On a timescale of thousands of years, forests come and go. While the deforestation and wildlife extinctions of the past two hundred years are our major focus, it is useful to understand longer timespans.

Background: The rise of forests

Some 20,000 years ago, the ice sheets which covered much of the northern hemisphere reached their maximum size and this point in time is called, appropriately the Last Glacial Maximum.  This condition persisted for thousands of years with sheets of ice 4 kilometers thick over much of Europe and North America. This meant that sea level was 120 meters lower than today.

Then about 16,000 years ago the melt began … but this was, in general, a slow process over thousands of years. Occasionally there would be a rapid burst such as an event called “Meltwater Pulse 1A” by scientists which happened about 14,000 years ago. In this pulse of warming the sea rose perhaps as much as a meter every 20 years for 200 years. As the ice sheets withdrew, the forests returned. The following graph shows sea level rise during the past 24,000 years. The sharp rise of Pulse 1A appears as a jumble of data points (Image courtesy of Robert H Rohde under Global Commons licence).



Background: the rise of humans

Humans and chimps shared a common ancestor about 6 million years ago.  That common ancestor wasn’t a chimp, of course. Since the split, chimps have been evolving for 6 million years just like we have. Our primate ancestry is as fruit eaters but somewhere along the evolutionary trail we became omnivores. This doesn’t mean, as is often supposed, that we need a balanced diet of meat and vegetables. It means that we are unspecialised eaters. We can eat anything. This is obvious today.  Most of the world’s population eats very little meat and high amounts of grains or tubers, but some people eat quite large amounts of meat and we can live almost entirely on almost any diet you care to mention … as long as we get enough food.

Some diets won’t give us a long and healthy life but they will get us to breeding age and beyond.

People can live almost entirely on fruit and vegetables and can live almost entirely on meat. The double appearance of “almost” in the previous sentence signals small exceptions. People in the Arctic circle living almost entirely on meat could die if the meat became too lean. This often happened at the end of winter when the animals they killed carried very little body fat. They needed to boil the bones to extract the marrow to add fat to their diet. The illness is called protein poisoning. Similarly, people living entirely on fruit can become deficient in vitamin B12 and die. This would have been less likely in our ancestors, who would have eaten dirt and insects and other material containing B12, but it is quite possible to be B12 deficient under modern conditions, where everything is clean.

B12 is made by soil bacteria  which also take up residence in some animal guts. So cattle get B12 from the soil bacteria in their rumen and fruit eating primates get it from dirt or insects in their diet. Most of our predecessors on the evolutionary tree of primates were fructivores … meaning they ate mostly fruit.

By about 10,000 years ago the forests were at their peak and we were beginning to develope agriculture and expand as a species.

The story of the subsequent deforestation is told in a remarkable recent book: Deforesting the Earth by Michael Williams. Starting about 8,000 years ago, humans began to deforest the planet and at about the same time carbon dioxide levels in the atmosphere began to rise. Scientists are still debating exactly when deforestation began to contribute to rising carbon dioxide, but it is clear that the deforestation of the last 2 thousand or so years has definitely had an impact (with other parts of the rise being due to non-human factors).

Modern times

About 200 years ago, the scale of deforestation rose rapidly. Australia by 1883 was using fully half the dynamite produced in the British Empire in its attack on Australia’s forests.  From the first settlement through to the past few years, we have ring barked, felled, burned and blasted large areas of the country.

In Australia, about 100 million hectares have been deforested since white arrival. This is a net figure, with some land being cleared multiple times. We crop about 24 million hectares and live on about 1.5 million hectares. This last figure is the extent of our urban occupation of the 770 million hectares of the continent with another 2 million being used for plantation timber. These figures show that about 70 percent of deforestation in Australia has been for livestock. This is very similar to the situation in the Amazon where cattle production is the driving force with 70 percent of previously forested areas of the Amazon now being under cattle and an addtional 10 percent being ex-cattle ranch grasslands that are no longer productive.

Already by the end of the 19th century the expansion of sheep was driving native wildlife to extinction. Twenty four mammal species disappeared from Western NSW alone in the 60 years since the first settlement in 1841.

The imperative to reforest the earth.

Climate change has brought with it an absolute necessity to undo at least the past 200 years or so of deforestation. This reforestation cannot happen without a substantial shift in both agricultural practices and correspondingly in our diets.