Pretend, for a moment, that heat is visible, that our naked eyes can see the infrared rays shedding from objects all around. At closest range, we observe the incessant buzzing of molecules—vibrating at 1,100 miles per hour in the air of a room at a pleasant 72 degrees Fahrenheit. Pull back, and whole masses of molecules are churning and drifting, always in flux. There’s no substance to heat; it is not contained in a cup of coffee or a warm breeze. The temperature scale measures how much motion, or kinetic energy, lives inside an object, and “heat” refers to the way this energy passes from one place to another—from hot to cold—in streams and swirls and currents. If we can look at heat, we see a world constantly in motion.
In the frigid waters of the Arctic and Antarctic, fish have evolved to survive 28.5-degree temperatures that would literally freeze the blood of less well-adapted creatures.
Start with our very own bodies. Under a microscope, we see warmth in cells, in tissues, molecules set atwitter passing heat around with little bumps and shoves as fats and sugars are broken up and reactions catalyzed in our metabolic furnace. We are like miniature power plants, burning fuel and pumping thermal energy through the pipes and valves of our circulatory systems, keeping core temperature hovering around 98.6 degrees. On a hot day, our bodies begin to overheat, and our skin brightens as blood flows up near the surface. Droplets of sweat seep from our pores, drawing off excess energy as they evaporate. A breeze cools us by picking up this flow. Even without sweating, our bodies naturally put out approximately 100 watts of power, emitted mostly in the infrared. From the outside, people look like tiny space heaters skittering across the globe: six-and-a-half billion appliances each set to about 91 degrees, the average temperature of human skin.
Take a step back, and we see currents of air moving heat all the way across the world. The wind itself starts to look a bit like the blood in our veins, set in motion by solar rays. As the sun flushes heat into our atmosphere at a mind-boggling rate of 175 quadrillion watts, the air near the equator absorbs more energy than the air near the poles. Daytime temperatures in the tropics rarely drop below about 77 degrees, whereas at the poles they never exceed that. As the equatorial air warms up, it expands and rises. At about six miles above the surface, it starts bending toward the poles. Heavier, colder air rushes in below and sets the winds in motion, gusting energy around the globe. The whole process, called Hadley cell circulation, powers the jet stream and drives Earth’s weather systems.
We also find heat sloshing around the world’s oceans, which absorb 93 quadrillion watts of the sun’s energy—a hundred thousand times more power than could be produced by all the power plants in the United States put together. Eighty-degree tropical waters cool off as they approach the poles and sink into the near-freezing deep ocean, driving global currents.
The flow of heat throughout the ocean creates temperature extremes that challenge the basics of life. In the frigid waters of the Arctic and Antarctic, fish have evolved to survive 28.5-degree temperatures that would literally freeze the blood of less well-adapted creatures. The guts of these animals produce a natural antifreeze—glycoproteins that bind to ice crystals as they begin to form and prevent them from growing large enough to fatally rupture the walls of cells.
