High wet-bulb temperature, a combination measure of heat and humidity, can make it hard to self-cool and maintain the temperature our bodies need to function properly. The higher the wet-bulb heat, the more dangerous it is to spend time outside of climate-controlled environments and the quicker its effects set in—especially for those with heightened health risks such as children and the elderly.
28°C (82°F) wet-bulb is a key threshold where the combined effects of heat and humidity can be dangerous for the human body. Exposure to high wet-bulb heat can eventually cause heat exhaustion, heat stroke, and even organ failure. Outdoors at 28°C wet-bulb, it can become unsafe for anyone to spend extended time working, exercising, or playing. Without good HVAC or air conditioning, even the temperatures indoors can become a health risk.
As the climate warms, dangerous wet-bulb temperatures are becoming more common, and occurring in locations that have never experienced them in the past. Miami, Florida, is one place facing this risk for the first time in the city’s history.
Miami at 0.5°C of warming
Climate zone:
tropical
Population:
450,000
Demographics:
16% over 65 years old; 0.8% population growth rate
Risk factors:
storms, flooding, heat, wet-bulb heat
Miami sits just outside the tropics in a band of mild climates on either side of the equator that are lush and warm and experience little variability in weather. Because of its proximity to the equator, the city received consistent sunlight and precipitation without much seasonal variation in temperature and weather patterns. The Gulf Stream running along the coast of Florida kept the weather especially warm and consistent. Summers were normally slightly hotter and wetter than winters, and with year-round temperatures between 17°C (63°F) and 32°C (89°F), being outdoors almost never meant enduring uncomfortable or dangerous heat.
Use the search function in the map of days above 28°C (82°F) wet-bulb to zoom into Miami, Florida at 0.5°C of warming.
The city’s reputation for pleasant weather made it synonymous with relaxation and comfort. Historically, Miami did not experience very high humidity simultaneously with very high temperatures. Even at 0.5°C of warming, dangerous wet-bulb heat wasn’t just an uncommon occurrence, it practically never happened.
This map of Miami, Florida depicts the number of days per year with an average maximum wet-bulb temperature exceeding 28°C (82°F) at 0.5°C degrees of warming. In the past at 0.5°C of warming, Miami was likely to experience zero days above 28°C wet-bulb in an average year and a cooler-than-average year. In a warmer year, Miami might have experienced up to one day where temperatures passed 28°C wet-bulb.
Migration from Central and South America and the Caribbean, along with an influx of retirees, supported a booming population over the past century. Even once greenhouse gas emissions began changing weather patterns, the comfortable climate of Miami continued to be a draw for visitors and residents. These warm, tropical, and predictable conditions helped define Miami through 0.5°C of warming. As global average temperatures continue to rise, the future is likely to be different.
Miami at 1°C and beyond
Between 1971 and 2000, Earth’s global average temperature was 0.5°C above the pre industrial average. More recently, we’ve moved past 1°C, so we use 1°C of warming as a proxy for the weather that we’re used to—the recent past.
At 1°C in Miami, there were a range of plausible outcomes in wet-bulb heat every year. You’ll see in the inspector box that there are three values across three kinds of years. In a coastal location like Miami, changes in air and ocean currents, or weather cycles like El Nino and La Nina, result in cooler years, warmer years, and average years. The inspector box tells you not just how many days above 28°C wet-bulb are probable for Miami, but how these outcomes might vary during a cooler year, an average year, or a warmer year. At 1°C, Miami had a likely range of one to three days per year above 28°C wet-bulb. An “average” year had one day per year above 28°C wet-bulb, while a warmer year had three. This is a relatively tight band of possible outcomes.
This map of Miami, Florida shows how the number of days per year with an average maximum wet-bulb temperature exceeding 28°C (82°F) changes over warming scenarios from 0.5°C to 3°C. 0.5°C and 1°C of warming represent the past, 1.5°C is impending, and 2°C, 2.5°C, and 3°C represent potential climates to come, or what is likely for Miami’s future.
The next warming threshold, 1.5°C, is impending (and the closest to the climate in 2024.) 2°C of warming is a potential future scenario.
At 1.5°C, the band of possible outcomes widens substantially. A cool year is projected to have zero days above 28°C wet-bulb, while an average year sees two days and a warmer year is projected to see 12. At 2°C, Miami can anticipate four days on average, with as many as 26 days in a warmer year. It’s hard to imagine 26 days—almost a month in which it’s too hot to work outside, play in a park, or spend the day in a classroom without air conditioning. The 22-day difference between an average year and a warmer year tells us critical information about what we need to prepare for at 2°C of warming: not just an uptick in the number of days of high wet-bulb temperatures, but an increase in the volatility in outcomes from year to year as global average temperature rises.
2.5°C and 3°C of warming are potential but avoidable, depending on the pace and scale of human greenhouse gas emissions. At 2.5°C of warming, an average year has 10 days above 28°C wet-bulb, and a hot year has 56 days. At 3°C of warming, these numbers increase to 21 days in an average year and 78 in a hot year. These numbers seem staggering, especially compared with the recent past or even present-day conditions. Because 2.5°C and 3°C are more distant thresholds of warming, these projections give us an impression of what we need to avoid. Data like this can be as much a tool for anticipating risk as motivation to avoid harmful outcomes by slowing and stopping warming.
What could 28°C wet-bulb mean for Miami?
Miami’s unique geography, economy, culture, and demographic makeup are all factors in assessing its wet-bulb risk. Thinking about those ranges in combination with Miami’s vulnerabilities is the first step to understanding its risks and making informed decisions about the future.
Here are a few guiding questions to start considering wet-bulb risk in Miami:
- 16% of Miami’s population is over 65 and 20% are children under 18 . What could it mean for their daily lives if outdoor activity becomes dangerous at certain times of the year? How will older people and school-age children adapt to this new constraint?
- Miami is the second most-visited city in the US, after New York City, and tourism accounts for more than 145,000 jobs in Miami-Dade County. What does it mean for a city so dependent on outdoor tourism if a week of the year is too hot for outdoor activity? What if an entire month per year becomes too hot for outdoor activity?
- The federal government currently only requires public housing to have heating, not air conditioning, and there are no federal mandates for limits on outdoor labor. What infrastructure and guidelines need updating to keep the population safe and healthy?
Examining the range of possible outcomes for a climate impact in a place, putting that data in context, and looking for vulnerabilities is a framework you can apply to any place with any Probable Futures map. We offer maps of 30+ climate variables at warming scenarios ranging from 0.5 to 3°C of warming. We invite you to explore and think about what climate impacts could mean in the places you care about.