Exactly How Much Life is on Earth?

What’s in a number?

According to a recent calculation by a team of biologists and geologists, there are a more living cells on Earth — a million trillion trillion, or 10^30 in math notation, a 1 followed by 30 zeros — than there are stars in the universe or grains of sand on our planet.

Which makes a certain amount of sense. The overwhelming majority of these cells are microbes, too small to see with the unaided eye; a great many are cyanobacteria, the tiny bubbles of energy and chemistry that churn away in plants and in the seas assembling life as we know it and mining sunlight to manufacture the oxygen we need to breathe.

Still, it boggled my mind that such a calculation could even be performed. I’ve been pestering astrobiologists lately about what it means. Could Earth harbor even more life? Could it have less? How much life is too much?

“The big take-home is this really sets up Earth as a benchmark for comparative planetology,” Peter Crockford, a geobiologist at Carleton University in Ottawa and the lead author of the report, which was published last month in the journal Current Biology, said in an email. The finding “allows us to more quantitatively ask questions about alternative trajectories life could have taken on Earth and how much life could be possible on our planet.”

For example, he said, what if photosynthesis — that miraculous transformation of sunlight into food and oxygen — had never evolved?

The question highlights the long, underrated relationship between geophysics and biology.

As Michael Kipp of Duke University, who was not part of the study, wrote in Current Biology Dispatches: “In the vast cosmic arena, there are perhaps planets that live fast and die young, while others are slow and steady. Where does Earth sit on this spectrum?” Caleb Scharf, an astrobiologist at NASA’s Ames Research Center in Mountain View, Calif., echoed Dr. Crockford. “There have been a number of interesting works in the last year or two where people have taken a step back to really think about the ways that life imprints itself on a planet,” he wrote in an email.

He called Dr. Crockford’s paper “a sort of neo-Gaian way of looking at things,” referring to the hypothesis, proposed in the 1970s by James Lovelock, that life and the environment work together to maintain a habitable planet.

According to the fossil record, geology and evolution have been engaged in a dance for 3.8 billion years, since our planet was only 700 million years old. It was then that the first single-celled creatures appeared, perhaps in undersea volcanic vents, feasting on the chemical energy around them.

The population of cells has been growing exponentially ever since, even through geological disasters and extinction events, which opened up new avenues of evolution.

The seeds for animal life were sown sometime in the dim past when some bacterium learned to use sunlight to split water molecules and produce oxygen and sugar. By 2.4 billion years ago, with photosynthesis well-established, the amount of oxygen in the atmosphere began to rise dramatically. The Great Oxidation Event “was clearly the biggest event in the history of the biosphere,” said Peter Ward, a paleontologist from the University of Washington.

Without photosynthesis, the rest of creation would have little to eat. But it is just one strand in a web of geological feedback loops by which weather, oceans, microbes and volcanoes conspire to keep the globe basically stable and warm and allow life to grow.

The carbonate silicate cycle, for example, regulates the amount of carbon dioxide in the atmosphere; the gas traps heat and keeps the planet temperate and mostly stable. Rain washes carbon dioxide from the air and into the ocean; volcanoes disgorge it again from the underworld. As a result, Dr. Crockford and his colleagues estimate, a trillion gigatons of carbon have been cycled from gas to life and back again over the millenniums. That’s about 100 times as much carbon as exists on Earth, which suggests that, in principle, every atom of carbon has been recycled 100 times.

The rise of cyanobacteria set off what is known as the Cambrian Explosion about 550 million years ago, when multicellular creatures — animals — appeared in sudden splendiferous profusion in the fossil record. We were off to the Darwinian races.

Dr. Crockford and his colleagues realized that they could trace the population growth of cells through time by measuring mineral isotopes and the amount of oxygen in old rocks. As a result, they were able to estimate the total life that Earth has produced since its beginning — about 10^40 cells, roughly 10 billion more than currently exist.

Although this number sounds huge, it represents only 10 percent of all the cells that will come about by the time the curtain falls on life on Earth a billion years from now. As the sun ages, it will brighten, astronomers say, amplifying the weathering and washing away of carbon dioxide. At the same time, as Earth’s interior gradually cools, volcanic activity will subside, cutting off the replenishment of the greenhouse gas.

As a result, Dr. Crockford said, “it is unlikely that Earth’s biosphere will ever grow beyond a time-integrated ∼10^41 cells across the planet’s entire habitable lifetime.”

But for now, Dr. Crockford and his colleagues wrote in their paper, “the extension of today’s relatively high rates of primary productivity will likely squeeze more life into less time.” The more cells there are, the more times they will replicate, producing more mutations, Dr. Crockford explained. We inhabitants of Earth’s biosphere have a billion years’ worth of surprises ahead of us.

As far as other planets go, he said, we still have only basic information about their sizes and habitability and our imaginations. Some of the candidates most likely to harbor extraterrestrial life are ice-covered ocean worlds that are the moons of Saturn and Jupiter — like Europa, soon to be visited by a new robot explorer, the Europa Clipper.

If there’s life those oceans, it’s likely to be primitive, Dr. Crockford said, as those cold environments lack sufficient energy to drive evolution.

“However,” he said, “it then gets extremely interesting to think about how the biosphere of such icy moons will change when the sun gets brighter.”