Pacific Climate Impacts Consortium

University House 1, PO Box 1700, STN CSC, University of Victoria, Victoria, British Columbia, Canada, V8W 2Y2 Phone: 250-721-6236 | Fax: 250-721-7217 | pacificclimate.org 1. To speak of air “holding” water vapour is to speak loosely. The partial pressure of water vapour in equilibrium with a solid or liquid body of water is described by the Clausius-Clapeyron equation and is independent of the presence of air or other gases. 2. As O’Gorman notes, this is only true as a first-order approximation: the type of precipitation that falls at the Earth’s surface is dependent not only on surface temperature, but also upon the local temperature structure of the lower troposphere. Recent research by P.A. O’Gorman (2014), in the journal Nature, uses an ensemble of global climate model (GCM) simulations to examine the projected changes in both mean snowfall and daily snowfall extremes in a high greenhouse-gas emissions scenario. He finds that, while both mean snowfall and extreme snowfall decrease as the climate warms due to the influence of greenhouse gasses, the reduction in daily snowfall extremes is smaller than the reduction in mean snowfall. O’Gorman suggests, based on a simple physical model, that this may be due to snowfall extremes occuring near an optimal temperature that is insensitive to climate change. Changes to mean snowfall and snowfall extremes can bring with them multiple impacts, altering river flow for snow-melt dominated rivers, affecting the rate at which sea ice melts, changing the amount of insulating snow that is beneficial for some plants and wildlife, and affecting transportation, the electrical grid and business. Because of these impacts and the fact that anthropogenic climate change affects both the mean state of the Earth’s climate and some climate extremes events—such as temperature and precipitation extremes—the potential effect of climate change on snowfall is of interest. Two physical quantities are useful for guiding our intuition about how snowfall may be affected by climate change. The first is saturation specific humidity; this is the amount of water vapour that a given amount of air can “hold”1 at a given temperature and pressure (past this point, water starts to condense faster than it evaporates). Saturation specific humidity is much more sensitive to changes in temperature than to changes in pressure. In general, in the troposphere, warmer air can hold more moisture than cooler air and so, as the climate warms, there is more moisture available for precipitation. The second quantity is snowfall fraction; this is the fraction of precipitation that falls as snow. Loosely, as temperature increases, the fraction of precipitation that falls as snow decreases2. To examine how mean snowfall and snowfall extremes may change as the climate changes, O’Gorman uses simulated Northern Hemisphere precipitation from an ensemble of 20 climate projections from 20 Global Climate Models (GCMs) participating in the fifth phase of the Coupled Model Intercomparison Project3 (CMIP5). The projections that he uses assume high greenhouse gas emissions4 that would lead to atmospheric carbon dioxide concentrations of approximately four times the preindustrial level by the PCIC SCIENCE BRIEF: CONTRASTING THE RESPONSES OF MEAN AND EXTREME SNOWFALL TO CLIMATE CHANGE