Logically, how is it possible to use more resources than Earth can replenish?

Since the 1970s, humans have used more resources than the planet can regenerate. This is known as overshoot. The WWF Living Planet Report has reported overshoot every two years since 2000.

However, this fact can inspire some confusion. How can it logically be possible for us to use more resources than Earth can produce, for decades on end?

There are two basic concepts at work here. One is our ecological footprint, which can be very loosely understood as a way of tallying up the resources we use from nature. The other is the planet’s ability to provide or renew those resources every year: its “biocapacity”.

When our ecological footprint exceeds Earth’s biocapacity, that’s unsustainable resource use. Unsustainable resource use can occur for some time. The environmental thinker Donella Meadows used a bathtub analogy to explain how.

Imagine a bathtub full of water, with the tap running and the plug out at the same time. It is possible for more water to flow out of the bath than into it for some time without the water in the tub running out. This is because the significant store of water in the bath acts like a buffer. The same goes for nature.

Because nature has accumulated resources – for example, in a forest – it’s possible for us to harvest nature at a greater rate than it can replenish itself for a certain amount of time.

But this leads to the question: if humanity’s ecological footprint exceeds Earth’s biocapacity, how long can we keep going without crossing a tipping point? Our recent research investigates this question.

Explaining the feedback system

It’s important to make the point that nature provides us with literally everything we need, through processes known as ecosystem services. Much of this is obvious because we buy and sell it, as food, shelter and clothing.

Other services go largely unnoticed. Forests provide protection from flooding by slowing down surface water runoff, for example, while mangroves absorb carbon dioxide from the air and store it. Until relatively recently, nature has continued to provide, despite our rapidly increasing ecological footprint.

In part this resilience comes from being able to buffer disturbance with the existing store of resources. But there’s an important mechanism that helps natural systems adjust – to a certain extent – to disruption. This is called a feedback mechanism, and if we take the bathtub analogy one step further we can see how it works.

Say we set up our bathtub so that the tap and the plughole communicate with one another. If more water suddenly starts flowing down the plug, then the tap increases the flow of water into the bath to compensate, thus maintaining the water level. This is an example of a “positive” feedback (more water exiting the bath) being moderated by a “negative” feedback (more water entering from the tap), thus maintaining the state of the system (water in the bath).

Let’s pick a real-world example. Clearing trees from a forest might mean that seeds from the soil have the chance to germinate. If they germinate before the landscape gets too degraded, they can potentially balance out the disturbance.

But harvesting forest also exposes the ground, causing soil loss. In turn, vegetation might find it more difficult to regrow – resulting in yet more soil loss, and so on. This is a “positive” feedback – one that reinforces and exacerbates the original problem.

Negative feedbacks can only adapt to a certain level of disruption. Once the disturbance is too large, they break down. Positive feedback loops can then prevail and the ecosystem is likely to cross a tipping point, resulting in permanent, dramatic and sudden transformation.

Crossing planetary boundaries

In our research, my colleagues and I compared future ecological footprints with research about planetary boundaries (points at which the risks to humanity of crossing a tipping point become unacceptably high). We found the discrepancy between the ecological footprint and biocapacity is likely to continue until at least 2050. We also found that our global cropping footprint is likely to exceed the planetary boundary for land clearing between 2025 and 2035.

This occurs in the context of atmospheric carbon dioxide concentrations that have already crossed the planetary boundary of 350 ppm. (As I write, the carbon dioxide concentration is over 400 ppm.)

By itself, both these points are serious enough. More seriously, we have no idea what happens when two planetary boundaries are approached simultaneously, or two tipping points interact.

We face the permanent loss of essential natural processes, putting, for example, our global food security at risk. Our research shows we need to address gradual, cumulative change, as the global resource buffer shrinks and stabilising feedback mechanisms are overwhelmed.

But there’s good news too. Ecological footprints decrease in response to human decisions. Our current trajectory towards tipping points is not fait accompli at all, but can be influenced by the choices we make now.