discuss how to build a more comprehensive one

An amendment to this paper has been published and can be accessed via a link at the top of the paper.