By Professor Steven Sherwood
UNSW Climate Change Research Centre

As populations grow, practices that were once sustainable cease to be so. Using rivers to carry away raw human waste, for example, is sensible until the population living in the settlement reaches a point where the river’s natural cleansing mechanisms cannot keep up any more.

One can imagine the battles between those pointing to dead fish and arguing for pit toilets, and traditionalists claiming that building so many pits would be too costly, and that the dead fish are a divine punishment having nothing to do with human waste which is after all plant food. Eventually, of course, in Australia and other developed countries, we did build pits, then sewers and sewage treatments plants. We did not really have a choice, and certainly don’t regret it.

Population growth brings economic benefits through economies of scale, knowledge development and transfer, and economic diversity, but some of the proceeds need to be responsibly reinvested to help to ease our footprint on nature as the body politic grows heavier. More importantly, we have to continuously reevaluate norms of behaviour. We often do not act until after the environmental damage becomes obvious – when we see dead fish in our rivers, for example. As a consequence we often suffer temporary economic setbacks, from disease or other ill-effects, but usually the natural system we have abused recovers and we move ahead - lesson learned.

Today we face a new and more difficult pollution problem: as the human population grows it is producing ever-increasing amounts of carbon dioxide by burning fossil fuel deposits that have been in the ground for hundreds of millions of years.

In the millennia prior to the industrial revolution, the amount of carbon dioxide in the air remained fairly constant, enough to make a pure layer 2.2 metres thick. We have now increased it to about three metres and are emitting enough to add over two more centimetres per annum (although about half of that seeps into oceans, soils and plants), a growth rate that is itself accelerating. The thicker this layer gets, the more effectively it insulates the surface of the earth from the cold of outer space, and the warmer the planet’s surface must become.

Calculations done by top scientific and academic organisations all around the world, including a few by staff here in the UNSW Climate Change Research Centre, show that continued emissions growth will lead by late this century to global temperatures that have not been seen on Earth since well before humans evolved from lower species. These calculations are supported by evidence from past warm episodes in Earth’s geologic history.

The consequences of this trend are very hard to predict, but will be made worse by the fact that it will be difficult for either human-built cities, or what is left of the natural biota, to shift in response to the almost certain shifts in precipitation, sea level and summertime heat stress. Here in Australia we are particularly vulnerable to all three, as we largely cling to the coasts of a generally dry continent with hot summers. We also will increasingly be seen as a haven for those fleeing from other nations to the north where political or, increasingly, climatic conditions are inhospitable. Humanity is conducting an unprecedented experiment caused by what may become the largest belch of carbon into Earth’s atmosphere ever in a single century. At present, Australians are the highest per-capita contributors to this among developed nations.

This is not the first time human societies have faced a problem like this, but it is almost certainly the first time we have experienced one on such a global scale. Unfortunately it will take hundreds of thousands of years for natural geologic processes on Earth to wash away the carbon we’ve put in, and we can’t just move up the river. So we really have only one go on this one.

Continued population growth obviously presents severe challenges for avoiding massive climate change in the future. To cut global emissions in half within a few short decades would be hard enough without population growth, but becomes significantly harder if we expect a couple billion more people, and harder still if all of them wish to approach current western living standards. It seems clear that no stone can be left unturned in finding ways to improve energy efficiency through conservation, changes to the design of both products and cities, and of course, decarbonising our energy sources as fast as we can. Even if all these things are done climate changes will grow more obvious as the years go by, and a greater population will mean more people suffering the consequences.


Climate scientist Professor Steven Sherwood says humanity is conducting an unprecedented experiment by polluting their atomsphere with vast quantities of greenhouse gases.

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"We really have only one go on this one."