We face an ecological crisis. The Industrial Revolution marked the start of massive human use of natural resources especially fossil fuels. Energy stored from several hundred million years of plant growth began to be used at roughly a million times the rate it had been formed. There was a euphoric (and totally unsustainable) surge of growth of both population and industrial production.

Meanwhile, carbon released into the atmosphere from the destruction of forests and burning of fossil fuels began to duplicate the conditions which led to the five geologically observed mass extinctions, during each of which more than half of all living species disappeared forever.

Industrialism and the rapid development of some kinds of science and technology have given people in some parts of the world a 200-year period of unbroken expansion and growth, but today this growth is headed for a collision with a wall-like barrier - the limits set by the carrying capacity of the global environment, and by the exhaustion of non-renewable resources.

Encountering these limits is a new experience for the industrialized countries. By contrast, pre-industrial societies have always experienced limits. Pre-industrial societies have already learned to live in equilibrium, in harmony with nature. Now the industrialized world too must soon replace the economics of growth with equilibrium economics.

It is assumed by many people in the industrialized North that if the developing countries would only learn mass production, modern farming techniques and a modern lifestyle, all would be well. However, a sustainable global future may require a transfer of knowledge, techniques and attitudes in precisely the opposite direction - from pre-industrial societies to highly industrialized ones.

The reason for this is that the older societies have cultures that allow them to live in a sustainable way, in harmony with nature. This is exactly what the highly industrial North (and the industrialised rich in otherwise pre-industrial societies) must learn to do.

How can we avoid the ecological megacatastrophe that is currently threatening both human society and the biosphere? How can we achieve a stable and sustainable global society? Voices from those who have thought deeply about the problems can help us. Here are a few of the things that they said:

Because the global ecosystem is a connected whole, in which nothing can be gained or lost and which is not subject to overall improvement, anything extracted from it by human effort must be replaced. Payment of this price cannot be avoided; it can only be delayed. The present environmental crisis is a warning that we have delayed nearly too long. —Commoner

Simplify your life. Don’t waste the years struggling for things that are unimportant. Don't burden yourself with possessions. Keep your needs and wants simple and enjoy what you have. Don’t destroy your peace of mind by looking back, worrying about the past. Live in the present. Simplify! —Henry David Thoreau

Among the herbicides are some that are classified as 'mutagens,' or agents capable of modifying the genes, the materials of heredity. We are rightly appalled by the genetic effects of radiation; how then can we be indifferent to the same effect in chemicals that we disseminate widely in our environment? —Rachel Carson

Despite the dazzling successes of modern technology and the unprecedented power of modern military systems, they suffer from a common and catastrophic fault. While providing us with a bountiful supply of food, with great industrial plants, with high-speed transportation, and with military weapons of unprecedented power, they threaten our very survival. —Commoner

The women we worked with recounted that unlike in the past, they were unable to meet their basic needs. This was due to the degradation of their immediate environment as well as the introduction of commercial farming, which replaced the growing of household food crops. But international trade controlled the price of the exports from these small-scale farmers and a reasonable and just income could not be guaranteed.

I came to understand that when the environment is destroyed, plundered or mismanaged, we undermine our quality of life and that of future generations. So, together, we have planted over 30 million trees that provide fuel, food, shelter, and income to support their children's education and household needs. The activity also creates employment and improves soils and watersheds. Through their involvement, women gain some degree of power over their lives, especially their social and economic position and relevance in the family. This work continues... —Wangari Maathai

The climate is a common good, belonging to all and meant for all. At the global level, it is a complex system linked to many of the essential conditions for human life. A very solid scientific consensus indicates that we are presently witnessing a disturbing warming of the climatic system. In recent decades this warming has been accompanied by a constant rise in the sea level and, it would appear, by an increase of extreme weather events, even if a scientifically determinable cause cannot be assigned to each particular phenomenon…

The problem is aggravated by a model of development based on the intensive use of fossil fuels, which is at the heart of the worldwide energy system. Another determining factor has been an increase in changed uses of the soil, principally deforestation for agricultural purposes. —Pope Francis

If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 parts per million to at most 350 ppm, but likely less than that. —James Hansen

Given the urgency of the climate crisis, we also quickly saw the need to move beyond education to confrontation - hence, in the US, the birth of the continent-wide Keystone pipeline fight. There was already a movement in place in the tar sands of Alberta and on the prairies of Nebraska through which the proposed pipeline would pass. But we nationalized the movement, with demonstrations in DC and pressure on President Barack Obama. So far, the pipeline remains unbuilt. Every project like this around the world (eg, fracking wells, coal ports, LNG terminals) is a target for opposition. We may not always win, but we always make life harder for the industry. —James Hansen

If our starting point is a respect for nature and people, diversity is an inevitable consequence. If technology and the needs of the economy are our starting point, then we have what we are faced with today - a model of development that is dangerously distanced from the needs of particular peoples and places and rigidly imposed from the top down. —Helena Norberg-Hodge

The old culture reflected fundamental human needs while respecting natural limits. And it worked. It worked for nature, and it worked for people. The various connecting relationships in the traditional system were mutually reinforcing, encouraging harmony and stability. —Helena Norberg-Hodge

Everything is now for sale. Even those areas of life that we once considered sacred like health and education, food and water and air and seeds and genes and a heritage. It is all now for sale.

We are committed with our lives to building a different model and a different future for humanity, the Earth, and other species. We have envisaged a moral alternative to economic globalization and we will not rest until we see it realized. —Maude Barlow

The future of life on earth depends on our ability to take action. Many individuals are doing what they can, but real success can only come if there's a change in our societies and our economics and in our politics. Surely we have a responsibility to leave for future generations a planet that is healthy, inhabitable by all species. —David Attenborough

Our house is on fire. I am here to say, our house is on fire. According to the IPCC, we are less than 12 years away from not being able to undo our mistakes. In that time, unprecedented changes in all aspects of society need to have taken place, including a reduction of our CO2 emissions by at least 50%.

No other current challenge can match the importance of establishing a wide, public awareness and understanding of our rapidly disappearing carbon budget, that should and must become our new global currency and the very heart of our future and present economics We are at a time in history where everyone with any insight of the climate crisis that threatens our civilization - and the entire biosphere - must speak out in clear language, no matter how uncomfortable and unprofitable that may be. We must change almost everything in our current societies. —Greta Thunberg

Edited extract from John Scales Avery’s freely downloadable ebook Lives in Ecology.