What rising temperatures mean for race-day performance

Race-day heat is no longer just a weather variable to plan around. The data connecting your finish time to global emissions is clear.

If you have been racing long enough, you have probably had at least one event that felt like it went wrong before you even crossed the start line. The legs were there, the training was done, but something about the day made everything harder than it should have been. For a lot of athletes, that reason is increasingly heat.

This is no longer an occasional bad-luck story. Climate change is a scientific fact, and the data behind it connects your race-day experience to something considerably larger than a tough finish.

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A warming world

Average global surface temperatures have risen by approximately 1.2°C above pre-industrial levels, according to the Intergovernmental Panel on Climate Change's Sixth Assessment Report.¹ That figure has become so frequently cited it has started to lose its weight, and many people perceive this to be a nominally small change.

For endurance athletes, it's important to translate it into something concrete: a 2017 analysis published in Nature Climate Change found that the number of days per year during which outdoor physical exertion becomes physiologically dangerous (defined by a threshold combining temperature and humidity) has already doubled since 1980 in many parts of the world, and is projected to increase under every emissions scenario modelled.²

Those are not abstract futures. They are race calendars.

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Heat makes racing harder

The research on heat and endurance performance is consistent. An analysis published in Medicine and Science in Sports and Exercise in 2007 examined finishing times across multiple years of major marathons and found performance declined by roughly 1 to 1.5 per cent per degree Celsius above approximately 10 to 12°C.³ Over 42 kilometres, that translates to several minutes. Over Ironman-distance triathlon, the compounding effect across a full day is considerably larger.

Rising average global temperatues means more athletes are racing slower, suffering more, and finishing less often as conditions worsen. Event medical teams are managing more interventions. Race directors are making harder calls. The trend is unambiguous.

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Humidity changes everything

Temperature alone does not tell the full story. Humidity determines whether the body can cool itself at all, because sweat only works if it can evaporate. In high-humidity conditions, that process slows or stalls: you sweat just as much, but the heat removal effect is dramatically reduced.

Wet-bulb temperature, which combines temperature and humidity into a single measure of cooling difficulty, is the metric that event medical teams now use to assess race-day risk. A 2010 position statement from the American College of Sports Medicine cited a wet-bulb temperature of approximately 28°C as the upper limit for safe mass-participation marathon racing.⁴ Several major events have since introduced formal protocols at which they will modify starts, shorten courses, or cancel entirely.

Under mid-century climate projections, a growing proportion of major race weekends in Europe, North America, and Australia will exceed these thresholds. Research published in The Lancet Planetary Health in 2021 by Plassman and colleagues modelled the future viability of outdoor endurance sport across global cities and found that several regions currently considered prime racing locations face materially degraded conditions by 2050 under a 2°C warming scenario.⁵

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The competitive landscape is already shifting

This is not a future risk. It is a present one, and the racing community is already adapting, however reluctantly.

The 2019 World Athletics Championships marathon in Doha was held at midnight in an attempt to avoid peak heat. Despite the precaution, the women's race saw a finishing rate of approximately 40 per cent, one of the lowest ever recorded at a global championship. Race director and medical reports cited heat and humidity as the primary factors.

Several Alpine cycling sportives have introduced modified or shortened routes in response to early-summer heatwaves. Fell running and mountain marathon events in the UK have faced course modifications when ground conditions, driven by prolonged heat and drought, become unsafe. These are not isolated incidents; they reflect a systematic shift in what race directors and safety officers are managing.

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Joining the dots

Here is where performance science and environmental science converge on the same point.

The warming that is affecting your race-day experience is caused by cumulative greenhouse gas emissions. The IPCC is unambiguous: the 1.2°C of warming observed to date is directly attributable to human activity, primarily fossil fuel combustion and land use change. Every fraction of a degree of additional warming shifts the distribution of difficult race days further upward and increases the frequency of conditions that exceed safe thresholds.

The World Weather Attribution project has concluded that virtually every major heatwave recorded since 2015 was made more likely, and more intense, by anthropogenic climate change.⁶ When the next hot race day rolls around, that context is not abstract. Human-generated greenhouse gas emissions are making you slower.

Climate change also increases the chance of extreme weather events like drought and wildfires. The degraded air quality from wildfire smoke that affected races across California, British Columbia, and southern Europe in recent years is the same story in a different form: climate-driven disruption landing directly in the sporting experience.

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What you can actually do

Climate change is already reshaping events, and short of adjusting your race-day approach there is not much any individual athlete can do about the conditions they arrive to. What you can do is hold the brands you rely on to get you to the finish line to account for their role in causing them.

The synthetic sportswear that makes up most of the endurance market is largely petrochemical in origin: a direct feedstock link to the emissions driving the warming that is making your racing harder. The nutrition supply chain carries its own footprint, across ingredient sourcing, packaging, and logistics. These connections run through the brands in your kit bag, whether or not those brands are transparent about them.

Knowing which brands are genuinely working to reduce their footprint, and which are deploying sustainability language without the evidence to back it up, is what the Ethical Endurance Framework is built to tell you.

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The longer view

Caring about where the sport goes over the next twenty years and caring about what the planet looks like over the next twenty years are, increasingly, the same concern.

The Ethical Endurance Framework independently scores brands across five categories: carbon and emissions, materials and nature, people and supply chain, governance, and transparency. Every brand in our buying guides has been assessed against it. No brand pays for inclusion. The scores reflect what brands can demonstrate, not what they claim.