Book Review

With ev­ery breath, we in­hale the story of our planet. Sam Kean, au­thor of Cae­sar’s Last Breath, tells us about the re­mark­able history of the air around us

BBC Earth (Asia) - - Contents -

Sam Kean, au­thor of Cae­sar’s Last Breath, tells us about the re­mark­able history of the air around us

Are we re­ally breath­ing in some of Cae­sar’s last breath?

The story goes that in 44 BC in Rome, Julius Cae­sar was as­sas­si­nated by a group of his own sen­a­tors, crum­pling to the floor with a fi­nal gasp. This last breath con­tained around 25 sex­til­lion (that’s 25 fol­lowed by 21 ze­roes) air mol­e­cules, which would have spread around the globe within a cou­ple of years. A breath seems like such a small thing com­pared to the Earth’s at­mos­phere, but re­mark­ably, if you do the math, you’ll find that roughly one mol­e­cule of Cae­sar’s air will ap­pear in your next breath.

And it doesn’t stop there. In the same way, you might cur­rently be in­hal­ing Cleopa­tra’s per­fume, Ger­man mus­tard gas and even par­ti­cles ex­haled by di­nosaurs. What ex­actly are these air mol­e­cules?

Ni­tro­gen and oxy­gen are the main in­gre­di­ents of air, mak­ing up 99 per cent, but that ex­tra 1 per cent is still re­ally im­por­tant. It’s like a glass of wine: most of the wine is al­co­hol and wa­ter, but there are all these ex­tra over­tones and flavours, too. In air, this 1 per cent is re­spon­si­ble for all of global warm­ing as well as all scents and per­fumes. It in­cludes car­bon diox­ide, ni­trous ox­ide (laugh­ing gas), as­sorted pol­lu­tants and vol­cano ex­haust.

The in­gre­di­ents of air re­veal the world’s en­tire history. Some of them have been around since the early days of the planet, while some only arose with the arrival of life, or with hu­man civil­i­sa­tion. Where did our at­mos­phere come from?

We’ve ac­tu­ally had four dif­fer­ent at­mos­pheres in the Earth’s history.

The first was a wispy left­over from our planet’s for­ma­tion, and soon got blown away. The next came from the ground, seep­ing out of cracks in the Earth’s sur­face – mostly car­bon diox­ide and wa­ter vapour, but also gases such as sul­phur diox­ide and hy­dro­gen sul­phide. At­mos­phere num­ber three was dom­i­nated by ni­tro­gen, emit­ted from vol­canic vents in rel­a­tively small quan­ti­ties, but ca­pa­ble of stick­ing around for a long time.

And fi­nally, oxy­gen be­gan to build up in the at­mos­phere thanks to early, pho­to­syn­the­sis­ing life forms. This paved the way for an oxy­gen-rich at­mos­phere that could sup­port com­plex life.

If you wanted to travel back in a time ma­chine to the Earth’s dis­tant past and take a deep breath out­side, you’d only be able to go back a few hun­dred mil­lion years – it’s only very re­cently in our planet’s history that there’s been enough oxy­gen to sus­tain us.

How is the air we breathe chang­ing?

The at­mos­phere is like a liv­ing thing – it’s con­stantly evolv­ing. The rates of car­bon diox­ide and other green­house gases are in­creas­ing, and the air is more ra­dioac­tive now be­cause we’re still deal­ing with fall­out from 1950s nu­clear weapons tests.

We also see a lot more com­plex, hu­man-made mol­e­cules in the air to­day. If aliens were to look at our planet’s at­mos­phere, the pres­ence of these gases would be a good sign that the Earth har­bours life. Like­wise, the next gen­er­a­tion of tele­scopes should al­low us to start look­ing for these com­pli­cated gases in the at­mos­pheres of dis­tant ex­o­plan­ets, help­ing us to search for the best can­di­dates for ex­trater­res­trial life.

I think it’s also in­evitable that we’ll find an ex­o­planet with a great mix of gases for us to sur­vive on. The hard part will be fig­ur­ing out how to get there.

What other in­ter­est­ing sto­ries did you un­cover about the air?

One of my favourites in­volves Charles Dick­ens. In his 1853 novel Bleak House, a char­ac­ter called Krook ap­pears to spon­ta­neously com­bust, turn­ing into a pile of ash. At the time, sci­en­tists were be­gin­ning to fig­ure out how breath­ing works, and they were also start­ing to make con­nec­tions be­tween burn­ing and the oxy­gen in air. So some peo­ple thought that if we have a lot of oxy­gen in our bod­ies, maybe we could spon­ta­neously ig­nite. It’s pos­si­ble that these ideas in­flu­enced Dick­ens, along with eye­wit­ness ac­counts. But now, of course, we know it’s not pos­si­ble. The body is up to 75 per cent wa­ter, and even the worst fever doesn’t get us hot enough to start a fire.




Julius Cae­sar has been dead for 2,061 years, but the air he breathed is still in circulation

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