The Earth’s atmosphere is nothing but freely roaming molecules. Left alone, they would drift and collide, and eventually even out into a mixture that’s dynamic, yet stable and broadly unchanging.
The sun’s rays complicate things. Energy enters the Earth system in daily cycles, the bulk of it going to whichever half of the planet is tilted toward the sun (and experiencing summer). The molecules in that half acquire more energy than others, which sets the global atmosphere steadily swirling. Depending on the season and location, molecules in our atmosphere might traverse warm land, then cold seas. They might encounter a mountain range that forces them to high altitudes, where the air pressure is low and water condenses. Then they might become part of large-scale phenomena, such as currents, atmospheric rivers, turbulent jet streams and continental fronts.
These phenomena are erratic. They interact at every scale and manifest as weather, from clear sunny days to blustery blizzards and the anomalous events — from hurricanes and polar vortices to hailstorms and tornadoes — that are happening with increasing intensity. Any thought of stability is illusory; no patch of molecules dances in isolation.
The result, from seemingly simple inputs of molecules and energy, is emergent, incalculable chaos. Some individual molecule in the room you are sitting in is careening about blindly and colliding with its immediate neighbors. Zoom out — block to city, field to landscape, region to continent — and patterns appear and intermix. Complexity abounds and compounds. Nothing in the atmosphere is untethered from the rest of the global picture.
We live with this unpredictable mess of an atmosphere every day. We tote around unopened umbrellas, or refresh weather apps and watch our weekend plans dissolve. Anticipating conditions any further out than a week or two is a fool’s errand. The Earth is a complex dynamical system — an interwoven mass of moving parts, each of which requires a different branch of science to understand. Even with advanced knowledge, sophisticated algorithms and modern instruments, it defies and eludes us.
Yet this engine of chaos is now under our influence. It is incontrovertible fact that we are changing the Earth’s temperature by adding more carbon dioxide to the atmosphere. We know exactly how we are changing it — that when we double the proportion of carbon dioxide in the thin layer that rests over the surface of the Earth, the planet will become 2 to 4 degrees Celsius warmer, overall, than it is today. This conclusion has remained essentially unchanged since 1896, when the Swedish scientist Svante Arrhenius arrived at an estimate of 2 to 5 degrees. (Using an extraordinarily simplified picture of Earth, he made a number of mistakes that, in the end, balanced out.) Some details may remain uncertain, some chaos untamable, but the basic conclusion is a matter of unwavering scientific agreement — 97% is a rare degree of consensus on almost any subject. We are nearly as sure of this as we are of the causes of infectious disease, or how stars form, or the fact that life evolves through natural selection.