finding 7.4 : key-finding-7-4

Northern Hemisphere spring snow cover extent, North America maximum snow depth, snow water equivalent in the western United States, and extreme snowfall years in the southern and western United States have all declined, while extreme snowfall years in parts of the northern United States have increased (medium confidence). Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States (high confidence).

This finding is from chapter 7 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Based on observational analyses of snow cover, depth, and water equivalent there is medium confidence in the observed changes, and based on model simulations for the future there is high confidence in snowpack declines in the western United States and medium confidence in the shift to rain from snow in the eastern United States.

Description of evidence base: Evidence of historical changes in snow cover extent and a reduction in extreme snowfall years is consistent with our understanding of the climate system’s response to increasing greenhouse gases. Furthermore, climate models continue to consistently show future declines in snowpack in the western United States. Recent model projections for the eastern United States also confirm a future shift from snowfall to rainfall during the cold season in colder portions of the central and eastern United States. Each of these changes is documented in the peer-reviewed literature and are cited in the main text of this chapter.

New information and remaining uncertainties: The main source of uncertainty is the sensitivity of observed snow changes to the spatial distribution of observing stations and to historical changes in station location, rain gauges, and observing practices, particularly for snow. Another key issue is the ability of climate models to simulate precipitation, particularly snow. Future changes in the frequency and intensity of meteorological systems causing heavy snow are less certain than temperature changes.

Assessment of confidence based on evidence: Given the evidence base and uncertainties, confidence is medium that snow cover extent has declined in the United States and medium that extreme snowfall years have declined in recent years. Confidence is high that western United States snowpack will decline in the future, and confidence is medium that a shift from snow domination to rain domination will occur in the parts of the central and eastern United States cited in the text.

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

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