Arizona 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

1956 was Arizona's driest year in this site's 1950-2025 record - 153 mm, 53% below its 1961-90 average. With New Mexico, this is the deepest rainfall deficit of any state in the 1950s drought - both states lost over half a normal year's rain.

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 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, Arizona averaged +1.25°C warmer across 2006-2025 than across the two decades before the Act, 1951-1970, the 10th-largest rise of the 49 states measured and above their median of +1.09°C. Cleaning the air was right for human health. It also uncovered warming that was already there.

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

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 Arizona's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Arizona's winter rainfall ran +56% against the 1961-90 average, with temperatures +0.2°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 does not show in Arizona's own record: 1993, two years after the June 1991 eruption, was an unremarkable year there, 32nd-coldest of 76. 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 Arizona's winter rainfall ran +66% against the 1961-90 average, with temperatures -0.4°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+)

Arizona shows the megadrought most clearly of any state: 2000-2021 rainfall averaged 13% below its 1961-90 average, the largest such shortfall in the Southwest. A 13% annual deficit sounds modest - the point is that it did not let up for 22 years, and soil moisture and reservoirs respond to the accumulation, not to any single dry year.

Williams et al. 2022

Colorado River flow decline / Lake Mead crisis

Arizona holds the most junior major allocation on the lower Colorado, which means it absorbs the first and deepest cuts when shortage is declared - and shortage has been declared repeatedly since 2021 as Lake Mead fell to record lows. The Central Arizona Project, which carries river water to Phoenix and Tucson, is the most exposed large system in the basin.

Udall & Overpeck 2017

The 2002 Western drought

2002 was Arizona's 3rd-driest year on record here - 177 mm, 46% below its 1961-90 average. 2002 is also the year of the Rodeo-Chediski fire, Arizona's largest until the Wallow Fire nine years later.

NOAA Drought.gov, NOAA NCEI

Start of the sustained rise in US heatwave frequency

Arizona's own Heatwave Days (WSDI) count rose from a 1961-90 average of 6.2 days a year to 27.0 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 Wallow Fire

The Wallow Fire started on 29 May 2011 and burned about 538,000 acres of the Apache-Sitgreaves National Forest in north-eastern Arizona. It is the largest wildfire in Arizona's modern record, passing the 2002 Rodeo-Chediski fire by around a thousand acres.

NASA Earth Observatory, CIRES

The 2012 drought and heat

2012 was Arizona's 8th-warmest year and its 10th-driest of 76, so the drought side of the year did reach here - but the heat did not come with it, with heatwave days only 30th-highest. The conditions that made 2012 notorious nationally were centred on the Plains and Corn Belt; what Arizona had was a dry year inside a much longer dry period, covered by its megadrought entry.

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

Post-2013 acceleration in US temperatures

8 of Arizona's ten warmest years on record have occurred since 2013, and all ten since 2000, with 2017 the warmest of all - a local acceleration closely tracking the national one.

NOAA Climate at a Glance

2015-16 El Niño

In 2015-16 Arizona's winter rainfall ran -31% against the 1961-90 average, with temperatures +1.1°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. Arizona's own record is a looser fit - seven of its eleven warmest fall in that span, with 2014, 2012, 2003 and 1996 outside it. Its warmest year is 2017.

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The West's air gets drier

Arizona's relative humidity has fallen 3.3 percentage points comparing 2015-2025 with 1950-2014 - the 5th-largest fall of any US state - while its annual temperature rose +1.11°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

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

Phoenix's 31-day 110°F+ streak, 2023

31 consecutive days at or above 110°F (June 30-July 31), shattering the prior record of 18 days (1974); the hottest calendar month on record for any major US city.

NWS Phoenix / NPR