The length of the frost-free season (and the corresponding growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western United States, affecting ecosystems and agriculture. Across the United States, the growing season is projected to continue to lengthen.

This finding is from chapter 2 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. Key message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection.

Description of evidence base: The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input. Nearly all studies to date published in the peer-reviewed literature (for example, bfc00315-ccea-4e7c-8a05-2650a07e4252 113b48da-06b9-415d-95a2-2c78a7d89368 cd205ef2-b503-4a5c-b0fc-c4a5b94efc08) agree that the frost-free and growing seasons have lengthened. This is most apparent in the western United States. Peer-reviewed studies also indicate that continued lengthening will occur if concentrations of heat-trapping gases continue to rise. The magnitude of future changes based on model simulations is large in the context of historical variations. Evidence that the length of the frost-free season is lengthening is based on extensive analysis of daily minimum temperature observations from the U.S. Cooperative Observer Network. The geographic variations in increasing number of frost-free days are similar to the regional variations in mean temperature. Separate analysis of surface data also indicates a trend towards an earlier onset of spring.bfc00315-ccea-4e7c-8a05-2650a07e4252 113b48da-06b9-415d-95a2-2c78a7d89368 cd205ef2-b503-4a5c-b0fc-c4a5b94efc08 2d1ffd71-6c31-4d2e-9867-bdf330be45c1

New information and remaining uncertainties: A key issue (uncertainty) is the potential effect on observed trends of climate monitoring station inhomogeneities (differences), particularly those arising from instrumentation changes. A second key issue is the extent to which observed regional variations (more lengthening in the west/less in the east) will persist into the future. Local temperature biases in climate models contribute to the uncertainty in projections. Viable avenues to improving the information base are to investigate the sensitivity of observed trends to potential biases introduced by station inhomogeneities and to investigate the causes of observed regional variations.

Assessment of confidence based on evidence: Given the evidence base and remaining uncertainties, confidence is very high that the length of the frost-free season (also referred to as the growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western U.S, affecting ecosystems, gardening, and agriculture. Given the evidence base, confidence is very high that across the U.S., the growing season is projected to continue to lengthen.

References :

• Evidence of increased net ecosystem productivity associated with a longer vegetated season in a deciduous forest in south-central Indiana, USA (113b48da)
• Recent warming by latitude associated with increased length of ragweed pollen season in central North America (2d1ffd71)
• Phenology shifts at start vs. end of growing season in temperate vegetation over the Northern Hemisphere for the period 1982-2008 (cd205ef2)
• Climate Change Indicators in the United States, 2012 (bfc00315)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)