The world has realized the danger of rising carbon dioxide (CO2) emissions. It has, in its own way, been fighting global warming. But what about the long-term fall in oxygen levels and its knock-on effects?

Compared to prehistoric times, the level of oxygen in the earth's atmosphere has fallen by over a third and in polluted cities the decline could be more than 50 percent.

This change in the makeup of the air we breathe has potentially serious implications for our health. Indeed, it could ultimately threaten the survival of human life on earth, says Roddy Newman, who is drafting a new book, The Oxygen Crisis.

So, what's the evidence? About 10,000 years ago, the planet's forest cover was at least twice what it is today, which means forests are now emitting only half the amount of oxygen.

And desertification and deforestation are rapidly accelerating this long-term loss of oxygen sources.

The story at sea is much the same. NASA reports that in the north Pacific Ocean oxygen-producing phytoplankton concentrations are 30 percent lower today, compared to even the 1980s. This is a huge drop in just three decades.

Moreover, the UN Environment Program said in 2004 that there were nearly 150 "dead zones" in the world's oceans where discharged sewage and industrial waste, farm fertilizer run-off and other pollutants have reduced oxygen levels to such an extent that most or all sea creatures can no longer live there.

Professor Ian Plimer of Adelaide University and Professor Jon Harrison of the University of Arizona accept that oxygen levels in the atmosphere in prehistoric times averaged 35 percent compared to only 21 percent today. The levels are even lower in densely populated, polluted city centers and industrial complexes, perhaps only 15 percent or lower.

Much of this recent, accelerated change is down to human activity, notably the industrial revolution and the burning of fossil fuels. Which means we are slowing down one process, oxygen generation, and speeding up another, carbon dioxide production.

Very interesting. But does this decline in oxygen matter? Are there any practical consequences that we ought to be concerned about? What is the effect of lower oxygen levels on the human body?

Surprisingly, no significant research has been done, perhaps on the following presumption: the decline in oxygen levels has taken place over millions of years of our planet's existence.

Surely, this mostly gradual decline has allowed the human body to evolve and adapt to lower concentrations of oxygen? Maybe, maybe not.

The pace of oxygen loss is likely to have speeded up massively because of global industrialization and as a result of the massive worldwide increase in the burning of fossil fuels.

Scaremongering? No. A reason for doomsaying? Not yet. What is needed is an authoritative evidence-based investigation to ascertain current oxygen levels and what consequences, if any, there are for the long-term well-being of our species - and, indeed, of all species.

(China Daily August 15, 2008)