Of the 12 overlapping periods between the late 1990s and early 2010s, during which previous studies identified a hiatus in warming of annual mean temperature 8,9,15, we found evidence for increases in pre-VGD temperature averaged over the NH (for details, see Supplementary Materials) over the single period of 2001–2014 (1.0°C decade− 1, P < 0.05), while in 8 of the remaining 11 periods with hiatuses in warming of pre-VGD temperature, there were advances in VGD (0.5–0.7 days decade− 1; P < 0.05) (Fig. 1a).
Analysis of the corrected Akaike information criterion (AICC) to test for turning points in pre-VGD temperature over the NH between 2000 and 2021 showed better estimation using a linear regression model than a piecewise model (see Supplementary Materials). Similarly, a t-test indicated that the turning point of the pre-VGD temperature trend estimated by piecewise regression was not significant (P = 0.25). Hence, there was no turning point in the pre-VGD temperature averaged over the NH during 2000–2021. Likewise, we found no evidence for turning points in the average pre-VGD temperature over Eurasia (AICC; t-test P = 0.39) or North America (AICC; t-test P = 0.54) and we identified turning points for 3.7% (AICC) and 7.3% (t-test P < 0.05) of the NH area (Fig. 2a); similarly, we found no evidence for turning points of average VGD over the NH (AICC; t-test P = 0.26), Eurasia (AICC; t-test P = 0.48), or North America (AICC; t-test P = 0.40) and identified turning points for 6.7% (AICC) and 11.0% (t-test P < 0.05) of the NH area (Fig. 2b).
Over the period 2000–2021, average VGD across the NH advanced by 2.5 days decade− 1 (P < 0.01) and there was an increase of 0.5°C decade− 1 in pre-VGD temperature (P < 0.05; Fig. 1b), despite a lack of significnat trends in pre-VGD temperature and VGD since 2009 (Fig. 1c–d). The main contribution to the pre-VGD warming and VGD advance across the NH over this period was from Eurasia, where average pre-VGD temperature increased by 0.8°C decade− 1 (P < 0.05) and VGD advanced by 3.1 days decade− 1 (P < 0.01; Fig. 1b). In contrast, there were no changes in average pre-VGD temperature or VGD in North America during the period 2000–2021 (P > 0.05; Fig. 1b).
These results indicate that changes in the pre-VGD temperature and VGD were continuous from 2000 to 2021, with warming pre-VDG temperature and advancing VGD recorded in most areas of the NH (Fig. 2c–d), where the temperature increased and VGD advanced in Eastern America, Alaska, western Canada and most areas of Eurasia, whereas the temperature cooled and VGD delayed in limited areas of Eastern Canada and Northern Europe.
In contrast to previous reports of the stalling of advances in VGD and identification of hiatuses in warming across the NH between the late 1990s and early 2010s 6,7, our analyses of different sources of satellite data revealed evidence for continued advances in VGD throughout these periods (Fig. 1a). Whereas the previous studies were based on analysis AVHRR NDVI3g.v1 data, for which quality issues have been identified for data collected post-2000 11, our analyses were based on high-quality MODIS NDVI data 16 collected using a sensor that lacks the problems associated with the AVHRR sensor (Supplementary Materials), leading to greater confidence. Similarly, our results showed that the continuous increases in pre-VGD temperature across the NH during the entire period 2000–2021 were associated with increase in pre-VGD temperature since the early 2000s and small temperature trends during the 2010s (Fig. 1b–c), rather than accelerated warming following a warming hiatus between the late 1990s and early 2010s 8,9. In response to the pre-VGD temperature, we found a continuing advance in VGD, including most periods starting from the end of 1990s, despite a lack of substantial warming of pre-VGD temperature in the corresponding period (Fig. 1a–d). The advancing rate of VGD (2.5 days decade− 1) averaged over the NH during 2000–2021 is close to 1.9–2.8 days decade− 1 over 1982–1999 or 1982–2002 when there was intensive warming 17,18.
Phenology of vegetation in spring is particularly sensitive to changes in temperature, so small temperature increases may lead to substantial advances in VGD 19; similarly, increases in precipitation that have been reported to advance spring phenology in some dry grasslands 20 may explain the advances in VGD shown here. Research in some cold areas has shown that warming outside the growing season may facilitate earlier fulfillment of chilling requirements, leading to earlier VGDs 21; indeed, our findings indicate that advances in spring phenology may not necessarily indicate overall climate warming, so we suggest caution be exercised in the inference of climate warming based on advances in spring phenology.
Nevertheless, our results show that spring temperature has continuously increased across the NH over the period 2000‒2021, resulting in a continuous advancement of spring phenology that might have contributed to subsequent increases in spring gross primary production 22 and accelerated net carbon uptake 23 during the warming hiatus. It is likely that earlier spring green-up may increase the likelihood of summer drought effects, due to enhanced transpiration rates 2, that then reduce vegetation productivity 24. Our finding of advances in spring phenology during warming hiatus also supports an increased risk of spring frost damage during ongoing global warming 25. Our results thus deepen understanding of spring phenology responses to recent climate change, especially during the warming hiatus that occurred between the late 1990s and early 2010s, with substantial implications for impacts on carbon-climate feedbacks.