Colorado’s Lake Dillon is Warming Rapidly — @CIRES

Grays and Torreys, Dillon Reservoir. Photo credit Greg Hobbs.

From CIRES:

CU Boulder researchers harness 35 years of data to uncover responses of a high-elevation reservoir to a warming world

The surface waters of Lake Dillon, a mountain reservoir that supplies water to the the Denver area, have warmed by nearly 5 degrees Fahrenheit (2.5 degrees Celsius) in the last 35 years, which is twice the average warming rate for global lakes. Yet surprisingly, Dillon does not show adverse environmental changes, such as nuisance algal blooms, often associated with warming of lakes. Researchers at the CIRES Center for Limnology, who have just published a multi-decadal study of Lake Dillon, conclude that the lake’s rapid warming and its lack of ecological response to warming are explained by the high elevation of the lake.

“The warming of Lake Dillon is a result of climate change but, in contrast with warm lakes, which respond in undesirable ways to warming, Lake Dillon shows no environmental response to warming, said William Lewis, Director of the CIRES Center for Limnology and lead author of the new paper published today in AGU’s Water Resources Research. “The explanation for the lake’s ecological stability lies in its low temperature, which serves as a buffer against ecological effects of warming.”

Since 1981, Lewis and colleagues in the CIRES Center for Limnology have collected detailed information not only on Lake Dillon’s temperature, but also on its water quality and aquatic life. Full vertical profiles of water temperature document changes in vertical distribution of heat over time. The record shows that warming of tributary water contributes to warming of the lake’s deepest waters.

“The 35-year data set allows us to see the complete warming pattern of the lake,” said James McCutchan, associate director of the Center. Natural events, including droughts and floods, create interannual variation that obscures the effects of climate change over short intervals, whereas multidecadal data sets can show more clearly the effects of climatic warming.

Dillon is the highest lake yet studied for full water column warming, as Lewis and his colleagues note in their paper. The study also is the first to analyze warming in a reservoir, rather than a natural lake.

“Reservoirs can differ fundamentally from other lakes in their response to warming because they often release water from the bottom as well as the top of the water column,” said Lewis. “They can warm not only from the top, in response to solar radiation reaching the surface, but also from the bottom, as tributaries subject to climatic warming replace cold bottom water with progressively warmer tributary water.”

The Lake Dillon study program is sponsored by Denver Water, which uses the water for treatment and delivery to Denver residents, and by the Summit Water Quality Committee, which represents the interests of local residents in preservation of Lake Dillon’s water quality.

#Denver was drier than #Phoenix in 2018

Denver City Park sunrise

From Westword (Chris Bianchi):

Denver received only 8.53 inches of precipitation (rain and snow-equivalent rainfall) in 2018, making it the sixth-driest year in the city’s recorded history. That’s a striking number for a bunch of reasons, but the main one is that it’s less than 60 percent of Denver’s average annual precipitation of 14.30 inches.

Here’s another way to think about it: Denver saw less rain in 2018 than true desert climates like Phoenix and Tucson, and Denver saw a rain total closer to America’s driest major city, Las Vegas (4.19 inches) than our average annual rainfall. Denver saw about 12 percent of Atlanta’s annual precipitation, 13 percent of New York’s and 17 percent of Chicago’s…

…only two months last year produced above-average moisture (January and March). The worst of the drought came in June (22 percent of average rainfall), July (48 percent), August (55 percent) and September (19 percent).

Why did 2018 lean on the drier side? When you’re measuring a full year’s worth of moisture, there are several factors to consider, but one thing stands out: an exceptionally dry late spring and summer, which is usually Denver’s wettest season. Summer storms usually help soak the ground. Droughts are often exacerbated by a positive-feedback loop, meaning dry soils and a lack of moisture in the air can suppress future rain and snow chances.

There was also a striking lack of snowfall in Denver for the second consecutive winter. DIA and Stapleton recorded less than half of Denver’s typical annual snowfall of about 57 inches. After the 2016-’17 winter produced the second-lowest snow total in Denver’s recorded history, the 25.7 inches DIA registered in 2018 was a slight improvement, amounting to the fifth-lowest snow total in the city’s recorded history.

December Climate Briefing: Atmosphere Resists #ElNiño Participation #ENSO, but El Niño precipitation patterns are showing up around the globe

From the IRI:

On the third Thursday of each month, the International Research Institute for Climate and Society holds a climate briefing, where it releases updated global seasonal climate forecasts as well as its forecast on the El Niño-Southern Oscillation. In this video, IRI’s Tony Barnston summarizes the key points from that briefing. For more, visit iri.columbia.edu/enso and follow #IRIForecast on Twitter.

#Snowpack news: Statewide % of normal = 91%

Click on a thumbnail graphic to view a gallery of snowpack data from the NRCS.

Ongoing #drought across the West is worse than many historical mega-droughts #ColoradoRiver #COriver #aridification #ActOnClimate

Concentric rings of various widths mark the annual growth of trees. Photo by Peter Brown, Rocky Mountain Tree-Ring Research. Photo credit: NOAA

From The Arizona Daily Star (Tony Davis):

The Colorado River — Tucson’s drinking water supply — carries nearly 20 percent less water than in 2000. Bark beetles are chomping away at our forests and killing off ponderosa pines. Wildfires are rapidly growing in intensity.

These problems have been linked to a drought that has stretched 19 years with no respite.

Now, a team of researchers concludes that the ongoing drought across the western U.S. rivals most past “megadroughts” dating as far back as 800 A.D. — and that this region is currently in a megadrought.

Using tree ring data as a proxy for drought conditions, the researchers say the current drought ranks fourth worst among comparable 19-year periods of megadroughts of the past 1,200 years.

A significant factor in this ranking is global warming triggered by human-caused climate change, says Park Williams, the study’s lead author and an associate research professor at Columbia University’s Lamont-Doherty Earth Observatory.

“The drought severity of the last 19 years is almost as bad as the worst 19-year period of the worst megadrought. It indicates that it’s very important that we develop more sustainable ways of dealing with water and allocating water across the western U.S.,” Williams said in a blog posted by Columbia’s Earth Institute…

The idea that this drought is among the worst compared to past megadroughts draws support from other researchers. There’s disagreement, however, as to whether the West is actually in a megadrought now.

It’s going to take more drought years before researchers Connie Woodhouse of the University of Arizona and Toby Ault of Cornell University — who also have studied megadroughts — are willing to use that term this go-around.

Ault’s threshold for a megadrought is 35 years, although he acknowledges that many researchers use 20 years. Woodhouse, a tree ring researcher who co-authored a pioneering study on megadroughts 20 years ago, said she hasn’t used a specific period to define megadroughts but that this drought hasn’t lasted long enough to qualify.

“The definition of megadrought technically is open to debate,” Jonathan Overpeck recently told The Atlantic in an article on Williams’ study. Overpeck is a University of Michigan climate scientist who formerly ran UA’s Institute for the Environment.

“The drought in the Southwest is right on the cusp of technically being a megadrought,” said Overpeck, who co-authored the 1998 study with Woodhouse and gets credit from her for coining the term…

One difference between this drought and most past megadroughts is that this one’s effects have been spread over the entire West, whereas the earlier droughts were more heavily concentrated in parts of the region, Williams told the conference.

Not a lot of places in the West are experiencing their worst droughts on record today, “but instead, a lot of places are experiencing pretty severe droughts,” Williams said.

Today’s drought pattern could have the fingerprints of human-caused climate change, he said. By using computer-based climate models, the Columbia researchers calculated that climate change has made the current drought 38 percent more severe…

Either way, the study said, “Water managers in the Southwest should plan for the possibility of a megadrought before the end of this century.”