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The future? Hotter, yes. Precipitation? A fuzzier picture


by Allen Best

What can Colorado expect of its changing climate going forward?

The Colorado Water Conservation Board has commissioned a study overseen by Becky Bolinger, the assistant state climatologist, and Jeff Lukas, whose business is called Lukas Climate Research and Consulting, to update projections from two previous studies, in 2008 and again in 2014.

Newer climate models have been issued, they explained in a presentation at the Colorado Drought Summit on May 31, and more weather data has been accumulated to compare against what had previously had been projected.

Rising temperatures in the last 20 years  roughly align with what climate models had suggested would happen. That falls short of verification of the models, said Lukas, but it does suggest stronger confidence in what the models today say about the future.

The base period is 1970 to 2000. Against the historical record, temperatures have been 1.5 degrees F higher in the 21st century. The best estimate is for another 2.5 degrees of warming by mid-century, but warming of 5 to 6 degrees is possible. “That is an uncomfortable future,” said Lukas.

“Climate models were very clearly telling us to expect more heat proportionate to the amount of emissions,” said Bolinger. If 2012—a year of wildfires—remains the warmest year in records, it likely won’t stand.

“By 2050 and beyond, things really be different,” she said, depending upon continued emissions. Earlier in the month, gauges on volcanoes in Hawaii recorded at 424 parts per million, the fourth highest rise since measurements began in 1958.

Another way of understanding this warming is to look at the warmest four-day periods of a year above a certain threshold. That threshold was achieved maybe once a year before the turn of this century. It has now accelerated and will increase to about five times a year by mid-century.

As for precipitation, that’s still unclear. It might produce more. The models have no consensus. Even if winters do produce more precipitation, though, that gain will be offset by impacts during other seasons beginning with earlier runoff. Warming alone also increases the thirst of the atmosphere, which dries out plants and soils and causes water to evaporate.

“It’s very certain that we are going to get a couple of degrees more warming over the next several decades, and that will continue to dry our watersheds, our water cycles, our crops,” said Lukas.

Impacts to river flows in summer and fall could be particularly severe. And droughts will be intensified.

“The worst droughts of the next several decades will likely exceed those of the past 100-plus years,” he said.

Jeff Lukas with climate projections

Jeff Lukas explains what can be said with high confidence about the evolving climate in Colorado and what can be said with only low confidence.

Lukas and Bolinger emphasized that their conclusions, a synthesis of other work, remained preliminary. Just prior to their presentation at the drought conference, they had sent their report for review by 50 others. The final report is to be issued this summer.

A few of the projections as defined by confidence levels: low, medium, high, and very high:

  • In runoff, the recent trend has been toward earlier in spring, and there’s high confidence that runoff will occur even earlier.
  • Evaporative demand similarly has been trending higher, robbing the soil and plants of moisture, and all the available literature points in the same direction with a
    very high confidence level.
  • Snowpack has been trending lower (this year being a notable exception), and that’s the projected future trend, too, but this projection has only a medium confidence level.
  • Heat waves? They’ve become more frequent and intense, and that is the projected change for the future — this coming with a very high confidence.
  • Cold waves. Fewer of them as compared to a half-century ago, and even fewer in the future. That comes with a high confidence.
  • Wildfire threat? The risk has grown, and it will continue to grow even more. This comes with a high confidence level.
  • Windstorms. The recent trend is uncertain, and the future change is uncertain. That comes with a low confidence level.
Allen Best
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