Climate Matters
Warmer Fall, Later Freeze, Longer Allergy Season
About 50 million people in the U.S. are allergic to ragweed pollen, which peaks in the fall.
Climate Central’s Monthly Briefing Highlights from September 2026
What do experts say?
Dr. Zachary Labe, climate scientist at Climate Central, said:
“One of the strongest El Niños on record is unfolding on top of a much warmer climate. We can expect many extremes to be further amplified, including hotter heat waves and a supercharged water cycle. These changes can have major impacts on local communities and ripple through the global economy.”
Global Climate
September 2026 Highlights: Temperature
September 2026 was the second-warmest September on record globally (for land and ocean temperatures combined). Records have been kept by NOAA since 1850.
Record warmth affected 16.7% of global land and ocean areas in September, which was the largest extent on record for any month. The previous record was set in May 2024 (14.2%).
The last time September’s global mean temperature was cooler than normal (compared to the 20th-century average) was in 1975.
The last four years have been the four warmest Septembers on record for global temperatures.
Global average temperatures over land areas were the third-warmest on record in September.
Global average temperatures over ocean areas were the warmest on record in September.
At 1.1°C above the twentieth-century average, this was the all-time largest monthly temperature anomaly on record for global ocean areas.
September 2026 was the warmest on record (tie) for land and ocean temperatures across the Northern Hemisphere. It was the second-warmest for the Southern Hemisphere.
The month was the warmest September on record for the Arctic, North America, and Asia.
September 2026 Highlights: Other Climate Indicators
The average September carbon dioxide level measured at Mauna Loa, Hawaii, was 426.1 parts per million (ppm).
Arctic sea-ice extent ranked as the 13th-lowest on record for September, while Antarctic sea-ice extent ranked as the third-lowest. Learn more on polar sea ice conditions in the Climate Moment of the Month.
Year-To-Date in Review: Global
Globally (for land and ocean temperatures combined), temperatures so far this year rank as the second-warmest on record (after 2024) for any January-September period since records have been kept.
Year-to-date temperatures rank as the second-warmest on record for the Northern and Southern Hemispheres.
Global average temperatures over land areas ranked as the fourth-warmest on record so far this year (January-September), while ocean temperatures ranked as the second-warmest.
U.S. Climate
September 2026 Highlights: Temperature
September 2026 was the warmest September on record for the contiguous U.S. Records have been maintained since 1895.
Alaska recorded its 25th-warmest September on record, and Hawaii recorded its second-warmest September.
The Ohio Valley, South, and Southwest climate regions saw their warmest Septembers on record.
September was the fourth-warmest (tie) for average maximum temperatures and warmest for average minimum temperatures.
Fourteen states had their warmest September on record for minimum (nighttime) temperatures.
Arkansas, Louisiana, Oklahoma, and Texas had their warmest September for maximum (daytime) temperatures.
September 2026 Highlights: Precipitation
September 2026 ranked as the 43rd-wettest September on record (tie) for the contiguous U.S.
Alaska recorded its 49th-wettest September on record, while Hawaii had its second-wettest.
Arizona (tie) and New Mexico observed their second-wettest Septembers on record.
September 2026 Highlights: Drought
As of October 6, 2026, at least 75% of the contiguous U.S. is experiencing abnormally dry or drought conditions.
More than 55% of the contiguous U.S. was in at least moderate drought in September, while a portion (about 9%) experienced extreme or exceptional drought.
Forty-one states had areas that experienced at least moderate drought conditions.
The average Palmer Drought Severity Index, a measure of longer-term drought conditions, was the second-lowest on record for September for the contiguous U.S.; only September 1934 recorded a lower value.
September 2026 Highlights: Energy
September 2026 had the highest count of cooling degree days on record when averaged across the contiguous U.S.
Cooling degree days provide a rough measure of energy demand for air conditioning, with higher values indicating hotter conditions and greater cooling needs.
Year-To-Date in Review: U.S.
The average temperature so far in 2026 is the warmest on record for the contiguous U.S. (January-September 2026).
Arizona, Arkansas, California, Colorado, Kansas, Mississippi, Montana, Nevada, New Mexico, Oklahoma, Texas, Utah, and Wyoming have had their warmest start to a calendar year on record.
Nine states recorded their warmest January-September period on record for maximum temperatures, while 17 states recorded their warmest for minimum temperatures.
Average precipitation so far this year in the contiguous U.S. is the 17th-driest on record.
Arkansas, North Carolina, and South Carolina ranked in their top five driest January-September periods on record.
So far in 2026 (January-September), the average number of cooling degree days is the highest on record for the contiguous U.S.
States including Arizona, Arkansas, Colorado, Mississippi, Nevada, New Mexico, Oklahoma, Texas, Utah, and Wyoming have experienced record-high counts of cooling degree days in 2026.
Climate Moment of the Month with Kaitlyn
By Dr. Zachary Labe, climate scientist at Climate Central
September is a key month for monitoring polar climate conditions. It’s the time of year when Arctic sea ice reaches its minimum extent, while the Antarctic typically reaches its maximum sea-ice extent. Both the Arctic and Antarctic have their own seasonal cycles in sea-ice area, which are opposite one another because of the timing of the astronomical seasons and differences in how heat is exchanged between the ocean and atmosphere.
In addition to differences in the timing of their seasonality, Antarctic sea ice has a larger seasonal cycle, meaning a greater difference between its annual maximum and minimum, and is generally younger and thinner. One of the key reasons for these differences in sea-ice characteristics is the geography of the two poles: Antarctica is a high-elevation, ice-covered land mass surrounded by ocean, while the Arctic is an ocean largely surrounded by land, with heat transported through relatively narrow openings from the Pacific and Atlantic Oceans.
Data for tracking sea-ice extent primarily come from satellite observations, which provide daily coverage of both poles and a consistent record dating back to 1979, allowing us to monitor changes over time. September has seen the largest declines in Arctic sea-ice extent, driven primarily by human-caused climate change. The Antarctic, however, has not shown a clear long-term trend over the satellite era, although that may be starting to change following several rapid declines over the last decade.
The effects of declining sea ice do not stay confined to the Arctic or Antarctic and can have broader consequences. For instance, less sea ice can contribute to additional warming, influence regional weather patterns, affect global commerce, and disrupt the livelihoods of Indigenous communities.
According to data from the National Snow and Ice Data Center, this year’s minimum Arctic sea-ice extent dropped to 1.78 million square miles (4.60 million square kilometers). This ranked as the 10th-lowest on record, tied with 2008, 2010, and 2025. In a clear example of the dramatic changes across the region, the last 20 years have seen the 20 lowest annual minimum ice extents in the satellite record. This year’s minimum was about 625,000 square miles below the 1981-2010 climatological average, which is equivalent to an area about the size of Alaska in missing ice. The lowest Arctic sea-ice extent on record was set in 2012, while the overall long-term trend is a decline of about 11.8% per decade.
Cooler temperatures across northern parts of the Arctic Ocean, associated with persistent low pressure and storminess, helped limit some of the ice melt this summer, although other parts of the Arctic experienced unusual warmth, especially around western Siberia. It remains an active area of research to better understand how natural climate variability and year-to-year weather conditions influence the amount of summer melt from one year to the next, even as the long-term trend in Arctic sea ice is projected downward.
For the Antarctic, which just ended its winter season, sea ice reached its annual maximum extent on September 14 at 17.59 million square kilometers (6.79 million square miles). This was the third-lowest Antarctic sea-ice maximum on record, with the other records set in 2023 and 2024. This year’s maximum was more than 2 standard deviations below the long-term average. After reaching its maximum, sea ice began to decline rapidly and dropped to the lowest extent on record for the time of year in early October. Although it remains uncertain how much climate change may be starting to influence overall Antarctic sea-ice trends, the region has experienced a number of notable extremes in recent years, particularly since 2016.
U.S. Weather Extremes
Notable temperature event: Persistent extreme heat affected parts of the southern U.S. in September, especially in states such as Oklahoma. Oklahoma City recorded its warmest September on record for mean minimum, average, and maximum temperatures. It was also the city’s first September on record with average daytime temperatures above 95°F, reaching 96.5°F, with records maintained there since 1890. The prior record was in 1939 at 93.6°F. Overall, this is consistent with the recent warming trend, as fall temperatures have increased by 2.6°F since 1970.
Notable precipitation event: Moisture from the remnants of Hurricane Polo brought a very active end to the monsoon season across the Southwest, especially in New Mexico. Albuquerque recorded its wettest September on record with 4.49 inches of rainfall (records start in 1891). In fact, September 16-29 was the wettest two-week period on record there, regardless of the time of year. This extreme rainfall also pushed seasonal totals at some locations, including Santa Fe, to their wettest monsoon season on record (June 15 to September 30). While some areas reported flash flooding, the rainfall also brought much-needed moisture to locations experiencing severe to extreme drought across the northern half of the state.
Climate Change Influence
A Climate Central analysis using the Climate Shift Index (CSI) shows that human-caused climate change is the primary driver of fall (September, October, November) warming trends in 82% of U.S. cities analyzed.
September alone has warmed in nearly all (191 out of 192) cities analyzed across the U.S. since 1970.
On average, Alabama, Arkansas, Louisiana, Mississippi, and Texas saw more than 20 days with CSI level 2 or higher in September 2026, while 13 additional states saw at least 10 such days.
Cities in Texas, including Victoria and McAllen, experienced CSI level 2 or higher conditions every day in September.
San Juan, Puerto Rico, reached CSI level 5 every day in September, indicating exceptional climate change conditions, while Honolulu experienced 21 such days that month.
Unusually warm overnight temperatures continued across the contiguous U.S. into September, with the September 1-15 average reaching CSI level 5 in parts of the South from Arizona to Florida.
Parts of southeast Texas experienced very dangerous humid heat during the first half of the month, meaning wet-bulb temperatures rose above 27.5°C. In mid-September, climate change made these conditions at least 100 times more likely.
Another large heat dome affected the southern U.S. in mid-September, bringing record-warm temperatures to some locations. Human-caused climate change made these temperatures at least three times more likely (CSI level 3), affecting more than 65 million people across the region.
Sea surface temperatures over the Gulf of Mexico reached record-high levels every day in September, with climate change making this ocean warmth up to 800 times more likely off the coast of Mexico.
During late September, a strong nor’easter brought strong winds, heavy rain, and coastal flooding to parts of the East Coast. A new Climate Central analysis found that some locations would not have experienced flooding without the added sea level rise from climate change.
Learn even more about how human-caused climate change altered September’s temperatures in our latest monthly attribution overview. Climate Central also has an explainer on attribution science and how it works.
Climate Outlook
Global Temperature:
The North American Multi-Model Ensemble (NMME) forecasts an increase in the global temperature anomaly over at least the next six months, linked to El Niño.
Seasonal forecast models, initialized in early October, project global temperature anomalies to peak around March 2027.
The multi-model mean reaches about 1.8°C above the 1850-1900 pre-industrial baseline at peak, with some individual ensemble projections reaching an anomaly above 2°C.
The average NMME forecast shows the average monthly global temperature anomaly staying above 1.5°C relative to pre-industrial levels through at least June 2027.
El Niño-Southern Oscillation (ENSO):
El Niño Current Conditions:
According to the latest NOAA Climate Prediction Center bulletin, El Niño continues to strengthen, with sea surface temperatures at least 4°C above normal in parts of the eastern Pacific.
The Niño 3.4 monitoring region has reached a relative sea surface temperature anomaly of +2.1°C.
The Niño 1+2 monitoring region has reached a relative sea surface temperature anomaly of +3.9°C.
Actual sea surface temperatures across the eastern equatorial Pacific are at record high levels for the time of year.
Preliminary data shows that sea surface temperatures set a new all-time record high in early October for the Niño 3.4 region, surpassing the prior record set on November 17, 2015.
The Southern Oscillation Index reached -2.0 in September 2026, indicating atmospheric circulation patterns across the tropical Pacific that are also consistent with a strengthening El Niño.
El Niño Forecast:
The latest ENSO bulletin gives an 83% probability of El Niño reaching a historic intensity for the October-December period, with records kept since 1950.
There is at least a 32% chance of a strong El Niño event continuing into February-April 2027.
The average NOAA forecast for the Relative Oceanic Niño Index (RONI) peaks above 2.7°C during the last three months of 2026.
The latest RONI projections from the NMME forecast models show an overall multi-model mean peaking at over 2.8°C for October-December 2026.
Some uncertainty remains in the exact strength across different seasonal forecast models.
El Niño Impacts:
El Niño can increase the odds of certain regional climate impacts, but it does not guarantee a specific outcome or explain every extreme event.
Some climate signals become more predictable during El Niño, giving communities more lead time to prepare for high-impact events.
Although El Niño often provides a temporary boost to global temperatures, these new climate records are ultimately being driven by human-caused climate change. Learn more in our recent Climate Matters.
El Niño Key Takeaways:
El Niño is a natural pattern of climate variability. It redistributes heat across the climate system, including changes in the exchange of heat between the tropical Pacific Ocean and atmosphere.
No two El Niño events unfold in exactly the same way. A stronger event does not necessarily mean stronger impacts.
Long-term warming has raised the baseline on which natural climate variability occurs, making new records increasingly likely.
U.S. Drought:
Shorter-term drought indicators, including soil moisture anomalies and the Crop Moisture Index, show extremely dry conditions across the Upper Midwest, including northern Michigan and Wisconsin, as well as across Arkansas, Louisiana, Mississippi, Missouri, and Oklahoma.
The Short-Term Composite Drought Indicator shows the most unusually dry conditions across the lower 48 in southern Arkansas. Meanwhile, the Long-Term Composite Drought Indicator shows the driest conditions across western Colorado.
Recent heavy rain has brought some drought relief to parts of Texas up through southeast Colorado.
Over the next month, some drought improvement is forecast across the southern half of the U.S., as well as in Maine.
Drought is expected to persist across most of the western U.S., especially around the Rocky Mountains.
For the October-December outlook, drought is forecast to worsen across Idaho, Montana, and Washington.
U.S. Temperature:
According to the NOAA Climate Prediction Center, above-normal temperatures are more likely than not across Hawaii and most of the contiguous U.S. through the end of October.
Meanwhile, a slight chance for below-normal temperatures is forecast for western Alaska.
Over the next three to four weeks, the strongest forecast signals for above-normal temperatures are found over the western U.S., Alaska, and Hawaii.
U.S. Precipitation:
According to the NOAA Climate Prediction Center, Hawaii and the southern U.S. have the highest probability of wetter-than-normal conditions through the end of October.
The northern U.S. has a higher likelihood of drier-than-normal conditions during this period.
Over the next three to four weeks, areas across the Northeast have the strongest forecast signal for below-normal precipitation.
Contact Experts
Our team of scientists can help interpret these findings and answer your questions. Contact our team.
To request a media interview with a Climate Central scientist about this briefing, please contact Abbie Veitch at aveitch@climatecentral.org.
Methodology
Global and U.S. climate statistics are provided by NOAA’s National Centers for Environmental Information (NCEI), including through the Climate at a Glance tool and NOAAGlobalTemp v6.1.0 dataset. Sea surface temperature data are from NOAA’s ERSSTv6 and OISSTv2.1. All climate regions and divisions follow the standard definitions established by NOAA NCEI. Station-level data are provided by the Applied Climate Information System (ACIS) using ThreadEx, which is developed, maintained, and operated by NOAA’s Regional Climate Centers. We recognize that climate ranking statistics can vary slightly between datasets, and there are higher uncertainties in temperature data prior to 1900.
Drought information is available through the U.S. Drought Monitor, western U.S. snowpack data are provided by the USDA National Water and Climate Center, and streamflow information is from the National Water Dashboard by the U.S. Geological Survey. Weekly and seasonal temperature and precipitation outlooks are from the NOAA Climate Prediction Center. Sea ice statistics are from the National Snow and Ice Data Center’s Sea Ice Index v4. Carbon dioxide concentration data are from the NOAA Global Monitoring Laboratory.
Additional Graphics
Additional Resources
Global Climate Report | September 2026 | National Centers for Environmental Information (NCEI)
National Climate Report | September 2026 | National Centers for Environmental Information (NCEI)
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