Iowa 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

1951: a cold, wet year and the Great Flood

1951 is Iowa's coldest year in this site's 1950-2025 record, and it was wet as well - 27% above its 1961-90 average. The same cold, rain-soaked year across the central US produced the Great Flood of 1951: in July the Kansas River and the Missouri River overwhelmed Kansas City, killing 17 people and displacing 518,000.

NWS Missouri Basin River Forecast Center, USGS

The summer of 1953

1953 gave Iowa 25.2 heatwave days, its 2nd-highest of the 1950-2025 record, though it ranks only 18th-warmest by annual mean - a summer event, not a warm year.

Northeast Regional Climate Center, LIFE

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, Iowa averaged +0.88°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 Mississippi flood of 1973

1973 was Iowa's 4th-wettest year on record here - 1090 mm, +30% against its 1961-90 average.

USGS Professional Paper 937, NWS St. Louis

The 1976-77 drought

1976 was Iowa's 3rd-driest year on record here - 599 mm, 28% below its 1961-90 average.

USGS, US GAO, USGS Water Data Labs

The cold winters of 1976-79

1979 is Iowa'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 and the third brutal winter in a row across the Midwest.

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

Pacific Decadal Oscillation flips to a warm phase

The Pacific Decadal Oscillation is a North Pacific pattern, and its influence on this side of the country is weak and indirect - mostly through how it nudges the winter jet stream downstream. Its 1976-77 flip marks a genuine shift in Pacific-basin climate, but for eastern states the Atlantic Multidecadal Oscillation is the ocean cycle that matters more.

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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 Iowa's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Iowa's winter rainfall ran +80% against the 1961-90 average, with temperatures +3.6°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

1988 Midwest drought / Yellowstone fires

1988 was Iowa's driest year in this site's 1950-2025 record (546 mm, 35% below its 1961-90 average) with 18.9 heatwave days, its 8th-highest.

NOAA

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

1990-91: mild winters and warm years

1990 is Iowa's 13th-warmest year in this site's 1950-2025 record. Mild winters did most of it: January-February 1990 was the warmest start to a year the contiguous US had recorded, a record that stood until 1998.

News & Record / AP (1991), NOAA NCEI

Mount Pinatubo erupts

Pinatubo's aerosol veil left a clear mark on Iowa. 1993, two years after the June 1991 eruption, is Iowa's 8th-coldest year of the 76 on record, though 1996 and 2014 have 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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The Great Flood of 1993 (Mississippi River)

1993 is Iowa's wettest year in this site's 1950-2025 record - 1216 mm, 45% above its 1961-90 average. The flood was not a single storm but a whole summer of rain falling on ground that never dried out.

NASA Earth Observatory / NWS

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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February 1996 Arctic outbreak, state record colds

The February 1996 Arctic outbreak reached well south of its record-setting core: 1996 was Iowa's 4th-coldest year in this site's 1950-2025 record and its 3rd for air-frost days, though the state records themselves were set further north in Minnesota and Wisconsin.

NWS La Crosse

1997-98 El Niño

In 1997-98 Iowa's winter rainfall ran +24% against the 1961-90 average, with temperatures +4.6°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 returned to a cool phase around 1998-99. Its direct effects are western - drier conditions across the south-west, changes in North Pacific fisheries - and its influence on this side of the country is weak and indirect. The Atlantic Multidecadal Oscillation, which flipped warm in 1995, is the ocean cycle with more bearing on eastern weather.

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2008 Iowa flood

9.03in of rain fell statewide over ~2 weeks (normal: 2.45in); Iowa's 4th wettest year on record (43.79in) - distinct mechanism and rivers from the 1993 Great Mississippi Flood.

NOAA NCEI

Start of the sustained rise in US heatwave frequency

Iowa's own Heatwave Days (WSDI) count rose from a 1961-90 average of 6.3 days a year to 14.2 in 2016-25 - more than doubling, close to the 2.4x averaged across the 50 states.

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

The 2012 drought and heat

Iowa recorded 27.7 heatwave days in 2012, more than in any other year of this site's 1950-2025 record, alongside its warmest year on record and its 8th-driest year (650 mm).

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

Post-2013 acceleration in US temperatures

6 of Iowa's ten warmest years on record have occurred since 2013, and 8 of the ten since 2000, with 2012 the warmest of all.

NOAA Climate at a Glance

The polar-vortex winter of 2013-14

2014 is Iowa's 6th-coldest year in this site's 1950-2025 record. In early January 2014 the polar vortex slipped south and Arctic air spread across most of the country, and the cold returned in waves into March.

Wikipedia, CBS NewsPolar Vortex Tracker

2015-16 El Niño

In 2015-16 Iowa's winter rainfall ran +109% against the 1961-90 average, with temperatures +3.7°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. Iowa does not follow it: its hottest year is still 2012, the drought year. Six of its own eleven warmest fall in the 2015-2025 span, with 2012, 2006, 1999, 1998 and 1987 outside it. Iowa is one of 15 states where 2012 still tops the list.

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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 2020 Midwest derecho

Iowa took the worst of the 10 August 2020 derecho. Gusts were measured at 126 mph at Atkins and estimated at 140 mph in Cedar Rapids, where around a fifth of the housing stock was damaged. Some 14 million acres of Iowa cropland were hit - roughly half of everything planted in the state that year. At $11.2bn it is the costliest thunderstorm event ever recorded in the United States, and none of it shows in Iowa's annual temperature or rainfall figures.

NWS Des Moines, NASA Earth Observatory

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