Day: July 29, 2026

How do scientists know if climate change made a heat wave, extreme storm or wildfire worse?

Two people walk in opposite directions on a city street, one carrying an umbrella for shade and the other wiping his face with a handkerchief.
People try to stay cool on the sweltering streets of Manhattan during a heat wave in July 2025. Spencer Platt/Getty Images

Kevin T. Smiley, Louisiana State University ; Deepti Singh, Washington State University; Jennifer Marlon, Yale University, and Jim Hurrell, Colorado State University

When a devastating heat wave, hurricane, flood or wildfire strikes, people often want to know: How much did human-caused climate change influence this event, if at all?

For many years, scientists could answer that question only in general terms. They knew that human-caused climate change was increasing the frequency or intensity of some kinds of extreme weather, but determining its influence on a particular event was much more difficult.

Today, researchers can provide a scientifically grounded answer that assesses whether climate change made a heat wave hotter, increased rainfall from a hurricane, or made a drought more severe. But the answers are not always straightforward, and they are not equally certain for every type of extreme event.

A person walks through a flooded street with damaged buildings, including a church, behind them.
Rapid attribution studies suggested that climate change made the heavy rainfall from Hurricane Helene’s remnants, which devastated mountain communities like Marshall, N.C., more likely than if the storm had occurred in a cooler climate. Jabin Botsford/The Washington Post via Getty Images

We served on the 14-member National Academies of Sciences, Engineering, and Medicine committee that produced a new report that reviewed where this rapidly developing field, known as extreme event attribution, stands today.

The report concludes that advances in weather and climate data, climate models and attribution methods have substantially strengthened scientists’ ability to assess the influence of climate change on many types of extreme events, while also identifying important scientific limitations.

How can scientists study a single event?

Weather events result from many interacting factors, including the day-to-day state of the atmosphere.

Scientists conducting extreme event attribution studies aren’t asking whether climate change caused a hurricane, heat wave or flood. Instead, they’re asking how climate change influenced that event: How would this event have differed in a world without increased greenhouse gases, and how have the chances of it occurring been altered by warming?

Researchers answer that question using two complementary approaches:

Both approaches combine weather and climate data, physical understanding of how the climate system works, and computer models. Scientists can compare results from multiple approaches to determine whether they point to the same conclusion and how much confidence to place in the results.

For example, studies of Hurricane Harvey, which dumped 50 inches of rain in parts of the Houston area in 2017 and caused widespread flooding, concluded that climate change increased the amount of rainfall the storm produced. Multiple studies using different approaches reached broadly consistent conclusions, illustrating how scientists conducting attribution studies can build confidence in the conclusions by drawing on several independent lines of evidence.

Scientists are more confident about some events than others

One of the report’s most important conclusions is that attribution science is not equally able to assess every kind of extreme weather.

Confidence is highest for events whose relationship to a warming climate is well understood and well represented in climate models.

Bubbles show which extreme events can be most easily assessed for the role of climate change
Scientists have the highest confidence in extreme attribution for events with temperature extremes and the lowest confidence for convective storms, such as tornadoes. The bubble size is proportional to the number of extreme event attribution studies conducted. National Academy of Sciences

Extreme heat is the clearest example. As greenhouse gas concentrations have increased with the burning of fossil fuels, just about every region of the world has experienced more frequent and more intense heat extremes. Scientists understand how increasing greenhouse gases warm the atmosphere and make heat extremes more likely and more intense, making it possible to attribute many individual heat waves with high confidence.

Confidence is also relatively high for attributing the contributions of climate change for large-scale heavy rainfall events, tropical cyclones, and droughts in some regions. A warmer atmosphere is thirstier – it draws more moisture, worsening droughts and fueling heavier downpours when storms form.

The contribution of climate change remains more difficult to assess for other weather hazards. Severe thunderstorms, tornadoes, large hail and damaging straight-line winds depend on small-scale atmospheric processes that global climate models still struggle to represent accurately. Data for these events is also less complete, making it harder to detect long-term changes and evaluate climate models.

Some events are becoming increasingly complex. For instance, multiple events can occur simultaneously or in succession, leading to compounding or cascading effects, such as drought that worsens wildfire risk. Attribution science is just beginning to account for these interactions.

These challenges do not mean climate change has no influence on these events. Rather, they reflect a fundamental principle of science: confidence depends on the strength of the available evidence.

Understanding how climate change influences extreme events helps communities, businesses and governments better assess changing risks, improve planning and infrastructure, and make more informed decisions about preparedness and adaptation. Just as importantly, knowing where confidence is lower helps ensure that decisions remain grounded in the strength of the available scientific evidence.

Why attribution science is getting better

The science of extreme event attribution has progressed significantly since the National Academies’ last assessment of it in 2016.

Researchers now have access to longer and better observational records from weather stations, satellites and other monitoring systems. Climate models have become more sophisticated, better representing the physical processes that produce extreme weather, thus representing many types of extreme weather more realistically.

At the same time, the number of attribution studies analyzing climate change’s role in extreme events has grown considerably, covering more types of extreme weather and climate events than a decade ago. International research groups now apply established methods to conduct rapid attribution studies, often within days of disasters, providing scientifically grounded information while events are fresh in the public mind.

Cattle gather around a pond with low water levels amid a very dry landscape.
Studies suggested that higher temperatures caused by anthropogenic climate change turned the 2020-2022 Western drought into an exceptional drought. In some areas, ranchers facing drying rangeland sold off cattle. Justin Sullivan/Getty Images

Scientists are also asking a new question: How did climate change affect the results of the event? For example, how did it affect the number of heat-related deaths, the extent of flooding, economic losses or ecological damage?

Hurricane Harvey again provides a useful example. The National Academies report highlights research showing that climate change increased Harvey’s rainfall by roughly 20% to 38%. Studies also found that the increase in rainfall attributed to climate change led to a proportionally even larger increase in the number of flooded properties, showing that relatively modest increases in the intensity of an extreme event can sometimes translate into much larger increases in societal impacts.

Extreme weather and climate change moving forward

As extreme weather affects communities around the world, attribution science is becoming an increasingly robust way to understand how climate change influences individual extreme weather events, providing information that can help communities prepare for and adapt to future risks.

Equally important, attribution studies identify where scientific confidence is high, where important uncertainties remain, and where additional research is needed. Together, these advances help ensure that scientific conclusions remain grounded in observations, physical understanding and careful analysis.

Kevin T. Smiley, Associate Professor of Sociology, Louisiana State University ; Deepti Singh, Associate Professor School of the Environment, Washington State University; Jennifer Marlon, Research Scientist, Yale University, and Jim Hurrell, Professor and Scott Presidential Chair in Environmental Science and Engineering, Colorado State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The surprising way that Chromium-6 got into in the Coachella Valley’s groundwater

Aerial view of rectangular water ponds surrounded by sandy terrain and hills in the background, under a clear blue sky.
The Thomas E. Levy Groundwater Replenishment Facility, operated by the Coachella Valley Water District. Photo by Robert Marcos.

by Robert Marcos, photojournalist

The half-million residents of Southern California’s Coachella Valley have a lot to worry about. Toxic dust storms sweeping up from the Salton Sea. New concerns about the Coachella Canal, after the USGS reported that the canal crosses directly over the San Andreas Fault which is 150 years overdue for an major earthquake.1

Residents also worry about the future viability of the Colorado River – which provides 430,000 acre feet of water that’s used to irrigate 76,000 acres of farmland, 120 golf courses, and to replenish the aquifer in order to prevent subsidence issues like those that first appeared in 1948. And it was this replenishing that caused toxicity problems to develop in the aquifer. 2

Colorado River water that’s diverted to the Coachella Valley is both highly-oxygenated and has a high pH, (meaning it’s alkaline). When raw Colorado River water was injected into the Coachella Valley’s aquifer, its oxygen chemically reacted with (naturally occurring) chromium-3 that’s bound up in the sediment. The new oxygen oxidized the chromium-3 and converted it into soluble, toxic, chromium-6. The chromium-6 was subsequently measured at levels that exceed California’s new limit of 10 parts per billion – at 33 of the Coachella Valley Water District’s 92 wells. 3

The worst contamination is found on the aquifer’s southeastern side, which not only contains high levels of chromium-6 but also arsenic, agricultural nitrates, and perchlorate runoff. When the aquifer on the Valley’s eastern side is replenished the river water’s alkalinity triggers a chemical reaction that releases (naturally occurring) arsenic from the sediment which then disperses into the groundwater. 4

Tough new standards: The State of California’s tough new standards for chromium-6 is causing major headaches for the Valley’s five water districts. California had formerly limited chromium-6 contamination to 50 parts per billion in drinking water. Conforming to the state’s new limit of 10 parts per billion is going to be costly. The Coachella Valley Water District estimates that it will cost them $350 million dollars, with no funding provided by the State. Meanwhile the standards that were established by the EPA for arsenic, which are also 10 ppb, have remained unchanged since 2001. 5

Mitigation Efforts

The four water agencies that manage the Coachella Valley’s groundwater – the Coachella Valley Water District, Desert Water Agency, Indio Water Authority, and Coachella Water Authority, have submitted structural compliance plans to the State Water Resources Control Board. They plan to mitigate naturally occurring chromium-6 to meet California’s strict limit through a mix of infrastructure overhauls, advanced filtration, and operational adjustments. Because California’s mandate depends on the size of the water system, these agencies are working against a rolling compliance deadline spanning from October 2026 through October 2027. 6

Advanced Water Treatment Infrastructure

  • Ion-Exchange Technology: Agencies like the Indio Water Authority (IWA) are expanding the use of strong base anion exchange systems. These systems route groundwater through tanks filled with resin beads that magnetically trap and remove chromium-6 molecules.7
  • Reduction Coagulation Filtration (RCF): The IWA has also piloted RCF treatment, which chemically alters the chromium-6 into a harmless, filterable solid form before water enters the distribution grid. 8

Well Offline Management and Decommissioning

• Taking Impacted Wells Offline: The largest provider, Coachella Valley Water District (CVWD), has identified 34 out of its 96 drinking water wells that exceed the 10 ppb threshold. Impacted wells across the valley are being selectively pulled from service or restricted during off-peak seasons to prevent contaminated groundwater from mixing into municipal supplies. 9

Drilling New Deep-Water Wells

  • Bypassing the Contaminated Strata: Chromium-6 in the Indio Subbasin is entirely naturally occurring, leaching out of specific geological rock layers and sediment over centuries. Water districts plan to construct new, deeper wells that tap into lower layers of the aquifer where chromium-6 levels are safely below the state limit. 10

Well Clustering and Pipeline Blending

  • Dilution and Interconnection: Agencies are designing “well-clustering” projects. This technique involves building miles of new connecting pipelines to merge water from higher-chromium wells with water from pristine wells. Blending the water dilutes the total chromium-6 footprint down to legally compliant levels before it reaches household taps. 11