Cloud cover and environmental datasets. The monthly mean MODIS cloud fraction at 0.05°used in this study was computed from the daily cloud mask **data** ("cloudy" label for the bits 0-1 of "state_1 km" band) included in the MODIS Surface Reflectance product (MYD09GA.006, overpass at local time of 13:30) of Aqua from 2002 to 2018, using the reduceResolution function with "mean" aggregation method on Google Earth Engine (https://earthengine.google.com/). The 1-km cloud mask was produced based on the MOD35_L2 cloud mask product, which had been extensively validated 71,72 . Before computing cloud fractions, a snow/ice flag (the bit 12 of "state_1km" band) was used to remove snow or ice pixels in the cloud record because the high reflectivity of snow/ice degrades the accuracy of cloud detection, especially during winter in the northern hemisphere. Therefore, the estimated cloud effect would have larger uncertainty in boreal winter than in summer.