We often hear it said that there are too many people on Earth, that 'overpopulation' is an existential threat, and that fewer people might consume proportionately less, resulting in some easy environmental gains such as less carbon output and fewer species extinctions. It's a seductive logic, but is it true?
This is an important question because it underlies many assumptions that organisations such as the International Energy Agency use in their projections for future resource demand, which are then used by governments in their strategic decision-making.
So, what's the answer? Well ... it's complicated!
One of the complicating factors is that there are very few places on Earth which have the right conditions in place to test such an expectation empirically. Nearly every country on Earth has a growing population. In addition, sometimes the data that we want just isn't available, or isn't reliable or robust enough. That being said, what do we know that can be of use in finding answers to our question?
It may surprise, but the world is in the midst of a major demographic transition that may deliver a shrinking world population by the end of this century. That might seem like a long way away, but it's a prospect that -hopefully - is achievable within the lifetime of my own seven year old daughter. Rather like climate change itself, depopulation is not something that might or might not occur at some distant point far into the future. It is already occurring, and in an increasing number of countries. Fortunately these are mostly high-consumption developed societies such as Japan, Germany and, the most recent addition to this special club, Spain. As yet rates of depopulation in these countries is slow, but it is likely to continue, and to accelerate and accumulate into a substantial decrease by mid-century. Japan, with a population of 127 million people, began to depopulate at the national scale in 2008 and is expected to lose up to a quarter if its population size by 2050. China, with a population more than ten times that of Japan, is anticipated to begin to depopulate by about 2030 - just fifteen years away.
That's the good news. The bad news is that the evidence, such that it exists, suggests that environmental gains from depopulation will be harder to achieve than many presently assume. And there are good reasons why the data point in this direction.
The stand out issue from evidence from Japan is that energy consumption and carbon output in areas with a shrinking population has been growing more rapidly than in growing areas (see graph). This is important empirical confirmation for what we have suspected for a while; that decreasing population density leads to increased resource consumption. Houses still need to be heated, even as occupant numbers decrease. When schools close due to lack of pupils, the remaining children's new school is likely further away, meaning they can no longer walk or cycle there and may need to be driven or bussed.
In terms of biodiversity, depopulation may not be as benign as some hope. Ecosystems need large areas of contiguous land to reach their maximum, for example to enable predators such as bears to find their optimum range. In depopulating communities abandoned buildings are often left standing, or unused agricultural land lies in a patchwork pattern and is rarely contiguous preventing a complex ecosystem from developing.
What's the take home message, then?
We can be fairly sure that a large reduction in the world's population would result in long-term environmental gains. But, to deliver the benefits that some expect - what I call a 'depopulation dividend' - we need action now. Simply stated, we don't have the time to allow markets to take their course, and there is no certainty the outcomes will be what we want. Climate change and massive biodiversity losses are already upon us. If we really are to gain the environmental benefits from depopulation that we hope for, then we need internationally coordinated structural intervention by government.
What form might this take?First, the counter-intuitive example of energy consumption from Japan tells us that we need much more research done on these issues. Japanese energy consumption data at the sub-national level is good, but few countries produce data at that level of complexity. It's hard to produce a convincing analysis on the basis of just one country's experience.
Second, we need a more robust approach to managing land and buildings. Empty or unused structures need taking down and abandoned land should be joined up to produce benefits for biodiversity.
Third, we need to revisit our value priorities. What is the point of resolutely pursuing growth where it's no longer possible? Why not value wellbeing instead, and in its broadest sense? This is not being anti-growth, but is a revaluing what we already hold dear but sometimes neglect - our health, our loved ones and our living environment.
The demographic transition to shrinkage that humanity is presently experiencing has the potential to contribute some real solutions to resolving the climate crisis. But, we shouldn't leave it to chance. We need specific evidence-based interventions. The alternative is just too awful to contemplate.
Dr Peter Matanle is Senior Lecturer at the School of East Asian Studies, University of Sheffield. He will be presenting his research in the Japan Pavilion at the COP21 meeting in Paris on 10 December. The panel theme is 'Population Decline and Climate Change in the 21st Century: Achieving A 'Depopulation Dividend' in the Asia-Pacific Region'.