US West 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

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

Among the most widespread and sustained droughts ever recorded in the US - worse than the Dust Bowl in several states. In the West it reached Nevada.

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

US Clean Air Act

The 1970 Clean Air Act cut sulphur dioxide and particulate pollution across the country, and removing those sunlight-reflecting aerosols also removed a cooling effect that had been partly masking greenhouse warming. Across the West, 2006-2025 averaged +1.14°C warmer than the two decades before the Act, 1951-1970.

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 1976-77 drought

A short, intense drought that ran from northern Minnesota to southern California. In the West it reached California.

USGS, US GAO, USGS Water Data Labs

Pacific Decadal Oscillation flips to a warm phase

The 1976-77 flip of the Pacific Decadal Oscillation to its warm phase arrived in the middle of a severe drought across California and Nevada, and the two decades that followed tilted the odds towards wetter winters in both, particularly when an El Niño coincided.

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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 usually brings the West a wet winter. In 1982-83 it was: winter rainfall ran +48% against the 1961-90 average, with temperatures +0.6°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 cooling is only faintly visible in the West's own record. 1993, two years after the June 1991 eruption, ranks 13th-coldest of the 76 years there, though 1998 has since 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

El Niño usually brings the West a wet winter. In 1997-98 it was: winter rainfall ran +78% against the 1961-90 average, with temperatures +0.1°C. 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 tilt the odds towards dry winters across California and Nevada, and the period that followed includes the long 2012-16 and 2020-22 droughts.

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Southwest megadrought (2000-2021+)

The driest 22-year period in the Southwest for at least 1,200 years, reconstructed from tree rings. It reached both states of the West, California and Nevada.

Williams et al. 2022

Colorado River flow decline / Lake Mead crisis

A decline in the river's flow of which roughly a third to a half is attributable to warming alone, not to reduced precipitation. California and Nevada both draw on it, through Lake Mead and the lower river.

Udall & Overpeck 2017

The 2002 Western drought

The sharpest single dry year of the early Southwest megadrought, and Colorado's driest year on record. In the West it reached Nevada.

NOAA Drought.gov, NOAA NCEI

2006: a national heat record that lasted six years

2006 was the warmest year ever recorded for the contiguous United States, edging past the 1998 El Niño record. A severe heat wave ran from mid-July into late August, killing at least 225 people; on 17 July all but three states reached 90°F or more, and California's central valley hit 110-115°F. In this site's data 2006 now ranks only 10th warmest nationally.

NOAA NCEI, Science

Start of the sustained rise in US heatwave frequency

Across the two states of the West, Heatwave Days (WSDI) rose from a 1961-90 average of 8.9 days a year to 23.1 in 2016-25 - more than doubling, with every one of them recording more heatwave days than in 1961-90. The two states are closely grouped, from 2.4x to 2.8x.

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

California droughts, 2012-16 and 2020-22

2012-16: the most severe drought in California's instrumental record - 2014 and 2015 statewide snowpack hit record lows (25% then 5% of average); $3.8bn in agricultural losses 2014-16. 2020-22: a second episode reaching comparably severe conditions over a larger area, $1.3bn-$1.7bn in annual crop losses.

California DWR / PPIC

Post-2013 acceleration in US temperatures

Across the two states of the West, an average of 9.0 of each state's ten warmest years have come since 2013, ranging from 8 in Nevada to 10 in California. Every state in the region has all ten of its ten warmest since 2000.

NOAA Climate at a Glance

"The Blob": the North Pacific marine heatwave of 2014-16

A vast patch of unusually warm water - nicknamed "the Blob" - spread across the north-east Pacific from late 2013 and lasted into 2016, at times more than 2.5°C above normal. It coincided with the PDO's swing to its warm phase and then the 2015-16 El Niño. Its effects ran through the food web: about 62,000 dead common murres washed ashore from California to Alaska in 2015-16, with total deaths estimated near a million birds - the largest seabird die-off on record.

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2015-16 El Niño

El Niño usually brings the West a wet winter. In 2015-16 it was: winter rainfall ran +16% against the 1961-90 average, with temperatures +0.9°C. 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. The West follows it closely: nine of its own eleven warmest fall in that span, with 2014 and 2012 outside it. Its warmest year is 2014.

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California's 500-year-record-low Sierra snowpack, 2015

Snowpack measured at just 5% of average on April 1, 2015 (the historic peak-measurement date). A tree-ring reconstruction back to 1500 CE (Nature Climate Change) confirmed this was the lowest Sierra Nevada snowpack in over 500 years.

The Conversation / Nature Climate Change

The West's air gets drier

The largest falls in relative humidity anywhere in the country, while the south-east barely changed. The drying shows in both California and Nevada.

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

The 2015 western snow drought

The year 81% of the West's monitored snow sites hit record-low snowpack - driven by warmth, not a shortage of precipitation. In the West it reached Nevada as well as California.

Mote et al. 2016, NOAA Drought.gov, USDA

The Camp Fire

The deadliest wildfire in California's history - and a November fire, in what used to be the wet season.

Cal OES, NIST

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

California's 2020 fire siege

8,648 fires burned 4,304,379 acres in California in 2020 - the largest wildfire season in the state's modern record. The August Complex passed a million acres to become the first fire in California history described as a 'gigafire', crossing seven counties and covering more ground than Rhode Island. On 16 August Furnace Creek in Death Valley recorded 130°F, potentially the highest reliably measured temperature anywhere on Earth.

CAL FIRE, CalMatters, NOAA NESDIS

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

Las Vegas's all-time record high, 120°F (2024)

Broke the prior all-time record of 117°F; also logged 3 consecutive days above 118°F and 7 consecutive days at or above 115°F, in data going back to 1937.

NWS Las Vegas / NPR

The January 2025 Los Angeles fires

A January firestorm that killed 31 people and destroyed more than 16,000 structures inside Los Angeles.

World Weather Attribution, NOAA Climate.gov, CarbonBrief