MODIS-based leaf area index (LAI) and normalized difference vegetation index (NDVI) are used to examine detailed information regarding actual growing season and total annual production for various regions. Overall, MODIS LAI has larger variability and demonstrates more information regarding the evolution and structure of the seasonal vegetation characteristics. In contrast, the NDVI saturates around 0.7 and tends to overestimate the growing season in regions where it is already long. Next, a climatic impact index (CII) is derived to provide additional information regarding the potential sensitivity of vegetation to changes in climatic variables by accounting for the length of growing season. By normalizing the growth rate to the biome-average growth rate, this index can identify fractional loss of annual production, as opposed to the absolute loss which may be strongly weighted by the overall growth rate for different ecosystems. Our index provides a quantitative framework for assessing the importance of the length of the growing season in determining climatic vulnerability. In the last part of the paper, we use the long time series AVHRR products as a substitute for the MODIS products and test the temporal characteristics of the CII. Major drought events are well-captured by the CII, suggesting potential use as a monitoring and evaluation tool. Furthermore, the strong positive correlation between the CII and the vegetation condition index (VCI) suggests that the CII can quantitatively identify the effects of climatic variability upon vegetation activity.