A major question in astrobiology is how we'll measure and interpret the atmospheric composition of any Earth-analog worlds we find out among the exoplanets. We pretty much know the technical requirements: big telescopes, excellent spectroscopic instruments, the right nearby planetary targets, and a whole lot of patience. But the interpretation piece is largely unexplored country.
We can certainly model the heck out of hypothetical atmospheric chemistries - with climate simulation, photochemistry, and even abstract models of biospheres. The trick is disentangling all of that three-dimensional, diurnal, seasonal, and chemical variation. Sometimes there is significant 'degeneracy', where similar, astronomically derived data (like a spectrum of starlight filtered through a planet's atmosphere) would present itself in quite different physical circumstances. Oxygen for example can be produced by life, but it can also be produced by non-biological atmospheric chemistry.
Carbon dioxide is another tricky compound. A planet can fill its atmosphere with CO2 from volcanoes, for example. It can also show strong seasonal variation in this gas. Both Earth and Mars do this, but for very different reasons. On Earth it's the biosphere that drives an annual oscillation in CO2 concentration. On Mars it's the thawing and freezing of CO2 at polar regions that causes its atmospheric density to vary over a year.
But Earth has also been increasing its greenhouse gas concentration pretty steadily over the past couple of hundred years. This is almost entirely attributable to the burning of fossil fuels (we can tell because of the isotopic mix of carbon in those fuels).
We've all read the reports, or seen the media discussion. Except it can be a little hard to grasp the sheer magnitude of what our species has accomplished. Dire warnings that we've crossed 400 parts-per-million of atmospheric CO2 (a level last seen tens of millions of years ago) may sound pretty bad, but it takes a little intellectual imagination to really grasp what's going on.
Let's give that a shot.
Current data (from direct measurements of the atmosphere to historical records of industry) tells us that between 1751 and 1987 fossil fuels put about 737 billion tons of CO2 into the atmosphere. Between just 1987 and 2014 it was about the same mass: 743 billion tons. Total CO2 from industrialized humans in the past 263 years: 1,480 billion tons.
Now, let's relate that to something a bit easier to visualize. A coniferous forest fire can release about 4.81 tons of carbon per acre. At the low end, about 80% of that carbon comes out as CO2. In other words, to release an equivalent CO2 mass to the past 263 years of human activity would require about 1.5 billion acres of forest to burn every year during that time.
That's 6 million square kilometers of burning forest every year for more than two centuries. That's a square patch about 2,450 by 2,450 kilometers, or about 1,500 by 1,500 miles. Here's what that'd look like:
Except that is for an average output, spread across 263 years. Estimates of today's CO2 production go as high as about 40 billion tons per year. That'd take something like ten billion acres of forest burning each year, which is about 42 million square kilometers. The entire continent of Africa is a mere 30 million square kilometers. So this, plus another third, on fire, each year:
Now here's the thing. If that was actually happening, if a continental forest area 30% larger than Africa burnt to a crisp each and every year, we might be a bit concerned. Yes, a raging fire of 42 million square kilometers has some additional, immediate hazards above and beyond what comes out of a car exhaust or a power plant. And yes, the total mass of emitted CO2 does not reflect the total amount of absorbed CO2 in a year or the net increase. But wouldn't we all be a little concerned about the impact on the global system?
Let's also, for a moment, do another thought experiment. You're a long-lived, budding astrobiologist on the planet Proxima Centauri b. You've just spent several hundred Proxima b years using that world's largest space telescope to take a spectrum of the light from an intriguing planet about 4 light years away. Some unknown process on this planet seems to be persistently adding carbon dioxide to its atmosphere. Seasons come and go, but the composition of the planet is changing during your own lifetime.
Your conclusion? If there's life on that world, something potentially catastrophic is happening to it.