Increasing terrestrial ecosystem carbon release in response to autumn cooling and warming

Articles https://doi.org/10.1038/s41558-022-01304-w Increasing terrestrial ecosystem carbon release in response to autumn cooling and warming Rui Tang1, Bin He 1 ✉, Hans W. Chen 2, Deliang Chen 3, Yaning Chen 4 ✉, Yongshuo H. Fu5, Wenping Yuan 6, Baofu Li 7, Zhi Li4, Lanlan Guo8, Xingming Hao4, Liying Sun9, Huiming Liu10, Cheng Sun 1 and Yang Yang11 Part of the Northern Hemisphere has experienced widespread autumn cooling during the most recent decades despite overall warming, but how this contrasting temperature change has influenced the ecosystem carbon exchange remains unclear. Here, we show that autumn cooling has occurred over about half of the area north of 25° N since 2004, producing a weak cooling trend over the period 2004–2018. Multiple lines of evidence suggest an increasing net CO2 release in autumn during 2004–2018. In cooling areas, the increasing autumn CO2 release is due to the larger decrease of gross primary productivity (GPP) growth than total ecosystem respiration (TER) growth suppressed by cooling. In the warming areas, TER increased more than GPP because the warming and wetting conditions are more favourable for TER growth than GPP increase. Despite the opposite temperature trends, there has been a systematic increase in ecosystem carbon release across the Northern Hemisphere middle and high latitudes. T errestrial ecosystems in the middle and high latitudes play a critical role in regulating the global carbon cycle and atmospheric CO2 concentration1,2. In recent decades, the overall climate warming has strongly enhanced ecosystem productivity by extending the growing season and increasing photosynthesis, although this warming effect on ecosystem carbon uptake has weakened since the latter half of the 1990s3–5. The Northern Hemisphere (NH) ecosystems act as a carbon sink in spring but a net carbon source in autumn due to the asymmetric response of ecosystem productivity and respiration in response to warming2. In spring, climate warming more strongly promotes photosynthesis than respiration, resulting in net carbon uptake, but an opposite condition occurs in autumn. A recent study reported widespread autumn cooling in the NH since 20046, and the ecosystem respiration is anticipated to decrease with cooling. Therefore, we hypothesize that the autumn carbon release would be suppressed by the emerging cooling. However, for those regions that experienced continued warming, an enhanced autumn carbon loss would be expected. To test this hypothesis, we use independent simulations and observation-derived estimates of net ecosystem exchange (NEE) to comprehensively examine the interannual variation of ecosystem carbon fluxes in response to different autumn temperature changes. The NEE datasets include nine simulations from the TRENDY ensemble of global vegetation models (Supplementary Table 1)7, one simulation from empirical models based on eddy-covariance observational data using machine learning algorithms (FLUXCOM)8–10 and three estimates from atmospheric CO2 inversion models: CarboScope11, CAMS12 and NISMON13. In addition, in situ atmospheric CO2 concentration records and eddy-covariance CO2 flux observations (Supplementary Table 2) were used as observational evidence to indicate net land–atmosphere carbon exchange14. Satellite-based normalized difference vegetation index (NDVI) and contiguous solar-induced chlorophyll fluorescence (CSIF), as well as a near-infrared reflectance- (NIRv-) based gross primary productivity (GPP) product (defined as NIRv_GPP), were also used to indicate the ecosystem productivity change. Widespread cooling has halted overall autumn warming Air temperature trends derived from the Climate Research Union (CRU)15 for the period from 1981 to 2018 indicate an apparent shift from warming to weak cooling in autumn around 2004 north of 25° N (Fig. 1a,b). The turning point of temperature that occurred in 2004 (P = 0.001) was confirmed by both piecewise linear regression and a moving t test (Methods). Before 2004, the average autumn temperature increased at a rate of 0.45 ± 0.14 °C decade–1 (Mann– Kendall, P = 0.004), which we define as the warming period (WP), whereas post-2004, there was a weak decreasing trend at a rate of −0.07 ± 0.13 °C decade–1 (P = 0.593), defined as the cooling period (CP). Cooling trends were widely observed in the high latitudes of North America and central Eurasia, accounting for about 51% of the total study area. This cooling pattern is consistent with the study of ref. 6 and was attributed to the strengthening of the Pacific Decadal Oscillation and Siberian High. For the convenience of description, we designated the regions where the autumn temperature shows a decreasing trend in the period 2004–2018 relative to the period 1982–2003 as the cooling College of Global Change and Earth System Science, Beijing Normal University, Beijing, China. 2Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden. 3Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden. 4State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China. 5College of Water Sciences, Beijing Normal University, Beijing, China. 6School of Atmospheric Sciences, Sun Yat‐Sen University, Guangzhou, China. 7School of Geography and Tourism, Qufu Normal University, Rizhao, China. 8School of Geography, Beijing Normal University, Beijing, China. 9Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China. 10Ministry of Ecology and Environment Center for Satellite application on Ecology and Environment, Beijing, China. 11Advanced Institute of Natural Sciences, Beijing Normal University at Zhuhai, Zhuhai, China. ✉e-mail: ; 1 Nature Climate Change | www.nature.com/natureclimatechange Articles Nature Climate Change a for the maximum temperature (−0.76 ± 0.23 °C decade–1, P = 0.005) than for the minimum temperature (−0.61 ± 0.22 °C decade–1, P = 0.016) (Supplementary Fig. 3). Thus, the decrease of temperature during the day in the last decade is faster than that during the night in the CAs. b Increasing land carbon exchange in recent decades <–1.5 –1.0 –0.5 –0.3 –0.1 c 0 0.1 0.3 0.5 1.0 >1.5 Temperature (°C decade–1) 3 NH 0.45 ± 0.14*** –0.07 ± 0.13 Z-score temperature 2 1 0 CAs 0.40 ± 0.19* –1 –0.69 ± 0.22*** WAs –2 0.51 ± 0.11*** 0.61 ± 0.14*** –3 1982 1986 1990 1994 1998 2002 2006 2010 2014 2018 Year Fig. 1 | Trends in mean autumn air temperature at 2 m north of 25° N during the periods 1982–2003 and 2004–2018. a, Trends in autumn (September–November) temperature during 1982–2003. b, Trends in autumn temperature during 2004–2018. c, Trends in mean autumn temperature anomalies during both periods. The air temperature data were obtained from the CRU Time S (...truncated)