Changes in Northern Hemisphere vegetation and carbon dioxide seasons

Seasons represent important climatic cycles that are associated with intra-annual flows of energy, water, and chemical components in the Earth system. In this study, Northern Hemisphere vegetation and carbon dioxide (CO2) seasons are defined using the NOAA AVHRR normalized difference vegetation index (NDVI) data and in situ CO2 data observed at Mauna Loa, Hawaii and at Barrow, Alaska. Their spatial patterns and temporal changes are examined. Analyses of NDVI times series data from 1982-2000 demonstrate that the Northern Hemisphere vegetation season has expanded by 4.7 days/decade due to an earlier onset (-2.5 days/decade) as well as a delayed offset (2.2 days/decade). There are spatial variations in the sign and magnitude of changes in the onset and offset of vegetation seasons across the Northern Hemisphere. An earlier onset is identified in coniferous taiga forest regions between 50°N and 60°N, while the offset is delayed in the corridor from the central United States to southern Canadian Rockies. Carbon dioxide seasons, which indicate high CO2 concentration periods resulting from the reduction of the photosynthesis in non-growing seasons, have shortened over the 1974-2003 period. This is due to the combined effect of advanced offsets in early spring and slightly delayed onsets in late fall. The changes in the onset and offset of carbon dioxide seasons at Barrow are statistically significant, while the magnitude and statistical significance of those changes at Mauna Loa are weak.