How do scientists actually calculate climate change's role in a single heatwave?
The core method is called extreme event attribution. Think of it as a forensic climate investigation: scientists use climate models to simulate two parallel worlds. One world is the actual climate we live in, with all the greenhouse gases humans have added. The other is a 'counterfactual' world — a version of Earth where those human-caused emissions never happened, representing pre-industrial conditions. By running thousands of simulations of each world, they can compare how often a heatwave of a given severity occurs in each. The difference tells them how much climate change altered the odds.
For the 2022 early-spring heatwave in India and Pakistan, researchers used this method and found that human-caused climate change made the event about 1°C hotter and roughly 30 times more likely than it would have been in a pre-industrial climate [3]. That is a direct, quantitative answer: the heatwave was not just 'worsened' by climate change — it was specifically made 1°C more extreme and 30 times more probable. This approach has been applied globally, and a 2023 review of all available attribution studies estimated that climate change is responsible for about $143 billion per year in global costs from extreme weather, with 63% of that coming from loss of life [5].
What do those numbers — '30 times more likely' or '1°C hotter' — actually mean for people?
These numbers translate directly into real-world consequences. When a heatwave is made 30 times more likely, it means an event that would have been a rare 1-in-100-year occurrence in a world without climate change is now expected to happen roughly every 3 years in today's climate [3]. That shift from rare to common has massive implications for public health, agriculture, and infrastructure.
The health burden is measurable. In China, researchers projected that under a high-emissions scenario (SSP5-8.5), the fraction of diabetes deaths attributable to heatwaves could rise from 0.8% in the 2010s to 9.3% by the 2090s — a more than tenfold increase [1]. Another study projected that heatwave-related deaths in China could reach over 72,000 per year by 2090 under the same high-emissions scenario, with climate effects alone driving 78% of the increase in the near term [2]. These are not abstract numbers; they represent lives lost that could be avoided if emissions are reduced. The same study found that limiting warming to 1.5°C could save about 3,500 lives per year in China alone by 2090 compared to a low-emissions scenario [2].
Are these estimates reliable? What are the limitations?
Attribution science is well-established, but it has important caveats. The estimates depend on the quality of climate models and the specific definition of the heatwave (e.g., which region, which months, which temperature metric). For the India-Pakistan 2022 heatwave, the study defined the event as the March–April average of daily maximum temperature over the most affected region, and found it had a return period of about 1-in-100 years in observations [3]. Different definitions could yield slightly different numbers.
Another limitation is that attribution studies work best for temperature extremes; they are more challenging for complex events like floods or droughts where multiple factors interact. Additionally, the projections of future heatwave deaths involve uncertainties from both climate models and population scenarios. For example, one study noted that while climate effects dominate the near-term increase in heatwave deaths, population aging and decline will also play a role, and under high emissions, population decline cannot offset the rising heatwave incidence [2]. Despite these uncertainties, the direction of the effect is consistent across all studies: climate change is making heatwaves more frequent, more intense, and more deadly.
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2022 to 2024, 1 from 2024 or later, 4 in Q1 journals, collectively cited 709 times — selected as the most relevant from 5 studies that passed quality screening, drawn from 42 papers retrieved from a database of over 500 million.
Sources used in this answer
Mortality burden of diabetes attributable to high temperature and heatwave under climate change scenarios in China
Projected that under a high-emissions scenario, the fraction of diabetes deaths attributable to heatwaves in 32 Chinese cities could rise from 0.8% in the 2010s to 9.3% in the 2090s, with lower emissions scenarios showing much smaller increases.
Projections of heatwave-attributable mortality under climate change and future population scenarios in China
Projected that heatwave-related deaths in China could reach 72,260 per year by 2090 under a high-emissions scenario, with climate effects driving 78% of the near-term increase, and that limiting warming to 1.5°C could save about 3,500 lives per year.
Attribution of 2022 early-spring heatwave in India and Pakistan to climate change: lessons in assessing vulnerability and preparedness in reducing impacts
Using event attribution, found that the 2022 early-spring heatwave in India and Pakistan was made about 1°C hotter and roughly 30 times more likely by human-caused climate change, compared to a pre-industrial climate.
Projected Changes in Socioeconomic Exposure to Heatwaves in South Asia Under Changing Climate
Projected that under high-emissions scenarios, heat stress in South Asia could exceed human tolerance limits by mid-century, with the largest increases in exposure in southern Pakistan and southwestern India.
The global costs of extreme weather that are attributable to climate change
Reviewed all available extreme event attribution studies and estimated that climate change causes about $143 billion per year in global costs from extreme weather, with 63% of that due to loss of life.