ClimateChange Assessment Guide.pdf - University of Waterloo

Executive Summaryv2007c). The magnitude of the warming is related tothe emission of GHGs and resultant concentrationsof those gases in the atmosphere and their radiativeforcing. The best estimate for the global averagetemperature increase for 2090-2099 relative to 1980-1999is 1.8°C to 4.0°C if all emission scenarios are considered(Intergovernmental Panel on Climate Change (IPCC),2007c).Warming is projected to be greatest over land and mosthigh northern latitudes and the least in the SouthernOcean and parts of North Atlantic Ocean. In mostareas of North America, the increase in annual meantemperature is greater than the global mean citedabove. The most warming is likely to occur in winter innorthern regions and in summer in the south westernUSA. Minimum winter temperatures are likely to increasemore than average temperatures in northern NorthAmerica.Warming generally increases the spatial variability ofprecipitation. Rainfall in the subtropics is projected todecrease and precipitation is projected to increase athigher latitudes (Christensen et al., 2007b). In Canadaand northeast USA, annual precipitation is very likelyto increase and it is likely to decrease in the southwestUSA. The seasonal distribution of precipitation is alsoaffected; winter and spring precipitation in southernCanada is likely to increase and summer precipitationis likely to decrease. The snow season length and snowdepth are expected to decrease except in northernmostCanada where the maximum snow depth will likelyincrease (Christensen et al., 2007b). The following tablelists some projected changes in global climate relevantto water.Projected changes in global climate as they relate towater (Meehl et al., 2007; Bates et al., 2008)Observed Changes• Hot extremes and heat waves will very likely be moreintense, frequent and longer in duration.• Number of frost days in middle and high latitudesdecrease and growing season lengthens.• Increased likelihood of summer drying in midlatitudesand associated risk of drought (increasingfrom 1% of present-day land area to 30% by 2100).Observed Changes• Increase in number of consecutive dry days.• Very likely that heavy precipitation events willbecome more frequent.• Increase in the amount and intensity of precipitationat high latitudes and in tropics; decreases in somesub-tropical and lower mid-latitude regions.• Area of snow cover projected to contract, glaciers/ice caps lose mass; in many areas summer melting isgreater than winter snowfall accumulation.• By 2050, permafrost area in Northern Hemispherelikely to decrease 20-35%; increases in thaw depth.• Potential evaporation projected to increase(atmosphere’s water-holding capacity increases withhigher temperatures but relative humidity stayssame); increase in water vapour deficit.• Runoff increases in high latitudes and wet tropics,decreases in mid-latitudes and some parts of drytropics.Observed Changes in Ontario’s ClimateChanges in the climate system have also been observedat the local scale – in Ontario and within the Great LakesBasin, as summarized below:• Annual average air temperatures across the provinceincreased from 0 to 1.4°C; the greatest warmingoccurred in the spring for the period 1948 to 2006,(Lemmen et al., 2008).• The number of warm days and night-time wintertemperatures increased between 1951 and 2003(Bruce et al., 2006a).• Total annual precipitation increased 5-35% since1900, (Zhang et al., 2000) and the number of days withprecipitation (rain and snow) increased (Vincent andMekis, 2006).• Water vapour in the Great Lakes Basin and SouthernOntario has increased more than 3% from 1973 to1995, contributing to higher intensity rainfall events(Ross and Elliott, 2001).