This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In cosmic terms the size and composition of planet Earth are not terribly unusual. Of course, we don’t yet know quite enough about other worlds to make that statement with absolute certainty. But measured by the most basic characteristics, Earth is not a crazy freak. In fact, rocky worlds of this size and general makeup appear likely to be incredibly abundant in our galaxy.
Things get more complicated if we look closer though. Earth orbits a type of star that is not as numerous as other less massive stars, and those low-mass stellar parents tend to have plenty of planets too. The solar system also has a configuration that may place it as a bit of statistical outlier – with wide orbits, and big worlds outside little worlds, but without more worlds close to its sun. Earth also has a sizable moon in comparison to the planet’s heft, and we suspect that moon came from a ferocious collision that converted a world that was ‘proto-Earth’ into something new.
Earth also has a relatively close balance of rock and water on its surface. A property that seems by no means guaranteed by our theories about planet formation. Worlds can be dry, and they can be very wet. Being like Earth may not be so common.
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Something else happened to the planet somewhere between about 3.5 and 4 billion years ago. It got life.
Although the deep innards of Earth probably don’t care at all about what’s happening thousands of kilometers above, for the surface of the planet, and perhaps even its upper mantle, life is a big deal. We think that the earliest living systems must have been intertwined with raw chemical energy in the environment. And there was plenty of that: From a youthful planetary atmosphere, perhaps containing hydrogen and lots of volcanically produced gases, to a young ocean that was likely much more caustic and reactive than ours today. There may also have been some bits and pieces of frozen minerals exposed to the atmosphere, the beginnings of continental crust, hot from the oven of planetary formation.
Life, with its myriad properties, has been part infection, part catalyst on Earth. From the earliest pieces of biochemistry to today’s tree of organisms, life has embedded itself in the way that energy is dissipated on this planet. In some respects life has complicated and dragged out the abiotic drift towards thermodynamic equilibration.
There are cycles and loops of chemical processing and reprocessing. Autocatalytic chemical pathways help maintain environments that might otherwise degrade into spent molecular piles – compounds reaching a static equilibrium with their surroundings. Life intervenes in the abiotic order of things. Sunlight doesn’t all go into warming a sterile surface, it gets intercepted and that energy is converted into complex molecular batteries. Living structures also grab photons by providing platforms for that energy to convert liquid water to vapor – pulling colossal amounts of fluid up through the gravity field in the act of transpiration. Eventually most of that intercepted energy does indeed end up contributing to the thermal state of the planet, but at a rate determined by life and its chemical and mechanical inefficiencies.
Because life happened to Earth, its atmosphere, surface, and planetary crust have been profoundly altered again and again for some 4 billion years. Billions of years ago microbial life built an oxygen-rich atmosphere. There are signs that part of that process may have helped plunge Earth into a ‘snowball’ state – a period of profoundly cooler global climate for hundreds of millions of years. Ocean chemistry has been reworked by life. Cloud formation is influenced by biologically produced particles. Some scientists have even posited that microbe produced material could influence plate tectonics; the slip and slide of planetary crust.
In some respects humans are entirely trivial by comparison. One can argue that the Earth doesn’t care about us, nor will our behaviors provide much more than a blip on a much deeper history and future. In a hundred million years time there will likely still be life on Earth, it just won’t be the same life that is here now.
Except those blips are not quite so inconsequential. There have only been 5 mass extinctions of life in the past 500 million years – times at which over 80% of all species of complex life have disappeared. Today there is considerable evidence that a sixth such extinction event is underway, directly correlated to human activity. A very large part of that activity is the active geo-engineering of Earth’s climate due to a relentless release of greenhouse gases from fossil fuels.
And therein lies the critical point. Our established order, our human flourishing, has to this point taken place under a range of environmental conditions that are quickly vanishing. There’s a chance that we’ll be among the species that don’t make it through.
Earth Day, this past April 22nd, first took place in 1970 as a call to environmental protection. Even at that point the basic tenets of both climate change and ecosystem stress had been understood for a long time. For example, Svante Arrhenius recognized in 1896 that burning fossil fuels could have a large enough impact on the atmospheric greenhouse to cause global warming that would affect human life. Admittedly he, sitting in Swedish weather, didn’t think that was such a bad thing at the time. But his world, his Earth, held a mere 1.5 billion humans compared to our 7.5 billion plus.
Earth, 4 billion years into its existence, has turned out to be a remarkable place. Our species is also a remarkable output of those layers of time and contingency. We are almost certainly the first life on this planet to be able to write the story I’ve sketched in the preceding paragraphs. We’re also the first to understand the options for the future. Planets change, planets get messy, and sometimes it’s life that does that. But we have a unique opportunity in Earth’s extraordinary history to engage with that reality – to grow up, to make things as good for ourselves as we possibly can in the window of time the cosmos has allowed us.