Colorado Climate – Sep 2026September 2026 Update

Novarupta (Katmai) eruption

The largest volcanic eruption anywhere in the world in the 20th century - 13 km3 of magma, about thirty times the 1980 Mount St. Helens eruption - at Novarupta in Alaska on 6-8 June 1912. Ash reached 100,000 ft and cooled the Northern Hemisphere by roughly 0.3°C over 1912-13. Unlike a tropical eruption such as Pinatubo, a high-latitude eruption's haze stays in its own hemisphere, so it did little to the Southern Hemisphere.

Smithsonian Global Volcanism Program, Alaska Volcano Observatory, National Park Service, Oman et al. 2005

The Dust Bowl

South-eastern Colorado, and Baca County in particular, was one of the hardest-hit corners of the whole Dust Bowl. Land abandonment there was severe enough that much of it was later bought back by the federal government and returned to grassland as national grassland.

Schubert et al. 2004, NCAR/UCAR

Black Sunday, the worst Dust Bowl storm

South-eastern Colorado lay on the track of the 14 April 1935 Black Sunday duster as it drove south out of the Plains. It arrived on what had been a clear, mild Sunday afternoon, which is part of why it caught so many people outdoors and unprepared.

NWS Norman

Post-WWII industrial expansion begins

After the Second World War, coal and oil use grew rapidly across Europe, North America and the Soviet Union. Global sulphur dioxide emissions rose from about 30 to about 130 million tonnes a year by 1980, and the sulphate haze they formed masked part of the greenhouse warming underneath - the effect known as 'global dimming'.

Klimont et al. 2013

The 1950s drought

1956 was Colorado's 4th-driest year - 347 mm, 24% below its 1961-90 average.

NOAA Drought.gov, NOAA Climate Program Office, Texas Water Development Board

Agung eruption

Agung's eruption in Bali put a veil of sulphate haze into the stratosphere that cooled global temperature by about 0.3°C over 1963-66.

Smithsonian Global Volcanism Program, NOAA Global Monitoring Laboratory

The South Platte flood of 1965

About 14 inches of rain fell near Castle Rock and Deckers on 14 June 1965, and two days later the South Platte tore through Denver. Peak discharge reached 40,300 cubic feet per second - 1.8 times the previous highest in records going back to 1889. Property losses came to $543m at the time, over $4.4bn in today's money, and it remains the most expensive flood in Colorado's history. 1965 is Colorado's 2nd-wettest year in this site's 1950-2025 record.

USGS Water Supply Paper 1850-B, Colorado Encyclopedia

US Clean Air Act

The 1970 Clean Air Act set strict national standards for sulphur dioxide and particulates, and American skies got measurably cleaner over the decades that followed. There is a climate sting in that success: sulphate aerosols reflect sunlight, so removing them removed a cooling effect that had been partly masking greenhouse warming. Measured here, Colorado averaged +1.04°C warmer across 2006-2025 than across the two decades before the Act, 1951-1970, against a median of +1.09°C across the 49 states whose records reach back that far. Every one of them warmed. The range runs from +0.66°C in Oklahoma to +1.67°C in Alaska, so the question a state's own chart answers is not whether it warmed, but by how much.

US EPA

The 1973-74 La Niña

The strongest La Niña of the 1950-present record, part of a multi-year cool phase that also included a second, separate La Niña in 1975-76.

ONI -2.0°C · RONI -1.9°C

NOAA CPCENSO Tracker

The cold winters of 1976-79

1979 is Colorado's 3rd-coldest year in this site's 1950-2025 record. Its first months were the end of the winter of 1978-79, the coldest on record for the contiguous United States.

Illinois State Water Survey, Missouri Climate Center, Monthly Weather Review (1978), Wikipedia

Pacific Decadal Oscillation flips to a warm phase

A warm Pacific Decadal Oscillation tends to bring the Southwest wetter winters, and the two decades after the 1976-77 flip were among the region's wetter spells of the century - which is part of why the drought that followed the flip back in 1998-99 came as such a contrast.

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Global SO2 emissions peak

Global sulphur dioxide emissions peak at about 130 million tonnes a year, then begin a long decline as Europe, North America and Japan clean up their air. As the aerosol mask thins, the rate of global warming picks up through the 1980s.

Klimont et al. 2013

El Chichón eruption

A sulphur-rich eruption in Mexico whose aerosol cloud stayed in the stratosphere for more than two years, cooling the Northern Hemisphere by about 0.2°C.

Smithsonian Global Volcanism Program

1982-83 El Niño

El Niño has only a weak pull on Colorado's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Colorado's winter rainfall ran -12% against the 1961-90 average, with temperatures +1.5°C. On NOAA's RONI index, which strips out background ocean warming, this is the strongest El Niño since 1950.

ONI +2.1°C · RONI +2.4°C

NOAA CPC, NOAA CPC (2)ENSO Tracker

Daytime warming catches up with the night-led pattern

Across 1961-90, 81% of the world's land saw nights warming faster than days. Across 1991-2020 the pattern reversed: 70% saw days warming faster, as the industrial haze that had been dimming daytime sunshine was cleaned up. The study compares two fixed thirty-year windows; measured as a turning point, this site's own global series turns around 1982.

Zhong et al. 2023

Mount Pinatubo erupts

Pinatubo's aerosol veil left a clear mark on Colorado. 1993, two years after the June 1991 eruption, is Colorado's 8th-coldest year of the 76 on record and no year since has been colder. A single large eruption can hold temperatures down for two or three years; it cannot reverse the trend.

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Atlantic Multidecadal Oscillation flips to a warm phase

The Atlantic Multidecadal Oscillation flipped to a warm phase in 1995. It is an Atlantic cycle, but its reach is not confined to the coast: a warm AMO alongside a cool Pacific has been tied to more frequent drought across the western and central United States, including the widespread droughts of 1996 and 1999-2002. It is a natural cycle of roughly 60-70 years, so it shifts the baseline rather than driving the long-term trend.

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1997-98 El Niño

In 1997-98 Colorado's winter rainfall ran -13% against the 1961-90 average, with temperatures +1.1°C - though ENSO's pull on rainfall here is weak. It is often treated as the textbook strong El Niño.

ONI +2.4°C · RONI +2.3°C

NOAA CPC, NOAA NCEIENSO Tracker

Pacific Decadal Oscillation flips back to a cool phase

The Pacific Decadal Oscillation flipped back to a cool phase around 1998-99. Cool phases favour dry winters across the Southwest, and the flip coincides with the start of the megadrought in 2000 - natural variability setting the stage, with warming then deepening it.

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Southwest megadrought (2000-2021+)

Colorado's own 2000-2021 rainfall averaged just 3% below its 1961-90 average - the smallest shortfall of the six megadrought states. Colorado's role in the megadrought is less about rain falling on it than about what leaves it: the Colorado River rises here, and warmer temperatures have cut its flow even in years with near-normal precipitation.

Williams et al. 2022

Colorado River flow decline / Lake Mead crisis

The Colorado River rises here, and most of its water comes from Colorado snowpack. Udall & Overpeck (2017) found roughly a third to a half of the river's 21st-century flow decline attributable to higher temperatures alone, independent of precipitation - warmer air takes more water from snow and soil before it ever reaches the channel. That is why Colorado can have a near-normal snow year and still send less water downstream than the same snowpack would have delivered fifty years ago.

Udall & Overpeck 2017

The 2002 Western drought

2002 is Colorado's driest year in this site's 1950-2025 record - 301 mm, 34% below its 1961-90 average. 2002 is also the year of the Hayman Fire, then the largest in Colorado's recorded history.

NOAA Drought.gov, NOAA NCEI

Start of the sustained rise in US heatwave frequency

Colorado's own Heatwave Days (WSDI) count rose from a 1961-90 average of 3.9 days a year to 18.5 in 2016-25 - more than a fourfold increase, well ahead of the 2.4x averaged across the 50 states.

World Bank CCKP (CCKP), NOAA Climate at a Glance

The 2012 drought and heat

Colorado recorded 21.5 heatwave days in 2012, its 4th-highest on record here, alongside its warmest year on record and its 3rd-driest year (313 mm).

NOAA NCEI, Summer 2012 Drought Update, NOAA NCEI, Drought

Post-2013 acceleration in US temperatures

8 of Colorado's ten warmest years on record have occurred since 2013, and 9 of the ten since 2000, with 2012 the warmest of all - a local acceleration closely tracking the national one.

NOAA Climate at a Glance

2015-16 El Niño

In 2015-16 Colorado's winter rainfall was close to its 1961-90 average (+6%), with temperatures +1.4°C - though ENSO's pull on rainfall here is weak. By the raw ONI index it was the strongest El Niño ever measured - a reminder that a strong index does not guarantee a strong local outcome.

ONI +2.6°C · RONI +2.3°C

NOAA CPCENSO Tracker

2015-2025: the eleven warmest years on record

Globally, the eleven warmest years on record have all happened since 2015. Colorado mostly follows it - eight of its own eleven warmest fall in the 2015-2025 span - but its hottest year is still 2012, the drought year. The three outside it are 2012, 1981 and 1954. Colorado is one of 15 states where 2012 still tops the list.

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The West's air gets drier

Colorado's relative humidity has fallen 3.6 percentage points comparing 2015-2025 with 1950-2014 - the 2nd-largest fall of any US state - while its annual temperature rose +1.00°C over the same comparison.

World Bank CCKP (CCKP), NOAA Climate at a Glance

Low-sulphur shipping fuel rule (IMO 2020)

New international shipping rules cut the sulphur allowed in ship fuel from 3.5% to 0.5% almost overnight, removing about 8 million tonnes of sulphur dioxide a year. With less sulphate haze over the oceans, it may have added around 0.1°C of warming by 2023-24.

Gettelman et al. 2024

The Marshall Fire, December 2021

Colorado's most destructive wildfire (1,000+ homes) - notable for occurring in WINTER, following a whiplash from a wet spring (fuel growth) to a record-dry, 10°F+-above-average fall/winter plus 100+mph winds.

NOAA Climate.gov

2023-24 El Niño

A strong El Niño arriving on top of long-term greenhouse warming, with the cut in shipping sulphur from the 2020 fuel rule adding a little more. Together they produced the record global temperatures of 2023 and 2024.

ONI +2.0°C · RONI +1.4°C

NOAA CPCENSO Tracker