From the Grand Junction Free Press (Hannah Holm):
While [Dr. Jeff Lukas of the Western Water Assessment team at the University of Colorado] said precipitation is much more difficult to predict than temperature, he noted that warmer temperatures will certainly intensify the drought conditions brought on by any dry years. That’s because warmer temperatures increase evaporation rates. It doesn’t help that in general, dry years tend to be hotter than wet years. The average results of climate modeling for precipitation indicate that conditions are likely to get wetter to the north and drier to the south, with Colorado right on the dividing line between the two.
Lukas was careful to emphasize that climate change models are not crystal balls, especially at the local level. Different models give very different outputs, so analysts look at each of them individually as well as the average results. The variations result from different assumptions about, among other things, the feedback responses of different elements in the climate system (oceans, ice caps, etc.) to increasing greenhouse gasses and higher temperatures. Adding to the uncertainties about how climate change will play out in any given location are regular, cyclical climate variations, like the “El Niño” and “La Niña” shifts in South Pacific Ocean temperatures that influence the tracks of our winter storms.
Despite these uncertainties, Lukas noted that warming on a global scale is happening already, and observations indicate some clear trends for Colorado. These include increased warming in spring and summer, a higher portion of precipitation falling as rain rather than snow, and earlier melting of the snowpack we do get. Further climate change is likely to intensify these trends. Even if overall precipitation levels didn’t change at all, these trends would pose significant challenges for water managers trying to meet existing demands, as well as increasing demands from anticipated population growth in the state.
Lukas also pointed out that tree ring studies indicate that there have in the past been more severe and more prolonged droughts than anything since formal record-keeping began around 1900, so even if climate change weren’t a factor, we could see more challenging drought conditions in the future than we are used to.
The climate change modeling results and observations Lukas discussed are dealt with in great depth in the Bureau of Reclamation’s Colorado River Basin Supply and Demand Study, as well as water supply studies undertaken by the state of Colorado. State water officials are also using them as they work on building a set of plausible scenarios to plan around as they and basin roundtables of stakeholders around the state negotiate about how to balance water supply and demand in coming decades.