Multiple afforestation programs accelerate the greenness in the 'Three North' region of China from 1982 to 2013

China has launched multiple afforestation programs since 1978, including the 'Three North' Shelterbelt Development Program (TNSDP), the Beijing-Tianjin Sand Source Control Program (BSSCP), the Nature Forest Conservation Program (NFCP), and the Grain to Green Program (GTGP). These programs focus on local environment restoration by planting trees in semi-arid and arid regions and by protecting natural forests. However, the effectiveness of these programs has been questioned by several previous studies. Here, we report an increasing trend of greenness in this region using the satellite-retrieved normalized difference vegetation index (NDVI) from GIMMS, GIMMS-3g and MODIS datasets in the past 32 years. The NDVI increase for the 'Three North' region was 0.28%-0.38% yr−1 in 1982-2000 and 0.86%-1.12% yr−1 in 2000-2013, which is much higher than the country's means of 0.060%-0.063% yr−1 and 0.27%-0.30% yr−1, respectively. Most of the increase occurred in low and sparsely vegetated areas; and enlarged the moderate vegetated area (growing season mean NDVI above 0.5) from 16.5% to 25.7% for the two time periods, respectively. We also analyzed changes in the length of the growing season and the climate conditions including temperature, precipitation and two drought indices. However, these environmental factors cannot completely explain the changes in vegetation activity. Our study suggests these multiple afforestation programs contributed to the accelerated greening trend in the 'Three North' region and highlight the importance of human intervention in regional vegetation growth under climate change condition.