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After two weeks on-site, the polar world comes into clearer focus, and what was initially disorienting becomes liberating. There is something almost magical about living in an environment where the sun never sets for a month or more while being disconnected from the rest of humanity. Running streams exiting melting glaciers hold water so pure we drink it unfiltered from the source. Every babbling glacial brook could be a water fountain or, for extraordinarily hearty souls, a bath. The continuous light of the sun, coupled with the vastness of the landscape, brings an almost boundless metabolic energy that powers long hikes, intense conversations, and deep companionship. The world becomes small and intense—defined by the colleagues on the expedition, the supplies and gear that we bring, and the rocks, rivers, and tundra that are accessible on foot. With these local and intimate interactions, the landscape—its shapes, textures, and rhythms—gains a personality, almost like a member of the expedition. The shifting weather and light of the Arctic and Antarctic shapes our work, lives, even our emotional states. With no news, internet, or external sources of information, the main distractions in camp are the ones we carry inside of us.

Much of what makes polar regions special is their relationship with the sun. Documentary filmmakers have had a field day with some of the misconceptions about this relationship, which most of us learn in elementary school only to forget by middle age. One film crew went to the commencement at Harvard University and asked students and faculty the question: What causes the changes of the seasons? Virtually everyone said that Earth is closer to the sun in summer, leading to its relative warmth, and farther away in winter, making it colder. Of course, they could not have been more wrong.

Earth's axis is tilted, and as the planet orbits the sun, light hits the surface at different angles at different times of the year. During winter, a region receives oblique light; in summer, the light hits more directly. Sunlight that travels at a low angle to Earth loses energy by the time it gets to the surface. Hence it is colder during these winter times. The opposite is true for summer: light that hits the surface closer to a 90-degree angle arrives with more energy than oblique light, making the environment warmer.

The same relationship between the angle of the sun's rays and energy explains the differences between the poles and the equator. The sun is, on average, about 93 million miles away from Earth. That vast distance means that the equator and Earth's poles are, for all intents and purposes, equidistant to the sun. The distinction between the poles and the equator lies in the angle at which the sun's rays reach the surface: light hits polar regions at a lower angle than at the equator. Accordingly, sunlight that arrives at the Antarctic continent at an angle of 30 degrees on a cloudless summer day carries half as much energy as that which falls on the surface near the equator. Because polar regions receive far less energy from the sun throughout the year than other parts of the Earth, the poles are low-energy places. Creatures that live there either find new ways to produce energy, conserve it, or learn to live with little of it.

Because the Earth's axis is tilted, the poles are continuously dark at the height of winter and light in summer. One definition of the Arctic and Antarctic Circles is the latitude at which the sun never rises, or sets, at least one day a year. The energy on the surface of the planet ultimately comes from the sun, and polar regions have no incoming solar energy for months at a time. Living things run on a kind of fuel reserve during these dark periods.

The reaction of the polar environment to light is an exemplar of its susceptibility to change. There's an illuminating analogy from chemistry and physics. Chemists talk about a system being "metastable" when any small change can push it to a new state. Imagine a ball delicately balanced on a stick—any tiny nudge can send it moving in a different direction. In an analogous way, small shifts in climate can transform polar regions. Because these regions function with so little energy from the sun, relatively subtle alterations in climate can move polar regions from an icy state to one lacking ice altogether.

Polar regions encompass 8 percent of the total surface of the Earth but hold an outsized influence on the state of the planet. Almost 70 percent of all the planet's fresh water is frozen in ice. On land, permafrost in the polar regions holds 1,600 billion tons of carbon—roughly double that in the entire atmosphere today. Locked in the soils and ice of the poles are clues to our past and things that will shape our planetary future. Every milestone of human evolution, from the origin of our species to the establishment of our social structures and technologies, arose during a time of ice at the poles. Freezing and thawing over millennia, the Arctic and Antarctica are like vaults that hold our planet's heirlooms. When polar regions melt, the vaults are thrown open—ancient water, carbon, and microbial life return to the surface to shape and change the world.

Layers of history permeate these landscapes. Valley walls hold layers of rock that reveal billions of years of changes on the planet. The landforms of hills, valleys, and mounds of gravel speak of more recent ways ice and wind have sculpted the surface. Ice, light, and wind are like an invisible hand that has carved these worlds over thousands of years. The ice itself contains atoms that tell how it grew and shrank over millions of years, while bubbles and ash caught inside the ice are relics of ancient worlds. And scattered across the surface of the tundra and coastlines are artifacts of human history, from the dwellings of the Indigenous Peoples who first settled here to implements left by European expeditions hundreds of years ago.