Human socioeconomic systems have evolved over thousands of years in concert with a changing natural environment. Every society is sustained by flows of energy and materials and can, thus, be seen as having a ‘socioeconomic metabolism’ akin to the physical metabolism of the human body. A historical perspective using these concepts can help us to understand not only the critical role played by energy in the evolution of human civilisation, but also humanity’s current predicament.

A specific type of socioeconomic metabolism supports a certain ‘mode of subsistence’ – a form of societal organisation characterised by distinct patterns of population growth, consumption of resources, economic systems and institutions, technologies and human culture. Social scientists identify three primary modes of subsistence from human history: hunter-gatherer, agrarian and industrial. A transition from one mode to another could be driven by socioeconomic factors or environmental conditions, or by interactions between them.

Hunter-gatherer societies are generally dated back to the emergence of Homo sapiens roughly 100 000 years ago. Their primary energy source was passive solar energy captured indirectly through photosynthesis, that is, through ingesting plants and animals that ate plants. This solar energy stored chemically in food was supplemented by a small amount of wood for cooking and heating.

Population size was limited to the local carrying capacity of the land and remained very small. Social organisation consisted of nomadic bands and small tribes possessing few artefacts and having very little division of labour. Tribes were very egalitarian because survival depended on everyone hunting or gathering food.

The ‘agricultural revolution’, which ushered in the agrarian mode of subsistence, began about 10 000 years ago – at the end of the last Ice Age – and involved the domestication of several plant and animal species. The socioeconomic metabolism relied chiefly on the deliberate management of solar energy through agricultural crops, with wood – as well as a bit of water and wind power – again playing a supportive role.

Agriculture gave rise to settled populations that gradually grew over time on the basis of raised food productivity. Permanent settlements and agri- cultural surpluses gave rise to more complex societies involving division of labour, such as the emergence of a ruling class, a military and artisans. But the vast majority of the population was still engaged in food production.

Many agrarian societies in the past ran into problems when practices such as irrigation gradually resulted in topsoil loss, land salinisation and declining productivity, or overharvesting of forests changed local climatic conditions. Most agrarian empires – including the Egyptian, Roman and Mayan – eventually collapsed.

The industrial era began in England around 1750 with the advent of new economic institutions and technologies, such as factory production and steam-powered engines. Fossil fuels emerged as the principal power source for industrial economies, beginning with coal in the eighteenth and nineteenth centuries and followed in the twentieth century by oil and natural gas. Today, over 80% of the world’s primary energy supply is derived from these nonrenewable resources.

Use of these abundant and dense fuels enabled prolific growth in the human population – from around one-billion in 1800 to over seven-billion today. From 1900, increasing use of fossil fuels in agriculture and transport enabled rapid urbanisation, as fewer people were required to produce the agricultural surplus. In most industrialised nations, over 80% of the population live in cities.

Cheap fossil fuels have allowed ever-growing extraction and use of other raw materials. The industrial economy has been spectacularly successful at raising living standards by producing diverse goods and services, albeit for a fraction of the world’s population – inequality has risen dramatically. Breathtaking technological advances, expanding transport and communication networks and cultural changes have greatly enhanced the complexity of human society.

But all this progress has come at a very high price for the natural environment, which, ultimately, supports our civilisation: resources are being depleted and environmental ‘sinks’ are being overwhelmed by our prodigious waste streams.

The industrial metabolism cannot persist indefinitely because the fossil fuels that power it are depleting. Already it seems that conventional oil production has passed its all-time peak, and there is a big debate about how long production of newly exploited unconventional oil resources can offset depletion of the easy oil – and the cost at which this can be done. Several studies suggest that global coal production could peak around 2030, and natural gas a decade or so later.

Further, climate scientists are telling us we are rapidly approaching the point of no return in terms of carbon emissions and their destabilisation of the global climate.

Over the coming decades, humanity will either make a successful transition to a new, more sustainable mode of subsistence, based on renewable-energy sources, or our civilisation will follow in the footsteps of its predecessors and collapse to a lower level of complexity. Future columns will explore the nature and requirements of the sustainability transition.