Global-scale vegetation greening has been demonstrated using nearly four decades of NDVI and leaf area index (LAI) greenness data derived from the Advanced Very-High-Resolution Radiometer (AVHRR) instrument (Fig. 1a,b). While early studies primarily used the NDVI to detect changes in global greenness, recent studies widely use the LAI, since it has clear physical interpretation and is a fundamental variable in almost all land-surface models (Box 1). An ensemble of LAI datasets has shown that 52% (P < 0.05) to 59% (P < 0.10) of global vegetated lands displayed an increasing trend in growing season LAI since the 1980s11 (Fig. 1a). Although some studies reported a stalling, or even a reversal, of the greening trend since 2000 based on AVHRR20 and collection 5 (C5) of the Moderate Resolution Imaging Spectroradiometer (MODIS) data21, this signal might be an artefact of sensor degradation and/or processing22,23,24. For example, using a revised calibration of the MODIS data in the most recent collection 6 (C6) dataset24, Chen et al.13 showed that leaf area increased by 5.4 million km2 over 2000–2017, an area equivalent to the areal extent of the Amazon rainforest13. Indeed, 34% of vegetated land exhibited greening (P < 0.10), whereas only 5% experienced browning (P < 0.10), that is, a loss of vegetation greening.

Fig. 1: Changes in satellite-derived global vegetation indices, vegetation optical depth and contiguous solar-induced fluorescence. a | Leaf area index (LAI) from four products: GIMMS13, GLASS192, GLOBMAP23 and Moderate Resolution Imaging Spectroradiometer (MODIS) C6 (ref.193). b | Normalized difference vegetation index (NDVI) from three products: GIMMS194, MODIS C6 (ref.195) and SPOT196. c | Enhanced vegetation index (EVI) from MODIS C6 (ref.195). d | Near-infrared reflectance of terrestrial vegetation (NIRv)197. e | Vegetation optical depth (VOD)119. f | Contiguous solar-induced fluorescence (CSIF)114. In parts a and b, the light-green shading denotes the range of LAI and NDVI across different products and the dark-green shading denotes the interquartile range (between the 25th and 75th percentiles). Only measurements during the growing season11 were considered. Full size image

New satellite-based vegetation indices also support the global greening trend observed since 2000 (Fig. 1), including the enhanced vegetation index (EVI) and near-infrared reflectance of terrestrial vegetation (NIRv) (Box 1). However, while vegetation greenness is increasing at the global scale, the changes vary considerably between regions and seasons.

Regional trends

In the high northern latitudes (>50°N), AVHRR and Landsat records indicate a widespread increase in vegetation greenness since the 1980s8,12,25 (Fig. 2a–d). Regions with the greatest greening trend include northern Alaska and Canada, the low-Arctic parts of eastern Canada and Siberia, and regions of Scandinavia12,25,26. Dendrochronological data and photographic evidence further corroborate these findings27,28,29,30. In general, the LAI over high northern latitudes will continue to increase by the end of this century31, based on the results of an ensemble of ESMs (Fig. 2e–h). However, although only 3% of the high latitudes show browning during 1982–2014 (ref.25), there is a growing proportion of Arctic areas exhibiting a browning trend32. Such trends first emerged in boreal forests, where a multitude of disturbances (for example, fires, harvesting and insect defoliation) prevail9,33,34,35,36,37. The North American boreal forests in particular exhibit browning areas nearly 20 times larger than the Eurasian boreal forests, showing heterogeneous regional greenness change38.

Fig. 2: Spatial patterns of changes in leaf area index. a | Growing season (GS) mean Advanced Very-High-Resolution Radiometer (AVHRR) leaf area index (LAI) trend during 1982–2009. The AVHRR LAI dataset is the average of three different products (GIMMS13, GLOBMAP23 and GLASS192). b | Change in the GS mean AVHRR LAI over four regions during 1982–2009. c | GS mean Moderate Resolution Imaging Spectroradiometer (MODIS) LAI during 2000–2018. d | Change in the GS mean MODIS LAI over four regions during 2000–2018. MODIS LAI is from collection 6 (ref.193). e | Relative change in GS mean LAI between 1981–2000 and 2081–2100 under the Representative Concentration Pathway 2.6 (RCP2.6), based on the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble. f | Relative change in GS mean CMIP5 LAI 2018–2100 under RCP2.6, relative to 1981–2000. g | Relative change in GS mean LAI between 1981–2000 and 2081–2100 under RCP8.5, based on CMIP5. h | Relative change in GS mean CMIP5 LAI 2018–2100 under RCP8.5, relative to 1981–2000. The number of CMIP5 models used in the calculation of the multi-model mean is 16 and 19, for RCP2.6 and RCP8.5, respectively (Supplementary Table S5). In parts a, c, e and g, the white land areas depict barren lands, permanent ice-covered areas, permanent wetlands, built-up areas and water. In parts b, d, f and h, blue represents the high-latitude Northern Hemisphere (NH) (50–90°N), green represents the temperate NH (25–50°N), purple represents the tropical zone (25°S–25°N) and yellow represents the extratropical Southern Hemisphere (SH) (90–25°S). The shading shows the ±1 inter-model standard deviation. Full size image

The northern temperate region (25–50°N) is another vegetation greening hotspot, experiencing faster rates of greening than the high latitudes since 2000 (Fig. 2b,d). Indeed, ~14 million km2 of the temperate region greened (P < 0.10), contributing about one-half of the global net leaf area increase over this time period13. The increase of vegetation greenness is especially strong in agricultural regions (for example, India13) and recently afforested areas (for example, China13,39); collectively, China and India alone contribute more than 30% of the total net increase in the global LAI13.

Tropical regions (25°S–25°N) are also greening (Fig. 2b,d), contributing about a quarter of the net global increase in leaf area since 2000 (ref.13). However, the tropics also have areas where significant browning has been reported, for example, in the Brazilian Cerrado and Caatinga regions and Congolian forests13,40. It is worth noting that substantial uncertainties remain in the tropical vegetation greenness estimations due to the saturation effects of greenness indices in dense vegetation41 and contamination by clouds and aerosols42. These uncertainties partly underlie the disagreement between the MODIS and AVHRR products13 when measuring tropical greenness and the debate on whether the Amazonian forests have greened or browned in response to droughts42,43,44.

The extratropical Southern Hemisphere (>25°S) has experienced a general greening trend since the 1980s13,45, but it is lower than that in the temperate and high-latitude Northern Hemisphere13 (Fig. 2a–d). Regional greening hotspots in southern Brazil and southeast Australia mostly overlap with the intensive cropping areas13, highlighting the increasing contribution of managed ecosystems to vegetation greening. Note that most of this region is dominated by semi-arid ecosystems46, where vegetation coverage is generally sparse. Thus, satellite vegetation indices over this region are generally sensitive to change in soil background. For example, browning was detected from the AVHRR dataset since the 2000s20 (Fig. 2b), but MODIS C6 data (which is better calibrated and can distinguish vegetation from background more accurately) instead showed an overall greening trend particularly since 2002 (ref.13; Fig. 2c,d).

Seasonal changes of greenness

In the northern temperate and high latitudes, greenness often shows distinctive seasonal patterns within a calendar year (Fig. 3). Several metrics of land-surface phenology have been developed to depict the seasonal cycle of greenness47, including the widely used start of the growing season (SOS) and end of the growing season (EOS)48. Although phenology dates can vary depending on the greenness product or algorithm used49,50,51, significant trends towards both earlier SOS (2–8 days decade−1) and later EOS (1–6 days decade−1) and, thus, longer lengths of the growing season (LOS) (2–10 days decade−1), have been observed in most Northern Hemisphere regions during the past four decades7,8,25,52,53,54 (Fig. 3a–c). These trends are corroborated by ground-based observation data in spring and autumn55,56,57. The increase in LOS is driven mainly by an advanced SOS in Eurasia (53–81% of LOS lengthening is due to SOS advance) and delayed EOS in North America (57–96% of LOS lengthening is due to EOS delay), with the more rapid total LOS increase seen in Eurasia25,58,59,60.

Fig. 3: Changes in the seasonality of vegetation greenness and atmospheric CO 2 concentration. a | Five-year mean seasonal variations of the normalized difference vegetation index (NDVI) over Northern Hemisphere high latitudes (>50oN) during 1982–1986 (black line) and 2008–2012 (green line). Start of the growing season (SOS) and end of the growing season (EOS) are shown as 50% of the maximum NDVI. The length of the growing season (LOS) is the difference between the EOS and the SOS. b | Frequency distribution of SOS change in the Northern Hemisphere during 1982–2012. c | Frequency distribution of EOS change in the Northern Hemisphere during 1982–2012. d | Five-year mean detrended seasonal CO 2 variations at Barrow, AK, USA (71oN) (NOAA ESRL archive: https://www.esrl.noaa.gov/gmd/ccgg/obspack/) during 1980–1984 (black line) and 2013–2017 (green line). Vertical lines mark the spring zero-crossing date (SZC) and autumn zero-crossing date (AZC). Horizontal lines mark the seasonal amplitude as the difference between the maximum and the minimum of detrended seasonal CO 2 variations. Shaded areas show the range of interannual variations in the NDVI in part a and the standard deviation of the detrended CO 2 mole fraction (ppm) in part d at the day of year. NDVI data are the updated dataset from Tucker et al.194. Parts b and c are adapted with permission from ref.48, Wiley-VCH. Full size image

In addition to longer growing seasons, satellite greenness data also reveal important shifts in the timing and magnitude of the seasonal peak greenness47,61. For example, the timing of peak greenness has advanced by 1.2 days decade−1 during 1982–2015 (ref.62) and 1.7 days decade−1 during 2000–2016 (ref.61) over the extratropical Northern Hemisphere (Fig. 3a), with the boreal region peak greenness advancing twice as fast as the Arctic tundra and temperate ecosystem peaks61. Since the 1980s, the magnitude of the peak greenness has also increased over the extratropical Northern Hemisphere by ~0.1 standardized NDVI anomaly per year62, with a stronger signal in the pan-Arctic region63,64.

Phenology changes, including the SOS advancement, EOS delay and peak greenness enhancement, can significantly change the Earth’s seasonal landscape. Northern high latitudes, which traditionally have high seasonality (that is, short and intense growing seasons), are exhibiting seasonality similar to that of their counterparts 6° to 7° south in the 1980s. In other words, the latitudinal isolines of northern vegetation seasonality have shifted southward since the 1980s. The diminished seasonality of the northern high-latitude vegetation10 is consistent with changes in the velocity of vegetation greenness (defined as the ratio of temporal greenness change to its spatial gradient)65, which showed faster northward movement of the SOS (3.6 ± 1.0 km year−1) and the EOS (6.0 ± 1.1 km year−1) than the peak greenness (3.1 ± 1.0 km year−1) during 1982–2011 (ref.65).