The notion of a “climate system” is the powerful idea that the temperature we feel when we walk outside our door every day of the year, that the wind blowing on our faces while taking a walk, that the clouds we see in the sky, that the waves we watch rippling on the surface of the ocean as we walk along the beach, are all part of the same coherent, interconnected planetary system, governed by a small number of knowable, deterministic physical laws.
The first explicit realization that planetary coherence is an attribute of many environmental phenomena we experience in our life probably coincided with the great explorations of the 16th and 17th century, when Halley—of comet fame—first postulated the existence of a general circulation of the atmosphere, from equator to poles, in response to differential heating of the planet by the sun. The reliability of the easterly winds, which ensured safe sailing westward on the great trade routes of the Atlantic, was a telling clue that something must have been explainable. That this must have had something to do with the shape of the earth and its rotation required the genius of Hadley, who—even without the necessary mathematics, only fully available over a century later—was the first to realize just how powerful a constraint rotation could be on the fluid dynamics of the planet.
But even without such explanations, a sense that there is such a thing as a coherent “climate,” in which, for example, weather appears to exhibit coherence over time and space, has followed us through history. After all, the saying “Red sun at night, sailor’s delight; red sun at morning, sailors take warning” must have captured some degree of coherent predictability to survive for centuries! Today we can explain that very rhyme as predicting mid-latitude weather, and we can quantitatively describe it with a particular solution to the simplified Navier-Stokes equations called a Rossby wave, after Carl-Gustav Rossby, the founder of modern meteorology.
We are so used to the coherent workings of the climate system that most people don’t even think of it as a scientific construct worth knowing. We take for granted—in fact we think it trivial—that in the mid-latitudes many of us enjoy summer, then autumn, followed by winter, then spring and then summer again, in a predictable sinusoidal sequence of temperature, wind, and rain or snow. That is just our “climate,” the sort of thing we read in the first page of a tourist guide to a new country we might visit. Yet we explain such climate with the impact on insolation of the tilt of the planet and its revolution around the sun, and that same simple solar cycle also produces two rainy seasons in some parts of the tropics, and highly predictable yearly monsoons in others. In fact, that the seasonal cycle could be such a complex and varied response to such a simple and predictable forcing is nothing short of astounding.
Even more striking should be the fact that our scientific understanding of fluid motion on that scale has enabled computational models of the atmosphere—the brainchild, at least in theory, of Lewis Fry Richardson, and, in practice, of John von Neumann and Jules Charney, based on relatively simple numerical version of the Navier-Stokes, continuity and radiative transfer equations—to simulate all those phenomena on a planetary scale to such precision that the untrained eye would struggle to tell the difference from the real thing. That the winds we hear blowing through the trees of our gardens should be part of a coherent system spanning thousands of miles across the planet, from the equator to the pole, responding to astronomical forcing, and that we can explain it quantitatively with equations derived from physical first principles and, within certain limits, predict its behavior should be awestriking.
Few know that the coherence of the climate system has also extraordinary function. For example, coherent movement on a planetary scale by both the ocean and the atmosphere ensures that heat is transported poleward from the equator at peak rate of almost 6 PW (or six million billion W—roughly a thousand times all installed power production capacity in the world), thus ensuring that when people say they are boiling in Nairobi or freezing in Berlin, they can be taken figuratively rather than literally.
The climate system is also a source of great wonder. It is astonishing, and a subject of a great deal of active research, that almost imperceptible differences in those same solar forcing conditions—the so-called Milankovitch cycle, tiny adjustments to the amount of sunlight reaching the earth because of small periodic changes in the shape and structure of the orbit of the planet around the sun—could result in ice ages, a climate response of such incredible force that can make the difference between several hundred meters of ice over Chicago and what we have today. And we know that its deterministic coherence does not necessarily imply simplicity: it is capable of generating its own resonant modes—such as those associated with El Nino and La Nina, phenomena that require the coupled interaction of the ocean and the atmosphere—as well as chaotic dynamics.
That all of this we could understand by measuring and studying our planet over thousands of measurements from meteorological stations, satellites, buoys, and cores, and that we could explain it by using the fundamental laws of physics is nothing short of miraculous and one of the great accomplishments of modern science. And it is all the more important because we spend our lives in it every day. The planetary state of the climate system is what determines how much water we might have available at any given time, what kind of crops we might be able to grow, which parts of the world will flood and which ones will be parched to death. It matters, a great deal, to all.