Growing uncertainty in projected spring onset variability in the
Plant phenology regulates the carbon cycle and land-atmosphere coupling.
Currently, climate models often disagree with observations on the
seasonal cycle of vegetation growth, partially due to how spring onset
is measured and simulated. Here we use both thermal and leaf area index
(LAI) based indicators to characterize spring onset in CMIP6 models.
Although the historical timing varies considerably across models, most
agree that spring has advanced in recent decades and will continue to
arrive earlier with future warming. Across the Northern Hemisphere for
the periods 1950-2014, 1981-2014, and 2015-2099, thermal-based
indicators estimate spring advances of -0.7±0.2, -1.4±0.4, and -2.4±0.7
days/decade, while LAI-based indicators estimate -0.4±0.3, -0.1±0.3, and
-1±1.1 days/decade. Thereby, LAI-based indicators exhibit later spring
and weaker trends toward earlier onset, leading to index-related
uncertainty being as large or larger than model uncertainty for a given
index. Reconciling index-related discrepancies is therefore critical for
understanding future changes in spring onset.