While (unsuccessfully) looking for an original, full source for Lord Kelvin's dire predictions of an end to breathable air, I found an interesting article from 1901-10-09 in New Castle News, quoted at length from a "London letter". It paints a disturbing picture of the vapory, frog-ruled world that mankind must survive in after peak oxygen is reached:

DESTROYING OXYGEN.

Will the human race and all animal life soon be left without air for breathing? will the world come to an end in the general asphyxiation of every living thing?

Lord Kelvin, the greatest authority today in mathematical physics, asserts that the oxygen supply of the world will be exhausted within the next five centuries.

Oxygen is the real force of the atmosphere so far as man and nearly all air-breathing animals are concerned.

Lord Kelvin has sounded an alarm which has created more discussion in scientific circles than any other pronouncement since Darwin put forth his 'Origin of Species.' No satisfactory reply has so far been offered. It is admitted that, theoretically, the oxygen in the atmosphere is diminishing. Every bucketful of coal in a furnace and every stick of wood in a cook stove burns up a portion of the world's supply of breathing air. How long will the oxygen hold out?

Is there any way in which the extravagant waste of the world's atmosphere can be checked?

Lord Kelvin's conclusions were stated in a lecture recently delivered before the British association for the promotion of science. He has made a study of the subject for many years. He is now past middle age, and ranks as the foremost living physicist.

The following is a summary of the important points of Kelvin's theory:

'The extravagant waste of oxygen by modern manufacturing processes may leave the inhabitants of the earth without air for breathing, and that within a short and calculable time. At the present rate of progress five centuries will exhaust the full supply of the world. This means the exhaustion of oxygen.

'The sum total of oxygen at our disposal is 1,020 millions of tons. Every ton of fuel used three tons of oxygen in combustion. Consequently the burning of 340,000,000 of tons of combustibles will destroy the world's air for breathing. The population of the earth is 1,500,000,000 persons. Each has to his credit 200,000 tons of combustibles. Burn this and we die, not from lack of fuel for keeping warm, but from lack of oxygen for breath. Considering the rate at which manufacturing and commerce are depleting the coal supply, less than 500 years may see the end of the human race.'

Science has rarely offered so strange and so terrible a picture of the end of the world as Lord Kelvin's theory suggests. From various scientific authorities in New York (Hallock, Woodward, Hovey, Van Ingen, Burgess and others) interesting speculation as to the gradual approach of the final catastrophe has been gathered.

With the decrease of oxygen in the air the heat of summer would become intense. This would not be the pitiless, parching heat of the desert. Moisture would hang heavy in the air. Steam would rise from the ground and the sun would be veiled in clouds of vapor.

Plants would spring up and flower in a day and trees grow almost in a night. With time for adjustment, the very luxuriance of vegetation would clear the air again and furnish breath to famished life.

But with the swift rush of Kelvin's calculations the mischief will have been accomplished in three centuries. Alarm will spread too late. As oxygen becomes precious the entire human race will strive madly for some means of increasing it. Every man will conserve his strength, because muscular effort requires the expenditures of much oxygen. Factories will not smoke any longer.

Huge electric plants will distill the seas into air. The banks of the ocean will be crowded with the humanity that comes to it to turn it by alchemy from water to breath. Every year the waters will recede under the drain of electrolyting process.

Man will become more puny with each generation. Death will confront the race and pride of power and trade and achievements in art and learning will give way to a desperate struggle for life.

Certain animals, on the other hand, will thrive apace. Huge and brilliant fishes will swim the sluggish streams. Serpents will grow to monstrous sizes and great frogs will croak in the swamps. All the lower nature may reach its flower again before the death of man, as it did before his birth.

The sturdiest of the human species will survive longest. Scarcely on the last day will the last men be able to distinguish the faces of each other in the thick vapor. They will move about in the dense atmosphere with slower and slower steps. A torpor will creep over them and they will die.

Professor William Hallock, department of physics and secretary of the faculty, Columbia university, asked about Lord Kelvin's theory, said:

'Lord Kelvin's contention rests upon a sound basis. It is true that modern manufactories are consuming fuel in larger amounts than the processes of nature now produce it.

'This combustion locks up practically that portion of the world's oxygen which was freed originally in the slow formation of this fuel through unknown ages.

'If we continue to use up our known supply of oxygen at the present rate, without in some way getting the stock reinforced, then our descendants must die of asphyxiation. But nature may in some hidden way discover a means of increasing the supply of oxygen. There may be sources of supply yet unknown to us. Man may invent an artificial process of freeing oxygen from its combinations. Or, lastly, man may become a cold-blooded animal and capable of existing upon an infinitesimal supply of oxygen.

'Vegetation upon the earth would probably have to be swept away before our supply of breathing air gives out. In that case it is a problem whether man would not starve to death before asphyxiation came upon him. Personally I do not anticipate any such catastrophe. It is one of those things interesting to speculate upon, because it is remote enough not to alarm us a great deal.

'One can always hope that something will happen before worst comes to worst.

'As animal life is now constituted it cannot live without oxygen. Vegetation, on the other hand, lives upon carbonic acid gas, which is useless to animals. This forms the main distinction between animal and vegetable life. Each supports a laboratory which works for the subsistence of the other.

'Fish and other cold-blooded animals live on an infinitesimal amount of oxygen. They use it only in muscular effort. Their body heat is the same as that of the element in which they live. Man, on the other hand, is not content with enough oxygen for this. He lives in a mean animal temperature of 98 degrees. he uses up a wasteful amount of oxygen in keeping his body temperature at 98 degrees. It is quite within the range of possibility that evolution may change this.'

UPDATE 2007-06-22: I have found a reference to Kelvin's original paper ("on the Fuel and Air Supply of the Earth", read at a meeting in 1897) that started all this, but it apparently was never published in full. For more details, read the "On the End of Free Oxygen" page.