has the highest average elevation in the nation. Winter temperatures substantially below freezing at higher elevations might protect the state’s snowpack. Still, across the West, including Colorado, the proportion of total annual precipitation falling as rain rather than snow has been increasing over the last 50 years. No clear trend in precipitation emerges over the last 100 years. There is evidence of increasing drought severity and length in the American Southwest (Figure 4). Streamflow runoff timing has advanced by up to 20 days in large areas of the West, in many cases mirroring the same locations with reductions in snowpack (Figure 5). Advances in timing are not yet evident in Colorado. Global Climate Projections for the 21st Century The IPCC projects global warming of about 0.7 F over the next 20 years, regardless of greenhouse gas emissions during the period. About half of the anticipated warming is ‘committed warming,’ reflecting the fact that the climate in general and the oceans in particular are slow to respond to the warming caused by existing greenhouse gasses. Beyond 2030, temperatures will increasingly depend on actual greenhouse gas emissions. The mid-range estimate for global temperature increases at 2100 range from 3.25 F to 7.2 F relative to the 20th century average. Warming is expected to be greatest over land and most at high northern latitudes. Snow-covered areas are expected to contract and sea ice to shrink. Heat waves and heavy precipitation events are likely to become more frequent. Tropical cyclones probably will become more intense, but it is unknown if they will become more or less frequent. Mid latitude storm tracks are projected to move poleward with associated changes in wind, precipitation and temperatures, continuing the broad pattern of observed trends over the past 50 years. At higher latitudes, increased precipitation is very likely. In most semi-arid subtropical land areas (Figure 6) precipitation decreases are projected. Additional climate change would continue well into the 22nd century, even if greenhouse gas concentrations were stabilized at 2100.
Linear Trends in April Snowpack (1957-1997) 80% 60% 40% 20%
Observed CT Trends (1948-2000)
60° >20d Earlier 15-20d Earlier 10-15d Earlier 5-10d Earlier <5d Earlier 5-10d Later 10-15d Later 15-20d Later >20d Later
Figure 3: Linear Trends in April 1 snowpack for the period 1950-97. Source: Mote et al., 2005.
Figure 5: Observed trends in streamflow runoff timing, 1948-2000, in days runoff occurs earlier. Source: Stewart et al., 2004
30° Trends in Drought Severity
Figure 4: Trends in Drought Severity 1915 to 2003. Upward trends are in red triangles, downward in blue. Note the increasing trend in the American Southwest. Source: Andreadis and Lettenmaier, 2006.
©IPCC 2007: WG1-AR4
Figure 6: Projected multi-model mean patterns of precipitation changes at 2100 under mid-range greenhouse gas emissions with December, January and February on left, and June, July and August on right. Colors are shown where at least 66 percent of the models agree and stippling where 90 percent of the models agree on the sign of the change. Note that many already dry areas such as the American Southwest and the Mediterranean are projected to dry further, and many wet areas in the tropics and high northern latitudes are expected to get wetter. Source: IPCC Working Group 1 Summary for Policy Makers, 2007.
Citizen’s Guide to Colorado Climate Change