New Scientist - 14.09.2019

40 | New Scientist | 14 September 2019

of the life-support systems that have sustained

the Holocene.

The changes could be abrupt and

irreversible. We don’t know where things

may end up. If the Holocene is our desired

reference point – the stable planet we know

and depend on – we need to find out where

those thresholds are, thus identifying our safe

operating space. That is what our research on

planetary boundaries tries to do.

So what are these boundaries?

We identify nine. There are three that

operate at a planetary scale: the oceans,

the atmospheric climate system and the

stratospheric ozone layer. Each has thresholds

beyond which danger lies. There are four more

that we call biosphere boundaries. They help

regulate the planetary systems. They are

biological diversity, the hydrological cycle, land

cover such as forests, and the flows of nutrients

vital to life, such as nitrogen and phosphorus.

Finally, we identify two categories of alien

things that don’t exist naturally: novel entities

including nuclear waste and gender-bending

chemicals, and aerosol air pollution, which

alters Earth’s energy balance and impacts

regional climate systems such as the

south Asian monsoon.

How do we know when we have crossed a

planetary boundary?

Our scientific understanding of Earth systems

has advanced tremendously over the past

30 years, but still we don’t know exactly where

the critical boundaries are for these systems.

So we apply a precautionary approach. We

identify safe zones and high-risk zones.

Between them, uncertainty ranges, within

which we don’t know what might happen.

We place the planetary boundary at the

lower levels of the uncertainty ranges.

What is the boundary for climate change,

for example?

For climate change, we chose the atmospheric

concentration of the most important

greenhouse gas, carbon dioxide. We

assessed the uncertainty range as being

between 350 and 450 parts per million,

so the planetary boundary is 350 ppm.

The world is now well above that, at 410 ppm,

and we are starting to see dangerous impacts.

In the oceans, there is heating, a slowing down

of the Gulf Stream and accelerated Arctic sea-

ice melting. In the atmosphere, we see a

serious impact on the jet stream and many

more extreme weather events. All these effects

may foreshadow the breakdown of other

systems, such as the hydrological cycle and

biodiversity. We are already into the high-risk

zones for biodiversity and nutrients.

Your critics say you don’t properly distinguish

between global and local systems.

Of course, before you reach any of our

planetary boundaries, you can have massive

problems locally. Lakes dry up, rivers fill with

pollution, ecosystems collapse and so on.

But when we talk about keeping within a safe

operating space, we mean safe for the planet

as a whole, not for every human or every

ecosystem. Even so, apparently local problems

can go global, especially if they occur widely.

Which problems have gone global in this way?

Water seems very local, for instance, but

there is a global hydrological cycle that is the

bloodstream of the biosphere. Take away the

rain and forests die, triggering massive releases

of carbon into the atmosphere and loss of

biodiversity. Or take away a major forest like

CLIMATE

CHANGE

Nitrogen

cycle

Phosphorus

cycle

BIOGEOCHEMICAL

FLOWS

NOVEL ENTITIES

Chemical pollution and

substances that don’t

exist naturally

(boundary to be

determined)

?

?

?

STRATOSPHERIC

OZONE

OCEAN

ACIDIFICATION

FRESHWATER

USE

CHANGE IN

LAND USE

ATMOSPHERIC

AEROSOL LOADING

(boundary to be

determined)

BIODIVERSITY

Extinction

rate

Below boundary (safe)

Biodiversity

intactness

(boundary to be

determined)

Overstepping the mark

We have already gone beyond four of the nine proposed safe operating levels for

humans on Earth, know as planetary boundaries

In zone of uncertainty (increasing risk)

Beyond zone of uncertainty (high risk) Outer boundary of safe zone

Start of high-risk zone

Johan Rockström is a global

sustainability expert whose research

focuses on water in tropical regions.

He is a former director of the Stockholm

Resilience Centre and is now co-director

of the Potsdam Institute for Climate

Impact Research in Germany

MIKAEL AXELSSON