The End of "Normal": Comparing 20th Century History to 21st Century Climate Reality

Abstract

The meteorological history of the United States is defined by its extremes. From the scorching droughts of the Dust Bowl to the inundating floods of the Mississippi River Valley, the continent’s diverse geography has always generated volatile weather patterns. However, the early 21st century has witnessed a statistically significant deviation from the historical baseline, characterized by an escalation in the frequency, intensity, and duration of extreme events. This comprehensive research report investigates the causal links between anthropogenic climate change and these observed anomalies. By synthesizing data from the Fifth National Climate Assessment (NCA5), the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6), and the burgeoning field of extreme event attribution, we examine whether the signal of human-induced warming has emerged from the noise of natural variability. The analysis reveals a complex landscape: while thermodynamic extremes such as heatwaves and heavy precipitation exhibit a robust and quantifiable dependence on greenhouse gas forcing, dynamic extremes like tornadoes and winter storms remain subject to significant uncertainties. Ultimately, this report argues that for the most damaging classes of weather events, the question is no longer if climate change is playing a role, but how much, quantifying the "climate tax" now levied on the American economy and infrastructure.

1. Introduction: The Shifting Baseline of the American Climate

1.1 The Historical Context of Extreme Weather

For much of the 20th century, extreme weather in the United States was viewed through the lens of stochasticity—random "acts of God" that occurred within a stable, albeit variable, climatic envelope. The Dust Bowl of the 1930s, the Great New England Hurricane of 1938, and the Super Outbreak of tornadoes in 1974 were seen as outliers, rare excursions from the mean that defined the boundaries of what was possible. Infrastructure, insurance models, and agricultural planning were all built upon the assumption of "stationarity"—the idea that the past is a reliable predictor of the future.

However, the observational record of the last four decades has systematically dismantled the assumption of stationarity. The baseline against which we measure "extreme" is shifting. What was once a 1-in-100-year flood event is now occurring with a frequency that defies 20th-century statistics. The "normal" climate of the United States has warmed, and with that warming has come a fundamental alteration in the energy dynamics of the atmosphere.

The data presented by the National Oceanic and Atmospheric Administration (NOAA) paints a stark picture of this acceleration. In the 1980s, the United States experienced, on average, a billion-dollar weather disaster every four months (adjusted for CPI). By the 2010s, this frequency had increased to one every three weeks.¹ In 2023 alone, the nation sustained 28 separate weather and climate disasters that each resulted in at least $1 billion in damages, setting a new historical record.² While economic factors such as inflation and increased development in hazard-prone areas contribute to these rising costs, they cannot fully explain the surge. The frequency of the events themselves—the physical storms, droughts, and heatwaves—has increased.

1.2 The Core Inquiry

The user's query strikes at the heart of the current scientific and public discourse: "Have there been enough changes (increase) in extreme events to correlate with a relation to climate change, or is it too soon?"

This report posits that for many types of extreme weather, the era of "too soon" has passed. We have entered the era of attribution. The scientific community has moved beyond general correlations to identify specific physical mechanisms—thermodynamic fingerprints—that link rising global temperatures to specific weather outcomes. We can now quantify how much more rain fell during Hurricane Harvey due to a warmer Gulf of Mexico, and how much more likely the Pacific Northwest Heat Dome was due to elevated baseline temperatures.

However, the answer is not uniform across all phenomena. The climate system is chaotic, and distinguishing the signal of climate change from the noise of natural variability (such as the El Niño-Southern Oscillation) requires rigorous statistical analysis. For some events, like heatwaves, the signal is deafening. For others, like tornadoes, the signal remains faint, buried in the noise of localized atmospheric dynamics.

1.3 Scope and Structure

This report serves as a deep dive into the history and physics of these changes.

• Section 2 explores the methodology of attribution science, explaining how researchers "interrogate" the climate system.

• Section 3 outlines the thermodynamic foundations—the physics of heat and moisture—that drive these changes.

• Sections 4 through 8 provide detailed analyses of specific hazards: heatwaves, precipitation, drought, tropical cyclones, and convective storms. Each section integrates historical case studies, such as the 2021 Pacific Northwest heatwave and the 2023 Vermont floods, to illustrate the human and economic toll.

• Section 9 examines the socio-economic implications, including the "climate gap" and the retreat of insurance markets.

By weaving together historical data, physical science, and narrative case studies, this report aims to provide a nuanced, expert-level perspective on the state of the American climate.

2. The Methodology of Attribution: Deciphering the Climate Signal

2.1 From Correlation to Causality

Historically, when a meteorologist was asked if a specific storm was caused by climate change, the standard answer was, "We cannot attribute any single event to climate change." This mantra was based on the understanding that all weather occurs within the context of the current climate, but separating the specific contribution of human forcing from natural chaos was computationally impossible.

This changed in the early 2000s with the advent of Extreme Event Attribution (EEA). This field of science treats the Earth's climate as a grand experiment. Since we cannot physically replicate the Earth to test different variables, scientists use high-resolution climate models to create simulations.

The methodology typically involves two sets of simulations:

1. The Factual World: A simulation of the current climate, including observed concentrations of greenhouse gases (carbon dioxide, methane) and current sea surface temperatures.

2. The Counterfactual World: A simulation of the world as it would have been without human interference. In this "World That Was," greenhouse gas concentrations are returned to pre-industrial levels, and the warming influence of fossil fuels is removed.³

By running these simulations thousands of times, scientists can compare the probability of a specific extreme event occurring in both worlds. If a heatwave of a certain magnitude occurs once every 10 years in the Factual World but only once every 100 years in the Counterfactual World, scientists can conclude with high confidence that climate change made the event ten times more likely.³

2.2 Probabilistic vs. Storyline Approaches

Two distinct schools of thought govern attribution science, both of which are critical for understanding the U.S. weather history.

The Probabilistic Approach: This method asks, "Has the likelihood of this type of event changed?" It focuses on the frequency and return periods. For example, NOAA scientists might analyze the statistics of heavy rainfall events in the Gulf Coast region to determine if the 1-in-50-year storm has become a 1-in-30-year storm.³ This approach is excellent for assessing risk and insurance premiums but can sometimes lose the nuance of specific meteorological setups.

The Storyline Approach: This method is more forensic. It asks, "Given that this specific storm formed, how did climate change alter its characteristics?" This approach takes the atmospheric circulation pattern (the "storyline") as a given and asks how thermodynamic factors modified the outcome.⁴ For instance, given the track of Hurricane Harvey, how much more rain fell because the Gulf of Mexico was 1.5°F warmer than normal? This approach allows for attribution even in events where the circulation pattern itself (e.g., a blocking high) might be rare or natural, isolating the human-amplified severity.⁴

2.3 The Concept of Signal-to-Noise Ratio

A critical analytical tool used throughout this report is the Signal-to-Noise Ratio (SNR).

• The Signal: The trend imposed by anthropogenic forcing (e.g., the steady rise in global mean temperature).

• The Noise: The natural, internal variability of the climate system (e.g., El Niño, the North Atlantic Oscillation, random weather chaos).

When the signal is strong and the noise is low (as with temperature extremes), attribution is straightforward and high-confidence. When the noise is high and the signal is weak or complex (as with tornadoes or hailstorms), the "fingerprint" of climate change is smudged and difficult to identify.⁵

Hazard Type	Signal Strength	Noise Level	Attribution Confidence
Extreme Heat	Very High	Low	Virtually Certain
Heavy Rainfall	High	Moderate	High
Drought (Aridity)	High	Moderate	High
Hurricanes (Intensity)	Moderate	High	Medium-High
Tornadoes	Low	Very High	Low
Winter Storms	Low	High	Low

Table 1: The spectrum of attribution confidence. Note that "Low" confidence does not mean there is no link, but rather that natural variability currently dominates the record, making the human signal difficult to isolate.⁵

2.4 The Evolution of Confidence

The IPCC AR6 and NCA5 reports reflect a maturation of this science. In previous assessments, language was often couched in uncertainty. Today, for hazards like heat and precipitation, the language has shifted to "established fact".⁷ The ability to run rapid attribution studies—sometimes within days of an event—has transformed the public conversation, moving it from theoretical future risks to immediate, quantified causalities.

3. Thermodynamic Drivers: The Physics of Excess Energy

To understand the history of extremes, one must understand the engines driving them. The changing weather is not magic; it is physics. The addition of greenhouse gases to the atmosphere has trapped excess energy, and that energy must go somewhere. It manifests primarily through two thermodynamic mechanisms: heat and moisture.

3.1 The Clausius-Clapeyron Relation

The most fundamental law governing the intensification of weather is the Clausius-Clapeyron relation. Discovered in the 19th century, this principle states that the water-holding capacity of the atmosphere increases exponentially with temperature. Specifically, for every 1°C (1.8°F) rise in air temperature, the atmosphere can hold approximately 7% more water vapor.⁸

This is not a linear relationship; it is exponential. As the atmosphere warms, its "thirst" increases dramatically. This has two opposing effects:

1. Where it is wet: When a storm system forms, it has access to a much larger reservoir of vapor. It can process this moisture into rainfall rates that were physically impossible in a cooler climate. This is the mechanism behind the increasing intensity of flash floods.¹⁰

2. Where it is dry: The same principle increases the evaporative demand of the atmosphere. Thirsty air sucks moisture out of soils, vegetation, and reservoirs at a faster rate, accelerating the onset of drought and curing forests into explosive fuels.¹¹

3.2 Ocean Heat Content: The Climate's Battery

While atmospheric temperatures fluctuate day to day, the oceans provide a memory of the warming climate. The oceans have absorbed over 90% of the excess heat generated by human emissions. This Ocean Heat Content (OHC) is the fuel for tropical cyclones.¹²

In the 20th century, hurricanes often churned up cold water from the deep ocean, which acted as a natural brake on their intensity. Today, the layer of warm water on the surface is thicker and extends deeper. When a storm churns the ocean, it often encounters more warm water rather than a cooling brake, allowing for rapid intensification cycles that were previously rare.¹³

3.3 Vapor Pressure Deficit (VPD)

A metric that appears frequently in modern fire and drought research is Vapor Pressure Deficit (VPD). Simply put, VPD is the difference between how much moisture the air holds and how much it could hold at saturation. High VPD represents "thirsty" air.

In the western United States, the rise in VPD is the leading driver of wildfire expansion. Attribution studies have shown that the increase in VPD is largely driven by temperature (anthropogenic warming) rather than just a lack of rainfall.¹⁴ This distinction is crucial: even in years with "normal" rainfall, the higher temperatures ensure that the landscape dries out faster and burns more intensely.

4. Extreme Heat: The Clearest Signal

If climate change has a signature, it is heat. The signal of anthropogenic warming is clearest in temperature records, where the shift in the mean global temperature has disproportionately increased the frequency of extremes at the "tail end" of the distribution. The 2010s were the hottest decade on record for the U.S., and the 2020s are on track to exceed that benchmark.¹⁵

4.1 The Statistical Shift

The Fifth National Climate Assessment documents a profound shift in the character of U.S. heat. It is not just that days are hotter; the season of heat is expanding. The frost-free season has lengthened, and heatwaves are occurring earlier in the spring and lasting later into the autumn. Furthermore, nighttime temperatures are rising faster than daytime temperatures, a critical trend that prevents the human body and the built environment from recovering after a hot day.¹

By mid-century, climate models project that the "1-in-20-year" extreme maximum daily temperature will occur every single year in many parts of the country.¹⁶ What was once a generational anomaly is becoming the summer baseline.

4.2 Case Study: The 2021 Pacific Northwest Heat Dome

In late June 2021, a meteorological event occurred that defied the statistical bounds of the historical climate. A massive ridge of high pressure, known as a "heat dome," parked over the Pacific Northwest—a region acclimatized to cool, maritime air. The descending air within the dome compressed and heated, creating a feedback loop of rising temperatures.

The Event: Records were not just broken; they were obliterated. Portland, Oregon, reached 116°F (46.7°C). Seattle hit 108°F (42.2°C). Most shockingly, the village of Lytton in British Columbia, Canada, reached 49.6°C (121.3°F), breaking the all-time national heat record by a staggering 4.6°C.¹⁷ The following day, a wildfire sparked by the extreme conditions consumed 90% of the town, reducing it to ash.¹⁸

Attribution Analysis: The World Weather Attribution (WWA) initiative conducted a rapid analysis of the event. Their findings were stark: the heat dome was "virtually impossible" without human-caused climate change. In a pre-industrial climate, such an event was estimated to be a 1-in-150,000-year occurrence. With the 1.2°C of warming the planet has already experienced, it became a 1-in-1,000-year event.¹⁹

The study further warned that if global warming reaches 2°C, an event of this magnitude could occur every 5 to 10 years.²⁰ The "impossible" is rapidly becoming the "inevitable."

Human and Economic Toll: The heat dome was a mass casualty event. It caused over 600 excess deaths in British Columbia and hundreds more in Washington and Oregon.²¹ Many of the victims were elderly or lived in housing without air conditioning, highlighting the infrastructure gap in a region historically unprepared for such extremes. Economically, the event was a disaster for agriculture. The intense heat "cooked" cherries and berries on the vine. British Columbia's agricultural sector lost at least $25 million in revenue, and an estimated 650,000 farm animals perished due to heat stress.²²

4.3 Case Study: Phoenix and the Persistent Heat of 2023

Two years later, in July 2023, the Southwest U.S. experienced a different kind of extreme. While the Pacific Northwest event was a sharp spike, the 2023 Southwest heatwave was a marathon of persistence.

The Event: Phoenix, Arizona, endured 31 consecutive days with maximum temperatures at or above 110°F (43.3°C), shattering the previous record of 18 days.²⁴ Perhaps more dangerously, the overnight low temperature did not drop below 90°F (32.2°C) for many days, denying residents any physiological relief.

Attribution Analysis: WWA analysis found that the heatwaves affecting the U.S. Southwest (and concurrently Southern Europe and China) in July 2023 would have been virtually impossible without climate change.²⁵ The study highlighted that unlike the statistical anomaly of the PNW heat dome, the 2023 heat in the Southwest is no longer rare. In our current climate, such events are to be expected approximately once every 15 years.²⁵

Implications: This event tested the limits of adaptation. Phoenix is a city built for heat, with nearly universal air conditioning. Yet, the sustained thermal load strained the power grid and resulted in over 500 heat-associated deaths in Maricopa County alone.²⁴ It demonstrated that even "climate-adapted" cities have breaking points when the baseline shifts too far.

5. The Hydrological Cycle Intensified: Floods and Atmospheric Rivers

The flip side of the heat coin is precipitation. As the Clausius-Clapeyron relation predicts, the U.S. is seeing a "wet gets wetter" pattern. The heaviest rainfall events are becoming heavier. The Northeast and Midwest, in particular, have seen massive increases (upwards of 50-70%) in the amount of precipitation falling in the top 1% of events since the mid-20th century.²⁷

5.1 Case Study: Hurricane Harvey (2017)

Hurricane Harvey serves as the archetypal example of a climate-amplified rainfall disaster. In August 2017, the storm made landfall in Texas and then did something unusual: it stopped. Stalled by a weak steering current, Harvey sat over the Houston metro area for days, acting as a firehose connected to the limitless moisture of the Gulf of Mexico.

The Event: Parts of Texas received over 60 inches (152 cm) of rain—the largest rainfall event from a tropical cyclone in U.S. history.²⁸ The weight of the water was so immense it actually depressed the Earth's crust locally by 2 centimeters.

Attribution Analysis: Attribution science was able to quantify the human contribution to this disaster with remarkable precision. Multiple studies concluded that anthropogenic warming increased the rainfall intensity of Harvey by approximately 15% (with an uncertainty range of 8-19%) and made the probability of such a rainfall event three times more likely.²⁸

• Mechanisms: Two climate factors converged. First, record-high Ocean Heat Content in the Gulf provided the moisture (thermodynamics). Second, the stalling pattern may be linked to a slowing of the jet stream, a dynamic trend associated with Arctic warming, though this dynamic link is less certain than the thermodynamic one.³⁰

Impact: Harvey caused $125 billion in damages, second only to Katrina. The attribution of 15% of the rainfall to climate change implies that billions of dollars of that damage were directly chargeable to human emissions. Survivors described the event as a war zone; Willie Williams, a resident of southeast Houston, recounted stocking up on supplies to ride out a storm, only to find himself trapped by rising waters that turned neighborhoods into lakes.³¹

5.2 Case Study: The 2023 Vermont Flooding

In July 2023, a different kind of flood struck New England. A slow-moving low-pressure system tapped into a plume of tropical moisture and funneled it directly into the Green Mountains of Vermont.

The Event: Rainfall totals of 4 to 9 inches fell on saturated soils. The Winooski River in Montpelier crested at levels exceeding those of the devastating 1927 flood and Tropical Storm Irene in 2011.³² The state capital was inundated, isolating communities and destroying infrastructure.

Attribution Context: While a formal attribution study for this specific event is complex, the setup fits the established pattern of climate change in the Northeast. The Atlantic Ocean was anomalously warm, providing excess moisture. The event highlighted a critical vulnerability: inland flooding. Climate change is not just a coastal threat; "atmospheric traffic jams" can dump extreme rain anywhere.³³ Comparisons of high-water marks showed that in many locations, the 2023 flood exceeded 2011 levels, suggesting a steepening trend in flood severity.³⁴

5.3 Atmospheric Rivers

On the West Coast, the primary delivery mechanism for extreme precipitation is the Atmospheric River (AR)—long, narrow filaments of moisture that transport water vapor from the tropics to the poles.

• Projections: NCA5 projects that ARs will become more intense. While the total number of ARs may not change significantly, the strongest ones—those capable of causing catastrophic flooding like the "Great Flood of 1862"—are expected to become more frequent and carry more moisture.³⁵ This poses a severe risk to California's levee system and water management infrastructure, which must balance flood control with water storage.

6. The Aridification of the West: Megadrought and Wildfire

While the East drowns, the West parches. Climate change is transforming the nature of drought from a temporary lack of rain to a chronic condition driven by heat.

6.1 The 1,200-Year Megadrought

Since the turn of the millennium, the southwestern United States has been gripped by a drought of historic proportions.

Analysis: A landmark study utilizing tree-ring data (dendrochronology) reconstructed soil moisture levels back to 800 CE. The findings were alarming: the period from 2000 to 2018 was the second-driest 19-year period in 1,200 years, exceeded only by a megadrought in the late 1500s.³⁶

Attribution: The study estimated that anthropogenic warming was responsible for 42% of the drought's severity.³⁶

• The Mechanism of Hot Drought: The driver is not just a lack of precipitation (which is driven partly by natural La Niña patterns) but the "thirst" of the atmosphere (VPD). Warmer temperatures increase evaporation and transpiration from plants. This means that even if rainfall remains average, the effective water available for rivers and reservoirs declines. This phenomenon has led scientists to term the current state not as "drought" (which implies a return to normal) but as aridification—a permanent transition to a drier baseline.⁶

6.2 Wildfires: A Compound Hazard

The aridification of the West has set the stage for the explosion of wildfire activity. Fire is a "compound hazard," situated at the intersection of climate, vegetation, and human ignition.

Trends: Since the mid-1980s, the area burned by wildfires in the western U.S. has doubled compared to what would have been expected without climate change.³⁷ The fire season has lengthened by months, and fires are burning at higher elevations that were previously too wet to sustain large conflagrations.³⁸

Attribution: Research confirms that the increase in VPD is the leading driver. A 2021 study supported by NOAA concluded that climate change has been the main driver of the increase in "fire weather"—conditions of heat, dryness, and wind—in the western U.S..³⁸ In California specifically, human-caused warming contributed to a 172% increase in burned area from 1971 to 2021.¹⁴

The Human Cost: The 2020 and 2021 fire seasons were cataclysmic. In 2020, the Creek Fire in California trapped hundreds of campers, necessitating a harrowing helicopter rescue by the National Guard. Survivors like Liz Lawrence described a chaotic escape through walls of flame, a scene reminiscent of an apocalypse.³⁹ The smoke from these fires blanketed the continent, reversing decades of air quality improvements and exposing millions to hazardous particulate matter.⁴⁰ Economically, the impact is destabilizing. The 2020 season in California caused insured losses of up to $9 billion, prompting major insurers to cease writing new policies in the state, creating a crisis of uninsurability.⁴¹

7. Tropical Cyclones: Intensity and Rapid Intensification

The relationship between hurricanes and climate change is one of "quality over quantity." Scientific consensus does not necessarily predict more hurricanes, but it predicts (and observes) stronger ones.⁶

7.1 The Thermodynamic Speed Limit

Hurricanes are heat engines. They extract energy from warm ocean water and convert it into wind. As Ocean Heat Content (OHC) rises, the "speed limit" for these storms increases.

• Attribution Confidence: There is high confidence that the proportion of tropical cyclones reaching Category 4 and 5 intensity is increasing. There is also medium confidence that the global frequency of tropical cyclones may remain stable or even decrease, but the destructive potential of the ones that do form is significantly higher.⁴²

7.2 Rapid Intensification (RI)

A particularly dangerous trend is the increase in Rapid Intensification (RI)—defined as an increase in wind speed of at least 35 mph in 24 hours.

• Mechanism: RI typically occurs when a storm passes over deep, warm water with low wind shear. Climate change is expanding the areas of the ocean that satisfy these conditions.

• Recent Examples: Hurricane Ian (2022) and Hurricane Milton (2024) both underwent explosive intensification cycles. Milton intensified by 120 mph in less than 36 hours.⁴³ An attribution analysis of the 2024 season found that the sea surface temperatures fueling Milton were made up to 400-800 times more likely by climate change.⁴³ This leaves coastal communities with less time to evacuate, turning manageable storms into catastrophic threats overnight.

8. The Frontiers of Attribution: Tornadoes and Winter Storms

While the signals for heat, rain, and drought are clear, other weather phenomena remain shrouded in the noise of natural variability.

8.1 Tornadoes: A noisy Signal

Attributing tornado activity to climate change is one of the most difficult challenges in meteorology.

• The Conflict: Tornadoes require two ingredients: instability (warm, moist air rising) and wind shear (winds changing speed/direction with height). Climate change increases instability (more energy) but may decrease wind shear (as the temperature difference between the Arctic and the equator lessens). It is a tug-of-war between these two factors.⁴⁴

• Observed Trends: There is no significant trend in the total number of U.S. tornadoes. However, there is a spatial shift. "Tornado Alley" appears to be migrating eastward from the Great Plains into the Southeast ("Dixie Alley"), where population density and tree cover make tornadoes more deadly.⁴⁵ There is also an observed increase in "outbreaks"—days with clusters of many tornadoes—though attributing this to climate change remains low-confidence.³

8.2 Winter Storms and the Jet Stream

Paradoxically, a warming Arctic may be contributing to severe winter weather.

• The "Wavy Jet Stream" Hypothesis: As the Arctic warms faster than the equator (Arctic Amplification), the temperature gradient that drives the jet stream weakens. A weaker jet stream meanders more, becoming "wavy." These waves can lock in place (blocking), allowing cold polar air to plunge south (the "Polar Vortex") and warm air to surge north.⁴⁶

• Uncertainty: This hypothesis is actively debated. While physically plausible, the signal is difficult to separate from natural variability. Studies of events like the 2021 Texas Freeze suggest a link to stratospheric disruptions, but confidence is lower than for heat or rain.⁴⁸

9. Socio-Economic Synthesis: The Cost of Inaction

The physical changes in weather patterns are translating into profound economic and social costs, reshaping the American landscape.

9.1 The Rising Cost of Disasters

The exponential rise in billion-dollar disasters—from $22 billion/year in the 1980s to nearly $150 billion/year in the 2020s—is a stark economic indicator.¹⁵ This represents a "climate tax" on the economy. These costs are not just federal expenditures; they represent lost businesses, destroyed homes, and disrupted supply chains.

9.2 The Climate Gap

The burden is not shared equally. The "Climate Gap" refers to the disproportionate impact of extremes on vulnerable populations. Low-income communities and communities of color often reside in flood-prone areas or urban heat islands with less green space. For example, projected increases in flood risk by 2050 are expected to disproportionately impact Black communities in the Southeast.⁴⁹

9.3 The Retreat of Insurance

Perhaps the most significant economic signal is the reaction of the insurance industry. Companies like State Farm and Allstate have paused or restricted new policies in California, citing wildfire risk.⁵⁰ In Florida and Louisiana, hurricane risk has driven premiums to unaffordable levels, forcing reliance on state-backed insurers of last resort. When the market signals that a region is "uninsurable," it is a powerful validation of the physical science: the risk has exceeded the bounds of the historical economy.

10. Conclusion: From "Too Soon" to "Too Late"

Returning to the guiding question of this report: Is it too soon to correlate these changes to climate change?

The evidence indicates that for the most damaging and pervasive types of extreme weather, the answer is no. The signal has emerged.

• Heatwaves: The link is unequivocal and virtually certain.

• Floods/Precipitation: The link is robust and physically consistent with basic thermodynamics.

• Drought: The link is strong, particularly regarding the role of temperature in driving aridity.

• Hurricanes: The link is evident in intensity and rainfall, if not frequency.

We are no longer looking for a needle in a haystack; we are looking at a haystack on fire. The "new normal" is a misnomer, as it implies a stabilization at a new baseline. Instead, we are in a state of transience, moving toward a climate of increasing volatility. The history of U.S. extreme weather is being rewritten, and the attribution is clear: we are living in the climate we have created.

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