Earth teemed with primitive life more 3½ billion years ago, at a time when the young Sun was too faint to keep our planet from freezing over. New climate models are converging on a solution to this longstanding paradox.
Ah, summer — the season when we're amply reminded that the Sun constantly bathes our planet with 1,367 watts of energy per square meter. And its radiant glow doesn't vary much at all, 0.1% or so throughout an 11-year solar cycle and perhaps as much as 0.5% over many centuries. That's a good thing — life depends on a steady flow of light and heat.
But the Sun was not nearly so shiny in its infancy, when it put out only about 70% of the energy it does today. (Reason: over time, as fusion converts hydrogen to helium, the Sun's core has gotten denser and must burn hotter to counteract collapse due to gravity. The young Sun had less helium, so the core was cooler and the fusion rate less furious.)
In 1972 Carl Sagan and George Mullen realized that, because of this "faint young Sun," the early Earth should have been completely frozen over — and yet it wasn't, based on all kinds of geologic and paleontologic evidence. In fact, it seems that Earth had liquid oceans and teemed with primitive life by 3½ billion years ago (and likely earlier still).
How Earth managed to avoid becoming the solar system's largest snowball in those Archean times is a climatic paradox that has challenged astrobiologists for decades. The most logical explanation is that our planet's primitive atmosphere contained enough greenhouse gases to boost the surface temperature and keep everything from freezing solid. But which gases? Thanks to the global-warming debate, we all know about the heat-trapping power of carbon dioxide (CO2), which has reached 360 parts per million. But water vapor (H2O) and, especially, methane (CH4) and ammonia (NH3) are greenhouse gases as well.
All Earth really needed was an atmosphere pumped up with enough CO2 and H2O. Just how much we don't really know. By one estimate, illustrated at right, 70 times the present level of carbon dioxide would have kept the surface just above freezing (273 K), and it might have required 700 times as much to reach today's global average of 288 K (59°F).
However, geologic evidence is telling a different story, one that caps the CO2 levels at no more than 50 times today's value. In fact, certain banded iron formations imply that the Archean atmosphere might have had no more than about 900 ppm CO2 — just three times the modern-day levels.
So what else, if not abundant CO2, might have been in the greenhouse formula? That's been a complicated and hotly debated issue. Lots of gas mixtures and alternatives have been kicked around over the years. Until his death in 1996, Sagan championed a mix of ammonia and a little methane — gases that might have proved crucial to the development of life. But ultraviolet sunlight breaks down ammonia very rapidly.
Potent methane would have worked in combination with CO2, but too much of it would have led to smoggy hydrocarbon hazes that would have reflected sunlight away, making Earth cooler. As an alternative to touchy gas formulations, in 2010 a research team led by Minik Rosing (Nordic Center for Earth Evolution) proposed that, since early Earth had less landmass, it was darker overall and absorbed more sunlight.
A key problem all along has been the computer-intensive programming needed to simulate long-ago climates. To get their models to run at all, researchers have had to simplify the computations to a single dimension — a vertical profile from Earth's surface to the top of its atmosphere. But these 1D models can't account for circumstances like the role of clouds or the fact that ice forms first at the poles, rather than everywhere at once.
Modern computational horsepower has finally made 3D modeling possible, if only barely. In July's issue of Astrobiology, Eric Wolf and Brian Toon (University of Colorado, Boulder) describe the results of thousands of hours of number-crunching using the university's Janus supercomputer. "Though we use a complex 3D model, our solution is at its heart very simple," Wolf explains. "We only need CO2 and a touch of CH4. No tricks, no high obliquity, no changes to clouds, land, total surface pressure, or exotic greenhouse species."
Wolf and Toon conclude that about 2.8 billion years ago, when sunlight was 20% weaker, an atmosphere with 1.5% CO2 (about 40 times what we have now) and a dash (0.1%) of CH4 would have produced a "hot", virtually ice-free Earth with mean temperatures close to what they are today.
Reducing the carbon dioxide to just 0.5% and eliminating the methane yields a global mean temperature of 260 K (9° F) — well below freezing. But even at this chilly temperature, the 3D modeling suggests that about half of the open ocean would remain ice free. "One key difference with 1D models is that they assume, automatically, that the whole planet must be frozen if the mean surface temperature dips below 273 K," Wolf says. "In 3D we find that this is not the case — large areas of open ocean can persist."
"The faint-young-Sun problem is solved well enough for the time being," observes James Kasting, a Penn State climate specialist who's been wrestling with atmospheric evolution since the 1980s. "But it's one of those problems that will never really be 'solved,' because the parameters for early Earth — particularly the surface temperature — will always be negotiable."
Interestingly, Sagan's ammonia-enriched mix isn't entirely dead. The ammonia might have been protected from destruction by tiny, fractal-shaped particles in a high-altitude organic haze derived from methane. So, if primordial microbes produced lots of methane, conceivably ammonia could have lingered long enough to warm the surface by a few degrees.
Both the ammonia and methane would have disappeared later on, reacting with the oxygen given off by photosynthetic organisms. That led to a temporary drop in temperatures — and episodes of widespread glaciation — but by then the Sun had apparently warmed enough to sustain Earth's emerging lifeline.