A heatwave can feel like a natural disaster.
So can a devastating flood, an unusually intense storm or a prolonged drought.
But there is a crucial question behind many of these events: how much of what we are experiencing is natural weather variability, and how much is being influenced by human activity?
The answer is more complicated than simply blaming a single storm, heatwave or flood on climate change.
Extreme weather has always existed. Hurricanes formed before industrialisation. Droughts occurred long before modern cities existed. Heatwaves and floods are not new phenomena.
What has changed is the background climate in which these events occur.
Human activities have increased concentrations of greenhouse gases in the atmosphere, warming the planet and altering the physical conditions that influence weather. The Intergovernmental Panel on Climate Change concludes that human-induced climate change is already affecting many weather and climate extremes around the world.
The science connecting the two is becoming increasingly sophisticated.
Weather and Climate Are Not the Same Thing
One reason climate discussions can become confusing is that weather and climate operate on different timescales.
Weather describes what is happening over hours, days or weeks.
Climate describes long-term patterns and changes.
A single cold day does not disprove global warming, just as one extremely hot day does not prove it.
But when scientists examine decades of observations, patterns begin to emerge.
According to the IPCC, hot extremes and heatwaves have become more frequent and more intense across most land regions since the 1950s, while cold extremes have generally become less frequent and less severe. Human influence is assessed as the main driver of these changes.
That is the distinction that matters.
Climate change does not mean that every individual weather event is directly “caused” by human activity.
It means the probability and characteristics of many extremes are being changed by a warming climate.
The Physics Starts With Greenhouse Gases
The fundamental mechanism is well established.
Carbon dioxide, methane and other greenhouse gases absorb and re-emit infrared radiation, affecting the balance of energy between Earth and space.
Human activities, particularly the burning of fossil fuels and changes in land use, have increased atmospheric greenhouse-gas concentrations.
More retained energy means a warmer climate system.
That additional energy does not simply produce warmer average temperatures.
It changes the conditions under which weather develops.
The atmosphere can hold more water vapour when it is warmer. Oceans become warmer. Ice melts. Sea levels rise. Soil and vegetation can lose moisture more rapidly under hotter conditions.
Those changes can influence extreme events in very different ways.
Why Heatwaves Are the Clearest Example
Heatwaves provide perhaps the clearest demonstration of the connection between human activity and extreme weather.
Imagine a climate in which the average temperature is gradually shifting upward.
An unusually hot day that might once have been rare can become much more common.
An extreme heatwave can also become hotter than it would have been in a cooler climate.
This is why scientists often describe climate change as loading the dice rather than directly creating every event.
Natural variability still determines when and where a particular heatwave develops.
But human-caused warming changes the odds.
The World Meteorological Organization notes that heat extremes have become more frequent and intense across most land regions, consistent with the IPCC’s assessment of human influence.
In some cases, the human influence is enormous.
A rapid attribution study of the extraordinary 2021 heatwave in parts of the United States and Canada concluded that human-caused climate change had made the event at least 150 times more likely.
That does not mean humans manufactured the weather system from nothing.
It means the extreme temperatures became vastly more probable because the climate had changed.
A Warmer Atmosphere Can Intensify Heavy Rainfall
Rainfall provides another important connection.
Warm air can hold more water vapour.
When atmospheric conditions cause that moisture to condense and fall, there can be more water available for intense precipitation.
This does not mean every rainy day becomes heavier.
But it creates conditions that can increase the intensity of some extreme rainfall events.
The IPCC has found that human influence has contributed to the intensification of heavy precipitation over land, with particularly strong evidence in regions including North America, Europe and Asia.
That matters because extreme rainfall can quickly overwhelm drainage systems, rivers and infrastructure.
A city designed around historical rainfall patterns may therefore face a very different risk when the underlying climate changes.
Flooding Is More Complicated
It is tempting to say that climate change causes floods.
The reality is more complicated.
Flooding depends on rainfall, river conditions, soil moisture, topography, drainage, land use and infrastructure.
A warmer climate can increase the likelihood or intensity of some heavy rainfall events, but the eventual severity of a flood also depends on what happens on the ground.
Urban development can increase runoff.
Deforestation can alter water movement.
Poor drainage can turn heavy rainfall into catastrophic flooding.
Coastal communities face another factor: rising sea levels.
The IPCC’s assessment of climate impacts shows that increasing weather and climate extremes are already producing widespread impacts on people, ecosystems and infrastructure, with vulnerability and exposure playing major roles in the eventual damage.
So climate change can increase the hazard without being the only reason a disaster occurs.
Drought Has a Different Climate Fingerprint
Drought illustrates why climate attribution cannot be reduced to one simple rule.
Higher temperatures can increase evaporation and dry soils and vegetation.
That can worsen the effects of insufficient rainfall.
But drought is also strongly influenced by natural patterns in the atmosphere and oceans.
Phenomena such as El Niño and La Niña can dramatically affect rainfall patterns across different regions.
The WMO notes that attribution of drought is generally less straightforward than attribution of heatwaves because of this substantial natural variability.
Scientists therefore have to examine individual events and regions carefully.
The conclusion is not that climate change causes every drought.
It is that warming can alter the severity, duration or probability of drought under particular circumstances.
What About Hurricanes and Tropical Cyclones?
Tropical cyclones are another area where the relationship is nuanced.
Climate change does not simply mean that every hurricane will become stronger.
Storm formation depends on several environmental conditions, including ocean temperature, atmospheric circulation and wind shear.
But warmer oceans provide more energy for tropical cyclones, while a warmer atmosphere can hold more moisture.
The IPCC finds evidence that human influence has affected some characteristics of tropical cyclones and has contributed to increases in extreme precipitation associated with some storms.
Another important factor is sea level.
Even if a storm itself did not become dramatically stronger, a higher starting sea level can make coastal flooding more severe.
This is an example of how climate change can increase disaster risk without simply changing the storm itself.
Wildfires Have Multiple Causes
Wildfires are similarly complicated.
A fire requires an ignition source, fuel and suitable environmental conditions.
Human activity can directly start fires through accidents, infrastructure failures or deliberate ignition.
But climate change can influence the conditions that allow fires to spread.
Higher temperatures can dry vegetation and soils. Longer periods of heat and dryness can increase the availability of combustible material.
The result is that climate change can contribute to fire weather, while local land management, vegetation, wind and ignition sources determine whether an actual wildfire develops.
This is why scientists distinguish between changing the conditions for fire and directly causing a particular fire.
Scientists Can Actually Test These Connections
Perhaps the most fascinating part of modern climate science is that researchers can investigate individual extreme events.
This field is known as extreme event attribution.
Scientists can compare what happened in the real world with simulations representing a world in which human greenhouse-gas influences were absent or much smaller.
They can then ask questions such as:
How likely was this event in today’s climate?
How likely would it have been in a pre-industrial climate?
Did human-caused warming make the event more intense?
Did it increase the probability of the event occurring?
The WMO describes this approach as a way of separating human and natural influences by comparing climate-model experiments under different conditions.
It is not a perfect system.
But it has transformed the conversation from speculation into quantitative analysis.
Scientists Cannot Attribute Every Extreme Event
This is one of the most important points to understand.
Climate science does not claim that every storm, flood, tornado or drought is caused by global warming.
Some events are too strongly influenced by natural variability.
Others occur on scales where current observations or models cannot provide a sufficiently confident attribution.
The IPCC specifically notes that some types of extremes, including tornadoes, remain difficult to attribute with current modelling and theoretical capabilities.
That uncertainty does not undermine the broader evidence.
It simply means the strength of the human influence varies from one event to another.
For heatwaves, the evidence is particularly strong.
For certain rainfall events, the evidence is growing.
For some other extremes, the answer remains less certain.
That is what scientific precision looks like.
Why Small Amounts of Warming Matter
Another misconception is that only dramatic increases in global temperature matter.
In reality, relatively small changes in global average temperature can produce significant changes in extremes.
The IPCC finds that regional changes in climate extremes generally increase with global warming and that even additional warming of around 0.5°C can produce statistically significant changes in many extremes.
This happens because extreme events sit at the edge of probability distributions.
Moving the average temperature slightly can move the entire distribution.
An event that was once exceptionally rare can therefore become considerably more common.
That is why a fraction of a degree can have consequences far larger than the number itself suggests.
The Real Danger Is Often the Combination of Extremes
Climate change can also create compound events.
A region might experience extreme heat followed by drought.
A heatwave can occur alongside exceptionally dry conditions.
Heavy rainfall can strike an area already vulnerable because of saturated soil.
Coastal flooding can coincide with a major storm.
These combinations can produce impacts much greater than any single hazard alone.
The IPCC specifically assesses compound and concurrent extremes as an increasingly important component of climate risk.
This is where climate change becomes more than a temperature story.
It becomes a risk-management problem.
Human Activity Changes More Than the Atmosphere
Greenhouse-gas emissions are the central driver of modern global warming, but human influence on extreme-weather risk can also occur through changes to landscapes and communities.
Cities replace vegetation with concrete and asphalt.
Wetlands are drained.
Forests are cleared.
People build homes and infrastructure in flood-prone areas.
Coastal populations grow.
These changes can increase exposure and vulnerability even when the underlying weather event remains similar.
That is why the same rainfall event can produce dramatically different consequences in two locations.
Climate change changes hazards.
Human development determines, in part, how much is exposed to those hazards.
The Evidence Is Getting Stronger
The science connecting extreme weather with human activity is no longer based solely on computer projections of what might happen decades from now.
Scientists now have decades of observations, improved climate models, better physical understanding and increasingly sophisticated attribution techniques.
The IPCC reports that evidence connecting human influence to changes in extreme weather has strengthened substantially, including for heatwaves, heavy precipitation, droughts, tropical cyclones and compound events.
The WMO likewise reports that human-induced climate change is already affecting weather and climate extremes across regions of the world.
The question, therefore, is no longer simply whether human activity influences extreme weather.
The more useful question is how much influence does it have on a particular event, and what happens as warming continues?
A Changing Climate Changes the Odds
Extreme weather is not new.
What is changing is the environment in which extreme weather occurs.
Human activity has warmed the atmosphere and oceans, changed the water cycle and raised sea levels. Those changes can make certain heatwaves hotter and more likely, intensify some heavy rainfall events, worsen some drought conditions and increase the risks associated with coastal storms.
But nature has not disappeared from the equation.
Weather remains chaotic.
Natural climate variability remains powerful.
And individual events can have multiple causes.
The most accurate way to understand the science is therefore not to ask whether humans “caused” every disaster.
It is to ask how human activity shifted the odds.
That shift is increasingly measurable.
And as the planet continues to warm, the distinction between an extraordinary weather event and a climate-changed extreme may become increasingly important not only for scientists, but for cities, governments, businesses and communities trying to prepare for what comes next.
