Climate change may well prove to be the single issue that forces us all to face the challenge of population. We cannot have more people, living longer and getting richer and expect the amount of emissions we produce to go down.
Used as we all are to hearing about habitats lost, species endangered, shrinking biodiversity and plunging numbers of wild animals, the 2016 Living Planet Report by WWF and the Zoological Society of London is still shocking.
Among countless terrifying facts and statistics, one stands out: the current rate of extinctions is 100 times what would be considered normal without the impact of human activity.
The root cause of all these problems is what our one species has done and continues to do to the natural environment on which every species depends. Put simply, more of us means more of that.
Each new human being arriving on Earth does not fill a niche already carved out for them and they don't bring a suitcase full of resources with them - they must elbow their way onto the planet and claim a share of what it has to offer.
We add 80 million people to our planet every year, another billion roughly every 15 years. The reduction by more than half of animal populations since 1970 can be mapped onto another graph - a doubling of the human population.
Exponential growth on a finite planet?
One driver of population growth is good new: global life expectancy at birth is estimated to have risen from 46 years to 72 years over the last six decades. Lifespan is projected to rise further for all and among the least developed countries, is expected to increase from around 64 years today to 72 years by 2050.
Today, too, our fertility rate is the lowest it has ever been - falling from 4.5 children per woman in the 1970s to 2.5 children per woman today - but there are a billion more of us than there were less than a generation ago and every four people still produce five new ones.
Our planet will get no larger, our non-renewable resources can only get smaller and our oceans, air and land have only a finite capacity to fill our needs and absorb the gases, waste and poisons we produce.
The impact comes not just from our numbers: people moving out of poverty inevitably consume more, while the affluent consume more than we need. A single Briton produces more CO2 than 30 people from Sierra Leone. Ending global poverty is a moral imperative but how many human beings there are and how much we take in addition to what we need are matters of choice.
The solutions: education, poverty reduction, reproductive health
Limiting our numbers will reduce the devastation we are inflicting on the natural world and make more of the Earth's finite resources are available to all of us. It needs to be done and it can be done - not through coercion but through doing what we need to do anyway.
We know that education works. In developing countries that invest in female education, women choose to marry later and have fewer children. Nigeria has one of the world's highest fertility rates - 5.7 children per woman. Each year of school attendance cuts the Nigerian birth rate by one tenth. Even here in the UK, we know that women with lower educational levels are more at risk of unplanned pregnancy.
We know that tackling poverty works - while an increase in national wealth does not guarantee a reduction in birth rate, the exceptions are vanishingly small.
We also know that contraception works. Overwhelmingly, women empowered to control their fertility choose to do so. Access to contraception is associated with greater gender equality, greater educational attainment, a reduction in child and maternal mortality, a reduction in the health care costs arising from unsafe abortions - the list goes on.
Every $1 invested in sexual and reproductive health and unmet need for family planning has the potential to save $120 in other development areas. This investment becomes even more astoundingly productive when the financial value of the ecosystem services protected and preserved by lower human population pressure is factored in.
Today, however, more than 220 million women want to avoid or delay pregnancy and have an unmet need for modern contraception. Right now, there is a shortfall of at least $847 million in the aid that is needed to provide contraception where it is most needed.
That is a problem that can be solved. The UK, in fact, has a strong record of supporting family planning though overseas aid but more must be done.
Smaller families in rich countries too
Education, women's empowerment, tackling poverty and providing access to contraception will all help limit our numbers in addition to all the other critical benefits they bring. It would be wrong, however, to duck the big issue: they are not likely to bring our numbers and consumption in line with what the natural world can provide for us and withstand.
To accept that population growth is a problem requires accepting the responsibility to do something about it (one reason for resistance to recognising the problem). As individuals, nations and societies, those of us who can choose, must choose to have fewer children.
While individual choice is the fundamental principle in limiting our numbers that does not mean family size is not also a matter for the nations and societies we live in. Governments, policymakers and opinion formers have a positive and legitimate role to play in influencing individual choice. Indeed, they have a responsibility to do so.
We expect our policymakers to tell us how much alcohol is dangerous to our health and to do something about the pollution on our streets, even though what we drink and how we get from A to B are matters of personal choice. We live in a world in which governments advise us to eat less meat, smoke fewer cigarettes, eat more fruit and drink less beer. They can advise us to have fewer children too.
We have been here before, however, and things are changing. Climate change is finally forcing and emboldening our leaders - and ourselves - to think long term, and to accept that actions may need to be taken which may not be popular. Indeed, climate change may well prove to be the single issue that forces us all to face the challenge of population. We cannot have more people, living longer and getting richer and expect the amount of emissions we produce to go down.
Limiting human population is not a goal, it is a means to an end. That end is a healthy planet that can sustain us all, human and non-human, now and for the generations to come. Limiting human population growth is a solution, and the Living Planet Report makes clear that we cannot afford to turn our backs on solutions.
Alistair Currie is Head of Campaigns at Population Matters.
Population Matters is a UK-based charity founded in 1991 which believes population growth contributes to environmental degradation, resource depletion and other problems. In support of our vision of a sustainable future with decent living standards, a healthy environment and a stable population size, we conduct research, inform the public and advocate improved family planning and sex education, women's empowerment, smaller families and moderate consumption. Visit our website.
- The WWF / ZSL Living Planet Report.
- Information on population trends, birth rates and death rates.
- The benefits of family planning to societies.
- For more information about the challenges of population growth and how to manage it, visit Population Matters' website.