Primordial Haze: a new vision of the early Earth
For people who study the Archean Earth – our world as it was from about 4.0 billion to 2.5 billion years ago – the Sun has always been a problem. More specifically, the young and dimmer Sun that shone down on our planet during the Archean. Three billion years ago the Sun was about 80% as bright as it is now, meaning that Earth received only 80% as much energy and should have been a lot colder… cold enough for more or less non-stop global glaciation. But Earth wasn’t frozen, then. Earth in the Archean was at least as warm as today, because there are plenty of rocks lying around from that era and very few show evidence of glacial activity anywhere. So what warmed the world, back then?
Carbon dioxide is a good guess. There’s a lot of geochemical evidence that the CO2 content of Earth’s atmosphere was much higher three billion years ago, and CO2 is famously a greenhoue gas. The problem is that there probably wasn’t enough CO2 in the Archean to keep the world from freezing, even so. The Archean atmosphere had about as much nitrogen in it as now, and N2 is most of the air now, so it’s likely that the total volume of the atmosphere was the same back then; about 1 bar of pressure at sea level. Based on independent lines of evidence there shouldn’t have been more than about 0.1 bar of CO2 in the air back then…. which is a lot compared with today, but still not enough of a heat blanket to keep an Archean world from icing over. So what gives?
Methane. A new study by Jacob Haqq-Misra and colleagues at Penn State shows that a good case can be made for methane as an important greenhouse gas three billion years ago. At that time there was still no molecular oxygen in the atmosphere (that would come later), but there was plenty of life: bacteria, including the eponymous methanogenic bacteria, which breathe out lots of methane. Today methanogenic bacteria must hide in oxygen-free places like deep mud because oxygen shuts them down… but back then they had free reign to proliferate, living off the waste products of oceans full of photosynthetic bacteria that didn’t produce oxygen.
Methane is a more potent greenhouse gas than CO2 – about 20 times more potent on a per-molecule basis – and in an oxygen-free Archean world it would not have broken down as quickly in the atmosphere, so it could have built up and provided a useful heat-trapping boost to world climate. Haqq-Misra and colleagues examined the ability of methane to trap heat in an anoxic atmosphere that also contained an appropriate amount of CO2, and found that about 1000 parts per million of methane would do the job. That’s a lot of methane… the present Earth’s atmosphere contains about a thousanth as much. But 1000 ppm is a realistic amount given a world ocean of methanogenic microbes and no O2 to spoil things.
And that’s not all. An interesting side-effect of having that much methane in an atmosphere devoid of O2 (or ultraviolet-shielding O3) is that a little bit of that methane would have reacted with incoming solar radiation to form ethane (C2H6), a simple hydrocarbon gas that is also a greenhouse warmer. Ethane production would have been entirely a function of how much methane was around, with more methane creating higher background levels of ethane… and at warmer temperatures more bacteria would have produced more methane, building up more heat in the atmosphere, which would have increased the rate of methane production… and so on… making a runaway greenhouse effect?
Not so fast. Above a low trace amount in air, ethane gas forms haze… a kind of smog, basically, which blocks sunlight and cools the Earth’s surface. So… too much ethane forms haze, cooling things slightly and slowing bacterial methane production, allowing methane concentrations to fade a bit, and ethane with them… letting the Sun back in, and warming things back up.
Ethane haze would have created a kind of global climate thermostat, where haze formation prevented a runaway greenhouse and kept climate stable. This kind of negative feedback loop is great for keeping climate – or anything – stable, because it has a tendency to wobble without falling down… sort of like a Weeble. Perturbations lead to counter-forces that push the system back to normal. Another feedback loop – involving CO2 and rock weathering – also helps to stabilize climate on Earth over geologic time. Feedback loops like that appear to be common in nature, because when they form they tend to persist through self-stabilization. The methane-ethane thermostat would probably still be operating on Earth today, if not for the evolution of oxygenic photosynthesis about 2.5 billion years ago. An oxygen atmosphere destroys methane, given time.
This new study is important because it provides a glimpse of how the ancient Earth worked, under a very different kind of atmosphere and in oceans containing very different kinds of life. It also shows us an example of a young life-bearing planet with a stable climate and a methane-rich atmosphere…. a useful model to look for among extrasolar terrestrial planets, perhaps.