Lake Tahoe’s average surface temperature last year was the warmest ever recorded, the latest evidence that climate change is altering California’s iconic Sierra Nevada landmark.

In a report released Thursday by UC Davis, scientists said that the lake’s waters in the past four years have been warming at 15 times their historic average.

The air temperature at the lake is becoming steadily hotter too. The winter of 2014-15 saw just 24 days where the average temperature dropped below freezing at the lake, according to the report, and only 6 percent of last year’s precipitation fell as snow — both all-time lows.

The ominous evidence threatens efforts in recent years to improve Lake Tahoe’s famed blue clarity by reducing pollution. That’s because the warming water will likely result in more algae growth, silt and invasive species, researchers said.

“The lake is changing, and it is changing at an increasing rate,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center.

The picture of a steadily warming lake — and a vacation wonderland with a relentless trend toward hotter weather, more rain and less snow — emerged from the 2016 “State of the Lake” report, a document the center publishes every year.

Straddling the California-Nevada border, Lake Tahoe is the second deepest lake in America. It’s 1,645 feet at its deepest point, behind only Crater Lake in Oregon. If the Empire State Building were submerged in Lake Tahoe, the top of its spire would still be below 200 feet of water. Roughly 3 million people visit each year.

“This year’s report is definitely a warning,” said Darcie Goodman Collins, executive director of the League to Save Lake Tahoe, an environmental group. “We need to improve our efforts.”

Last year, the lake’s average clarity was 73.1 feet, the UC Davis scientists reported. That’s a 4.8-foot decrease from the previous year.

The worst recorded average clarity was 64.1 feet in 1997, and the best was 102 feet, in 1968, the year measurements began. As the lake’s crystal clarity began to suffer in the 1970s and ’80s from fertilizer, polluted runoff and erosion, environmentalists launched a Keep Tahoe Blue effort. Federal officials and scientists began devoting more money and attention to the lake.

Local rules were tightened and education campaigns launched to reduce erosion and the amount of nitrogen and phosphorus — chemicals that can cause algae to increase in the lake — from fertilizer and old septic systems and air pollution from vehicles. Homeowners were required to capture stormwater from their properties — channeling water from gutters into filters in gardens, for example, to keep it from running into the lake.

The work paid off: Lake clarity has generally improved over the past decade.

But now the warmer water and weather are presenting a new challenge.

Colder water, from the melting of snow, is dense, so it sinks in the lake, Schladow said. But when precipitation comes mostly from rain, it’s warmer, and the silt and other tiny debris it washes into the lake stay near the surface.

Also, UC Davis scientists in recent years have noticed that the lake isn’t mixing as thoroughly as it did when they began studying it nearly five decades ago. That means oxygen isn’t cycling to the bottom as much, increasing the risk of “dead zones.”

The 10 hottest years globally since 1880, when modern records began, have all occurred since 1998, with 2015 the hottest. Humans burning coal, gasoline and other fossil fuels have sent carbon dioxide concentrations worldwide to the highest level in at least 400,000 years, according to NASA. And that carbon traps heat in the atmosphere.

The impacts at Lake Tahoe have been dramatic, measured by exhaustive UC Davis research.

Consider:

In 2015, the average surface temperature of the lake was 53.3 degrees, up from 50.3 in 1968. That was the warmest recorded since consistent measurements began that year.

Daily readings taken since 1911 at Tahoe City show that the daily minimum air temperature has increased 4.3 degrees Fahrenheit, and the daily maximum is up 2 degrees since then.

The winter of 2014-15 had only 24 days where the average daily temperature was below freezing. That’s the fewest in 105 years of measurements. From 1910 to 1930, every year had more than 55 days below freezing.