Before we get too deep into this article, it is worth recalling that our civilization is preparing to leave the fossil fuel business far behind and that we will use the next one thousand years of so to rebuild forests in the worst deserts. In time mature forests will even drive up the available oxygen.
All that fossil fuel will be reformed as large trees as far as the eyes can see. I can even imagine deliberately burning fuels deliberately to provide extra CO2.
At least we are able to measure a slight change in the right direction as predicted.
One last comment. The last paragraph observes evidence of charcoal resulting from forest fires because of high oxygen. Not so fast here. The charcoal is evidence of a lack of oxygen which allows production of charcoal. Our forest soils are bereft of charcoal because all the fuel was burned out. There was obviously plenty of oxygen
O2h no! Is our oxygen running out?
21 July 2010 by Stephen Battersby
I HAVE been feeling a little short of breath on mountain paths lately, and it took me ages to light the barbecue last week. I wonder why? Perhaps it's something to do with a lack of exercise, an excess of beer, some damp charcoal... But wait, there is a much simpler explanation: these days, there isn't enough oxygen in the air.
So say many websites - sites that just happen, by wild coincidence, to sell solutions to this atmospheric catastrophe in the form of "liquid oxygen" supplements, personal oxygen generators, even oxygen-laden skin-creams. As New Scientist's Feedback column has noted over the years, these snake-oil salesmen exploit a bafflingly persistent myth, that industrial activity has sucked much of the oxygen out of the air.
Some claim that a couple of hundred years ago the "natural" level of oxygen in the atmosphere was 38 per cent, others that large cities such as Tokyo, Japan, now have oxygen levels as low as 5 per cent. In fact, the oxygen content of the air worldwide is about 21 per cent, the same as it was for the Victorians, the Romans and the Neanderthals. All those claims that the sky is falling are utter nonsense. Well, almost.
Measurable decline
It is true that human activity is causing a steady, measurable decline in the oxygen content of the world's air, although as yet this decline is negligible. But if we continue to burn fossil fuels for centuries more, levels could fall much further. Should we be worried?
Since 1989, Ralph Keeling has been monitoring oxygen levels around the globe. As well as continuous measurements at permanent monitoring stations, flasks of air are captured in some of the wilder parts of the planet, such as Cold Bay in Alaska and Cape Grim in Tasmania, and sent to Keeling's lab at the University of California, San Diego, for analysis.
Originally, Keeling measured the speed of light in the air samples with a laser beam. Because light moves faster in oxygen than nitrogen, this reveals the oxygen content. Now he and his collaborators use several methods, including mass spectroscopy and ultraviolet probes. Some of these techniques are being used on board a plane that is zigzagging from pole to pole. All the methods give the same result: the concentration of oxygen is declining everywhere at the same rate, about 20 parts per million (ppm) per year. In other words, for every million molecules of O2 in the air in 1989, there are now only about 999,600.
This fall comes as no surprise. When you burn a hydrocarbon fuel such as oil, its hydrogen and carbon atoms combine with oxygen from the atmosphere to create water and carbon dioxide. As we drive up levels of CO2 by burning fossil fuels, we also deplete oxygen.
Brief bonanza
In fact, Keeling's measurements have shown that oxygen is declining less rapidly than expected, probably because plants are enjoying a brief bonanza. As they have exploited the higher levels of CO2, the total amount of biomass on the planet has increased, and in the process extra oxygen has been pumped out.
This probably won't last. Studies of Earth's past suggest that the total biomass will soon stabilise - or more likely start to decline. That means the drop in oxygen will depend largely on how much more fossil fuel we burn.
Say we guzzle all the easily accessible fuels - all the coal, oil and gas that can be economically hacked or pumped from the Earth today. In total, that is estimated to be roughly 1200 billion tonnes of carbon, mostly in the form of coal. Burning the lot would mean we lose 3600 ppm of our oxygen, so the level would fall from today's 20.95 per cent of the atmosphere down to 20.87 per cent.
Hardly suffocating, then, but that may not be the end of the story. The Earth holds other, less-accessible fuels, such as tar sands, and a more exotic possibility in the form of icy methane hydrates. In the pessimistic "A2 scenario" looked at by the Intergovernmental Panel on Climate Change, a world undergoing slow technological change but high population growth burns 3700 billion tonnes of carbon over the next two centuries. This would translate to a loss of about 1.1 per cent of our present stock of oxygen - down to 20.7 per cent of the atmosphere.
Natural lows
To get a feel for what this would mean, bear in mind that the amount of oxygen in a given volume of air changes with atmospheric pressure, and the local pressure changes with the weather. When it is raining, one lungful of air will often have a few per cent less oxygen than on an average day - a larger reduction than in our pessimistic scenario above. Or you could get much the same reduction by climbing from sea level up a hill just 100 metres high. In other words, a fall to 20.7 per cent would make hardly any difference.
When it rains, each lungful of air contains less oxygen than normal
Estimating fuel reserves is far from a precise science, however. In fact, there is undoubtedly much, much more buried carbon than any estimates of fossil fuel reserves suggest - more than enough to consume every last bit of oxygen in the atmosphere if it was burned (see "Suffocating saurians"). Perhaps fortunately, the vast majority of this carbon is spread very thinly, forming only a very minor ingredient in rocks. It is useless as fuel, because it would take more energy to extract than would be gained by burning it.
Still, according to some analyses as much as 25,000 billion tonnes of carbon might be recoverable. These estimates are based largely on old figures for the total amount of methane hydrates that are now thought to be too high, but let's be extremely pessimistic, and assume that they are right - and that we are ingenious enough and unwise enough to extract and burn all this carbon, or trigger its release. That would consume nearly 8 per cent of our oxygen, causing levels to drop to just 19.4 per cent of the atmosphere. Surely that would cause serious problems?
Peak performance
Probably not, says physiologist Mike Grocott of University College London
Athletes would find it harder to break records, says Grocott, and climbing high mountains will become a little more difficult. "Everest is already on the threshold of what's possible without supplemental oxygen. It might become impossible for most people."
Somewhat more seriously, people living in parts of the high Andes and the Himalayas would find life even tougher. Physical labour would become harder, for instance, and infant mortality would increase. That would be worrying - if it weren't for the vastly greater peril of extreme climate change caused by burning all that carbon. With the ice caps rapidly melting, today's coasts being inundated and the tropics turning into desert, the least of the world's worries will be a few wheezing yaks.
Real crisis
While there is far more reason to worry about rising CO2 than falling O2, we could still create a real oxygen crisis - not on land, but in the oceans. As atmospheric oxygen levels fall, less oxygen will dissolve in seawater, depleting levels slightly. Worse still, the warmer seawater gets, the less oxygen it can hold.
The effect of both these processes has been modelled by Gary Shaffer and his colleagues at the Danish Centre for Earth System Science in Humlebaek. In their worst case, based on the A2 scenario, oxygen levels in the top 500 metres of the oceans could drop by more than 20 per cent by the year 4000 (Nature Geoscience, vol 2, p 105).
"Dead zones" with almost no oxygen already make up around 2 per cent of the oceans by area, and they could expand seven-fold. "Fish can swim away, but the area of ocean they inhabit will become smaller," says Shaffer. Some species are more tolerant to low oxygen levels, and of course marine mammals and seabirds get their supply from the atmosphere, but they would all fall victim to an indirect menace. Low oxygen levels encourage the growth of bacteria that destroy nitrate - a vital nutrient for the ocean's microscopic plants, or phytoplankton. The phytoplankton are the base of the main marine food chain, eaten by zooplankton, which are eaten by fish, and so on. Take them away and the whole ecosystem collapses.
As if that weren't bad enough, the bacteria that thrive in low oxygen conditions emit a powerful greenhouse gas, nitrous oxide. If we suffocate the sea, it might take its revenge.
Suffocating saurians
Three hundred million years ago, dragonflies and millipedes grew to frankly disturbing sizes - thanks in part to levels of oxygen in the atmosphere as high as 30 per cent. What created this abundance, and what brought oxygen back down to today's less heady levels?
It is all to do with the fate of plants. Plants emit oxygen while they are photosynthesising, but the same amount of oxygen is used up if the carbon compounds they make are broken down. To boost oxygen in the long term, organic matter must be buried beyond the reach of hungry bugs and beasts. Most of the oxygen in our air came from plants whose remains - or those of the animals that fed on them - became entombed in sedimentary rocks.
As the crust shifts, however, it can bring sediments containing ancient corpses back to the surface, where they decompose, removing oxygen from the atmosphere once more. Gases from organic material buried deep in the Earth can also escape to the surface, where they react with oxygen. So oxygen levels can go down as well as up, depending on the balance between burial and exhumation.
When plants conquered the land they found new places to grow and new ways to be buried, gradually boosting atmospheric oxygen. This eventually resulted in the oxygen-rich atmosphere of the Carboniferous period around 340 to 280 million years ago. "The Carboniferous had lush swampy forests, perfect for burying carbon" says Tim Lenton of the University of East Anglia in Norwich , UK
Later, the continents moved together to create large, dry supercontinents much less hospitable to verdant plant life. The balance swung against burial, and oxygen levels fell.
What happened next, during the age of the dinosaurs from 230 to 60 million years ago, is more controversial. According to Lenton and his colleagues, oxygen levels increased once more before falling to present day levels. However Robert Berner of Yale University
So did the dinosaurs have to breathe heavily despite their efficient bird-like lungs, or could they take leisurely sniffs of the air? The jury is still out, though fossil charcoal shows that forest fires were common at the time. This would be difficult to explain if oxygen levels fell below 15 per cent, as Berner's model suggests, because fires should not occur below this level.
No comments:
Post a Comment