Significance Rising arctic temperatures could mobilize reservoirs of soil organic carbon trapped in permafrost. We present the first quantitative evidence for large, regional-scale early winter respiration flux, which more than offsets carbon uptake in summer in the Arctic. Data from the National Oceanic and Atmospheric Administration’s Barrow station indicate that October through December emissions of CO 2 from surrounding tundra increased by 73% since 1975, supporting the view that rising temperatures have made Arctic ecosystems a net source of CO 2 . It has been known for over 50 y that tundra soils remain unfrozen and biologically active in early winter, yet many Earth System Models do not correctly represent this phenomenon or the associated CO 2 emissions, and hence they underestimate current, and likely future, CO 2 emissions under climate change.

Abstract High-latitude ecosystems have the capacity to release large amounts of carbon dioxide (CO 2 ) to the atmosphere in response to increasing temperatures, representing a potentially significant positive feedback within the climate system. Here, we combine aircraft and tower observations of atmospheric CO 2 with remote sensing data and meteorological products to derive temporally and spatially resolved year-round CO 2 fluxes across Alaska during 2012–2014. We find that tundra ecosystems were a net source of CO 2 to the atmosphere annually, with especially high rates of respiration during early winter (October through December). Long-term records at Barrow, AK, suggest that CO 2 emission rates from North Slope tundra have increased during the October through December period by 73% ± 11% since 1975, and are correlated with rising summer temperatures. Together, these results imply increasing early winter respiration and net annual emission of CO 2 in Alaska, in response to climate warming. Our results provide evidence that the decadal-scale increase in the amplitude of the CO 2 seasonal cycle may be linked with increasing biogenic emissions in the Arctic, following the growing season. Early winter respiration was not well simulated by the Earth System Models used to forecast future carbon fluxes in recent climate assessments. Therefore, these assessments may underestimate the carbon release from Arctic soils in response to a warming climate.

High-latitude ecosystems contain vast reservoirs of soil organic matter that are vulnerable to climate warming, potentially causing the Arctic to become a strong source of carbon dioxide (CO 2 ) to the global atmosphere (1, 2). To quantitatively assess this carbon−climate feedback at a regional scale, we must untangle the effects of competing ecosystem processes (3). For example, uptake of CO 2 by plants may be promoted by longer growing seasons in a warmer climate (4, 5) and by fertilization from rising levels of atmospheric CO 2 (6). These gains, however, may be offset by carbon losses associated with permafrost thaw (7), enhanced rates of microbial decomposition of soil organic matter with rising temperature (8), a longer season of soil respiration (9), and the influence of increasing midsummer drought stress on plant photosynthesis (10). Warming may also trigger carbon losses by expanding burned areas and by allowing fires to burn deeper into organic soils (11).

Model simulations disagree on the sign and magnitude of the net annual carbon flux from Alaska (3). Observational constraints are limited: Year-round measurements of CO 2 fluxes are very few, and the spatial scales are generally small (e.g., eddy flux and chamber studies) relative to the large extent and diversity of arctic and boreal landscapes. Incubation experiments show higher respiration rates as soils warm (8, 12, 13), which could more than offset any future increases in net primary productivity (14).

Summertime chamber measurements as early as the 1980s hinted that tundra ecosystems could potentially change from a sink to a source of carbon (15), and some modeling studies predict that carbon losses from soil will start to dominate the annual carbon budget by the end of the 21st century (16). A recent synthesis of eddy flux data from Alaskan tundra and boreal ecosystems calculated a neutral carbon balance for 2000–2011 (17), whereas a study of eddy flux towers in tundra ecosystems across the Arctic calculated highly variable carbon sources (18).

In this paper, we present a three-part synthesis to assess carbon fluxes and carbon−climate feedbacks in arctic and boreal Alaska. (i) We combine recent in situ aircraft and tower CO 2 observations, eddy covariance flux data, satellite remote sensing measurements, and meteorological drivers to estimate regional fluxes of CO 2 . By taking advantage of the spatially integrative properties of the lower atmosphere, we use CO 2 mole fraction data to constrain spatially explicit, temporally resolved CO 2 flux distributions across Alaska during 2012–2014. We compute the annual carbon budget for Alaska, partitioned by season, ecosystem type, and source type (biogenic, pyrogenic, and anthropogenic, Figs. 1 and 2). (ii) We analyze the 40-y record of hourly atmospheric CO 2 measurements from the land sector at Barrow, AK [BRW tower, operated by the National Oceanic and Atmospheric Administration (NOAA)], to place recent regional carbon fluxes in a historical context. (iii) We evaluate the Alaskan CO 2 flux simulated by a representative set of Earth System Models (ESMs) against our CO 2 fluxes for Alaska in 2012–2014, focusing on the net carbon budget, early winter respiration flux, and the duration and magnitude of the growing season carbon uptake. By combining and comparing these complementary approaches, we gain a more complete understanding of the Alaskan carbon budget and insight into how arctic carbon fluxes may respond to future climate change.

Fig. 1. Time series of biogenic CO 2 fluxes for Alaska during 2012–2014 calculated from CARVE aircraft data. (A) Mean daily net biogenic CO 2 flux for Alaska during 2012–2014, with modeled (PVPRM-SIF) CO 2 flux (green) and the aircraft optimized net CO 2 flux (red) and interpolated aircraft optimized net CO 2 flux (black). Our approach for estimating these fluxes and the uncertainty range (shown with shading) is described in SI Appendix, Calculation of the Additive Flux Correction. (B) Optimized biogenic CO 2 fluxes for different regions in Alaska: NS tundra (blue), SW tundra (orange), and boreal forests (green).

Fig. 2. CO 2 budget for Alaska 2012–2014. (A) Net biogenic carbon budget for boreal forests (light green), Yukon−Kuskokwim Delta of southwest Alaska and the Seward Peninsula to the west (SW tundra) (orange), NS tundra (blue), and the mixed areas or areas of Alaska not included in other regions (gray) (in teragrams of carbon per year) calculated from the aircraft optimized CO 2 flux for 2012, 2013, and 2014. (B) Map of the regional areas described in A. Negative fluxes indicate uptake of CO 2 by the biosphere. (C) Net carbon fluxes for Alaska during 2012–2014 from biogenic (dark green), biomass burning (white), and fossil fuel (black) components.

Conclusions We find that Alaska, overall, was a net source of carbon to the atmosphere during 2012–2014, when net emissions from tundra ecosystems overwhelmed a small net uptake from boreal forest ecosystems. Both ecosystems emitted large amounts of carbon in early winter. Our results suggest that October through December respiration has increased by about 73% over the past 41 y from organic carbon-rich soils on the North Slope of Alaska, correlated with increasing air temperatures. The ESMs used to forecast future carbon fluxes in the CMIP5 and IPCC studies did not represent early winter respiration, especially when soil temperatures hover near 0 °C. Hence these assessments may underestimate the carbon release from arctic soils in response to warming climate.

Acknowledgments We thank the pilots, flight crews, and NASA Airborne Science staff from the Wallops Flight Facility for enabling the CARVE science flights. We thank J. Budney, A. Dayalu, E. Gottlieb, M. Pender, J. Pittman, and J. Samra for their help during CARVE flights; M. Mu for CMIP5 simulations; J. Joiner for GOME-2 SIF fields; and J. W. Munger for helpful discussion. Part of the research described in this paper was undertaken as part of CARVE, an Earth Ventures investigation, under contract with NASA. Part of the research was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with NASA. We acknowledge funding from NOAA, NASA Grants NNX13AK83G, NNXNNX15AG91G, and 1444889 (through JPL), the National Science Foundation Arctic Observation Network program (Grant 1503912), and the US Geological Survey Climate Research and Development Program. Computing resources for this work were provided by the NASA High-End Computing Program through the NASA Advanced Supercomputing Division at the Ames Research Center.