Since 1901, regional averages of both the consecutive number of frost-free days and the length of the corresponding growing season have increased for the seven contiguous U.S. regions used in this assessment. However, there is important variability at smaller scales, with some locations actually showing decreases of a few days to as much as one to two weeks. Plant productivity has not increased commensurate with the increased number of frost-free days or with the longer growing season due to plant-specific temperature thresholds, plant–pollinator dependence, and seasonal limitations in water and nutrient availability (very high confidence). Future consequences of changes to the growing season for plant productivity are uncertain.

This finding is from chapter 10 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Changes in growing season length and interactions with climate, biogeochemistry, and land cover were covered in 12 chapters of NCA3dd5b893d-4462-4bb3-9205-67b532919566 but with sparse assessment of how changes in the growing season might offset plant productivity and subsequent feedbacks to the climate system. This key finding provides an assessment of the current state of the complex nature of the growing season.

Description of evidence base: Data on the lengthening and regional variability of the growing season since 1901 were updated by Kunkel.be68e291-15a1-41ce-bfaa-23d2858e4ded Many of these differences reflect the more general pattern of warming and cooling nationwide (Ch. 6: Temperature Changes). Without nutrient limitations, increased CO₂ concentrations and warm temperatures have been shown to extend the growing season, which may contribute to longer periods of plant activity and carbon uptake but do not affect reproduction rates.5ce42823-d14b-46ab-96a3-9da6e2dc14ae However, other confounding variables that coincide with climate change (for example, drought, increased ozone, and reduced photosynthesis due to increased or extreme heat) can offset increased growth associated with longer growing seasons07cbf202-f653-4cb4-aa82-053a3e93547a as well as changes in water availability and demand for water (e.g., Georgakakos et al. 2014;3ff0e30a-c5ee-4ed9-8034-288be428125bHibbard et al. 2014aa1fec1f-b5c3-48b8-b17e-ca88da35eb4c). Increased dry conditions can lead to wildfire (e.g., Hatfield et al. 2014;e3a33475-9c92-40f7-9745-3d5afcadb3f1 Joyce et al. 2014;212b019e-f046-40a4-bc19-5e752527fb1c Ch. 8: Droughts, Floods and Wildfires) and urban temperatures can contribute to urban-induced thunderstorms in the southeastern United States.6e7498ed-db7b-4997-b4b1-f3c0a7e70777 Temperature benefits of early onset of plant development in a longer growing season can be offset by 1) freeze damage caused by late-season frosts; 2) limits to growth because of shortening of the photoperiod later in the season; or 3) by shorter chilling periods required for leaf unfolding by many plants.8e871120-f0e4-4b68-a404-7f831973bc71 5d949044-ccb3-4d40-a2f1-688f8e856d2b

New information and remaining uncertainties: Uncertainties exist in future response of the climate system to anthropogenic forcings (land use/land cover as well as fossil fuel emissions) and associated feedbacks among variables such as temperature and precipitation interactions with carbon and nitrogen cycles as well as land-cover change that impact the length of the growing season (Ch. 6: Temperature Changes and Ch. 8: Droughts, Floods and Wildfires).07cbf202-f653-4cb4-aa82-053a3e93547a 5ce42823-d14b-46ab-96a3-9da6e2dc14ae e3a33475-9c92-40f7-9745-3d5afcadb3f1

Provenance

This finding was derived from figure -.2: Confidence / Likelihood