Understanding the future of heat begins with latitude and longitude.
When we look at maps of the world, the continents and oceans flatten out. But the earth is spherical and tilted, spinning around its axis and rotating around the sun. The shape, tilt, spin, and orbit are what account for the differences in our days and nights, positions of the sun, the seasons we experience, and temperatures we dress for.
Let’s start in the tropics.
On the longest day of the year, the June solstice, Singapore receives 12 hours and 12 minutes of sunshine. On the shortest day, the December solstice, it receives 12 hours and 3 minutes of sunshine. Since Singapore receives almost exactly the same amount of solar energy all year round, its temperature barely fluctuates.
Singapore
An average day in the 1980s in Singapore had a high temperature of 30.5°C (87°F) and a low temperature of 24.5°C (76°F). The warmest month, May, had an average daily high temperature, at 32°C (89°F), while the “coldest,” December, had an average daily high temperature of 29°C (84°F). These temperatures are often uncomfortable due to the high humidity in this tropical climate.
Wet-bulb temperature is calculated using temperature and humidity. High wet-bulb temperatures can impair the human body’s ability to self-cool through sweating. 26°C (79°F) wet-bulb can occur at 30°C (86°F) air temperature and 75% relative humidity, or 38°C (100°F) and 36% humidity.
Wet-bulb temperature amplifies extreme heat, transforming a hot day into an insufferable—or even deadly—one. The combined highs of heat and humidity generate unprecedented conditions in which outdoor labor is impossible and indoor spaces must be climate-controlled.
This map of Singapore depicts the number of days per year with a daily maximum wet-bulb temperature exceeding 26°C (79°F) at two different warming scenarios.
As we’ve seen across the world, Singaporean nights have warmed more than days. This is one of the many ways we know that the atmosphere’s warming is due to increased greenhouse gases. The amount of solar energy that reaches the surface of the earth has had only very small fluctuations, but the amount of energy that leaves the atmosphere is diminishing, so nights don’t cool off the way they used to. 2019 was the warmest year on record in Singapore. The average day passed 32°C (90°F) in the afternoon, and nighttime low temperatures averaged 25.9°C (79°F), 1.5°C higher than the average of the 1980s.
While the equator receives a consistent amount of sun during the year, areas closer to the poles experience dramatic change over the course of the year.
Fairbanks, AK
Fairbanks’s latitude is 65°N, slightly more than 1° south of the Arctic Circle.
On the June solstice, Fairbanks receives 21 hours and 50 minutes of sunshine. The sun isn’t directly overhead the way it is near the equator, but that is still a lot of energy over the course of the day, so temperatures can be quite warm. Remarkably, Fairbanks has the same record high temperature as Singapore: 37°C (99°F).
In the depths of winter, however, the surface receives precious little energy. The nearby Arctic Circle, at 66.6°N, is the latitude at which the sun doesn’t cross the horizon at all on the December solstice. Fairbanks gets some daylight on that day, but just 3 hours and 42 minutes, and even then the sun never rises much above the horizon.
With so little energy coming in, temperatures drop far below freezing. Fairbanks’s record low temperature is -54°C (-66°F).
As the earth’s climate has warmed, extremes reached in places that already experience a wide range of temperatures are becoming even more volatile in predictable ways. Since the big change isn’t the amount of energy coming in from the sun, summers are only slightly warmer, while spring, fall, and especially winter are much warmer. It’s less that the Arctic is getting hotter and more that it is losing its cold.
For example, the historical average high temperature for December 1 was -15°C (5°F). On December 1, 2020 Fairbanks registered a high temperature of 3.9°C (39°F).
Number of days maximum temperature below 0 °C (32 °F)
Freezing days have wide-ranging implications, from the availability of snowmelt used for water supply to cultural events like winter sports. In addition, with fewer weeks below freezing and fewer extremely cold days, permafrost that has been frozen for thousands of years has more time and energy to thaw, causing the dead plants and animals trapped in that ice to decompose and release their carbon into the atmosphere.
Milan’s latitude is 45° North, halfway between the tropics and the Arctic, and its climate is temperate—the climate in which most humans have lived. On the June solstice, Milan receives 15 hours and 41 minutes of sunshine, and on the December solstice it receives 8 hours and 42 minutes.
Milan
Milan has a distinct winter, spring, summer, and fall, but historically none of them were severe. In the peak of summer, daily highs were typically around 29°C (85°F). In the depths of winter, nights historically dipped just below freezing, with a record low temperature of -15°C (4°F) in 1956. In the past, Milan’s record high temperature was—like Singapore and Fairbanks—about 37°C (99°F).
Distribution of summer and winter temperatures in Milan
In these charts you can see how Milan’s summer and winter temperatures would shift if global average temperatures were to rise from roughly 1°C above the preindustrial average to 3°C.
In a 3°C world, the average occurs less frequently, and the extremes are wider.
Mid-latitude locations like Milan typically experience a wide range of temperatures throughout the year, and even within one season. But as the earth’s climate warms, people living in temperate climates must be prepared for not just extreme temperatures, but much more variability. Summer heat waves are becoming common, and in recent years summer temperatures have passed 40°C (104°F). For Mediterranean climates accustomed to cooler winter temperatures, including frosts and snows, winters are shorter and significantly warmer.
Changes in global temperature are going to transform these familiar places into different climates. The stable climate that has been so kind to our species could be irrevocably transformed if we don’t cease carbon emissions. Shifts between climates have historically been associated with large-scale extinctions, as even the most adept and dominant species cannot survive certain physical conditions.
Imagine the things that make human society special—art, agriculture, sports—existing in a world so hot that our bodies couldn’t sweat to cool us down or so cold that ice covered all our oceans. Consider the damage to global health, political stability, and the global economy that would be wrought if Earth exited the only conditions humans have ever known.
The loss of our special climate would touch everything we are familiar with, and irrevocably transform Earth into an unrecognizable place. That planet wouldn’t allow for human ingenuity, creativity, or life.
As human activity adds greenhouse gases to the atmosphere and the global temperature warms, critical cycles in Earth’s climate system are starting to be disrupted. The activation of “tipping points,” like the loss of the Greenland Ice Sheet or Arctic Sea Ice due to rising temperatures, threatens to create a cascade of climate impacts and generate even further warming.
These dynamics were initiated by human-caused climate change and are close to progressing beyond our control. They risk forcing the earth’s climate into a state of perpetual warming with no chance to achieve new equilibrium. Many parts of the resulting “Hothouse Earth” would be unrecognizable and uninhabitable for humans, like Dinosaur Earth and other hot climates of the past. It is unknown at what temperature level Earth and its atmosphere may tip into Hothouse Earth, but there is strong evidence that some of the key mechanisms of Hothouse Earth have already begun.
A Hothouse Earth is not inevitable, and not instantaneous. It is a pathway with a timeline and, for the first time in the long history of humans on Earth, we are in a powerful position of understanding.
The work we need to do to prevent tipping points is urgent, but it is work we are capable of. As a species, we have to cease greenhouse gas emissions entirely, and we need to do it as soon as possible. Our understanding of Earth, climate science, and its workings, along with a willingness to change the way we live, sometimes fundamentally, can slow this timeline and give us opportunities to adapt in ways that allows us to thrive, in conscious relationship with our physical surroundings.
To confront this future, we are going to need knowledge and imagination. We invite you to explore our maps and envision what our world might be like.