Climate science is based on theory. And that is more than enough for scientists. COURTESY NASA

I used to be an astronaut, a spacewalker on the International Space Station. Naturally, most of my fifteen-year crew career was spent on the ground, working with engineers to get the Station built and fully crewed for scientific research, but the day-in, day-out flow of this ground work was punctuated by the occasional illuminating, even eye-shattering experience, when I was launched into orbit and saw Earth through my spacesuit visor.

The Station goes around the world in an hour and a half, which means it flies through fifty minutes of day, followed by fifty minutes of night, endlessly repeating. This means that during a seven-hour spacewalk, you may see four sunrises and sunsets. I remember holding onto a handrail on the outside of the Station, which was flying silently up the Atlantic, from south to north, and as we moved toward Europe I could see the terminator—the line between night and day—rolling up over the horizon ahead of us. The white sun sank quickly behind us in a showy flurry of orange, pink, and red horizon bands, and then we were suddenly in twilight, floating into the dark half of the world. The terminator flicked over us, and, in the deeper darkness ahead and below us, I could see a huge lit-up city, glued to the curved Earth, sliding up over the rim of the world to meet me. I could see the structure of the city, with its glowing heart, its network of roads and its halo of suburban lights fading into the dark countryside. The city was Cairo, looking beautiful and radiant. Down there, millions of people were trying to make a living, find or keep a job, provide for themselves and their families, and worrying about the future. I could cover all of them with one outstretched hand. Cairo slid under and then behind me, and more cities rolled into view over the horizon, all tacked to the outside curve of the planet surface, and all seemingly moving toward us, rather than us toward them.

To me, the city lights below represented human energy and hope. Most people work hard to better their own and their families’ lives by struggling to get a bit more than they have. It’s a laudable impulse; it’s what got us out of caves and into villages, towns, and cities. This process has propelled civilizations forward: art, philosophy, engineering, and science all came from the cities where people interact, discuss, argue, and push the human reach a little further. In this century, for the first time in history, more than half of us live in cities, and most of us in the affluent West are busy driving cars, flying from place to place, and heating and cooling our houses and offices. Hence all those glittering lights.

All of this is placing quite a demand on the resources of our little home planet. The facts of climate change are straightforward: there’s been a warming surge over the past hundred years, with a dramatic uptick in this new century. We are seeing the effects in the shrinking of the summer Arctic sea ice and the melting of the Greenland glaciers. That melt, in turn, has been partly responsible for the three-inch rise in sea levels since 1992. The Earth is warming, the ice is melting, and sea level is rising. These are observed facts.

Are we humans the cause of these changes? The answer is an emphatic yes. Many climate-research groups around the world have calculated the various contributions to climate change, including those not related to humans, like volcanic ash. It has been shown repeatedly that it is just not possible to explain the recent warming without factoring in the rise in anthropogenic greenhouse gases. If you left the increase in carbon dioxide out of your calculations, you would see a wobbly but, on average, level temperature trend from the eighteen-nineties to today. But the record—the reality—shows a steeply rising temperature curve which closely matches the observed rise in carbon dioxide. The global community of climate scientists, endorsed by their respective National Academies of Science or equivalents, is solid in attributing the warming to fossil-fuel emissions. Humans are the cause of the accelerating warming. You can bet your life—or, more accurately, your descendants’ lives—on it.

As a scientist, I would like to think that the political discussion of climate change and how to mitigate its worst effects would be sober and fact-based. Unfortunately, this is not the case. Climate-change deniers in the United States have done a first-class job in spreading confusion and misinformation. As a result, many prominent politicians insist, and get away with insisting, that climate change is a hoax, a mantra that has gained some credibility through sheer repetition. Climate deniers are also fond of saying that global warming is not resolved in science or is “just” a theory. This is a perfect example of Orwellian Newspeak which also flies in the face of three hundred years of scientific progress, in which intellectual argument and conviction must be based on facts and substantiated theories, rather than personal beliefs or biases.

It is also dangerous. If nothing is done to reduce carbon emissions over the next couple of decades, our climate models predict that there will be massive changes in the global precipitation and temperature patterns, with huge effects on water and food security, and dramatic sea-level rise. This is why the signers of the international Paris climate agreement called for limiting the rise of the global surface air temperature to two degrees Celsius above pre-industrial levels.

The science behind the predictions made by these climate models is not always easy to explain, and this prompted me to think more about how scientists communicate what we know to the wider community. When we talk about why the climate has changed, and what the future climate is likely to be, scientists use analyses and predictions that rest heavily on results from computer models, which in turn rest on layers and layers of theory. And there’s the rub—a lot of the confusion about what is known and unknown about the changing climate can be traced to people’s understanding of the role of theory in science.

Fundamentally, a theory in science is not just a whim or an opinion; it is a logical construct of how we think something works, generally agreed upon by scientists and always in agreement with the available observations. A good example is Isaac Newton’s theory of gravitation, which says that every physical object in the universe exerts a gravity force field around itself, with the strength of that field depending on its mass. The theory—one simple equation—does a superb job of explaining our observations of how planets orbit around the sun, and was more than good enough to make the calculations we needed to send spacecraft to the moon and elsewhere. Einstein improved on Newton’s theory when it comes to large-scale astronomical phenomena, but, for everyday engineering use, Newton’s physics works perfectly well, even though it is more than three hundred years old.

Newton’s ideas, and those of his successors, are all-pervasive in our modern culture. When you walk down a street, the buildings you see are concrete and steel molded to match theory; the same is true of bridges and machines. We don’t build different designs of buildings, wait to see if they fall down, and then try another design anymore. Engineering theory, based on Newton’s work, is so accepted and reliable that we can get it right the first time, almost every time. The theory of aerodynamics is another perfect example: the Boeing 747 jumbo-jet prototype flew the first time it took to the air—that’s confidence for you. So every time you get on a commercial aircraft, you are entrusting your life to a set of equations, albeit supported by a lot of experimental observations. A jetliner is just aluminum wrapped around a theory.