State of the Planet

News from the Columbia Climate School


Have We Crossed the 9 Planetary Boundaries?

Photo credit: NASA

The world needs to awaken itself to the looming catastrophe of global warming,” said Jeffrey Sachs, Director of the Earth Institute, at a recent meeting in Muscat. “We must provide a safe operating space where vested interest and lobby-driven policies will not see the world marching into disaster.” Just what is a safe operating space for human civilization?

Twenty-eight scientists from around the world defined a “safe planetary operating space” circumscribed by 9 planetary boundaries within which humanity can continue to thrive and develop. The 2009 report by the group led by Johan Rockström, Director of the Stockholm Resilience Centre, examined the non-negotiable planetary conditions that humanity needs to respect and maintain in order to avoid catastrophic environmental changes.

For the last 10,000 years, the Holocene geological epoch, conditions on earth have been stable and favorable to the development of human civilization. But some scientists believe that mankind entered the Anthropocene epoch during the Industrial Revolution in the 18th century, as human population and development began to critically impact the global environment.

While scientists have warned of specific environmental tipping points in the past, the 9 planetary boundaries concept looks at the global system as a whole and how separate biophysical processes interact. One of the report authors, Jonathan Foley, Director of the Institute on the Environment at the University of Minnesota, likened man’s ignorance of the planetary gestalt to driving full speed at night over a mesa without lights or a map. We know the cliffs are out there, but we don’t know where. The 9 planetary boundaries are an attempt to create that much needed map.

Photo credit: Christine Clifstock/Stockholm Resilience Centre

The boundaries delimit: climate change, ocean acidification, stratospheric ozone depletion, disruption of the nitrogen and phosphorus cycles, global freshwater use, land use changes, biodiversity loss, aerosol loading in the atmosphere, and chemical pollution. Many of these systems may react abruptly once a certain threshold is breached, so boundaries are set at a safe distance from the tipping points. Human beings have already crossed the boundaries for climate change, biodiversity loss, and interference with the nitrogen cycle; and we are fast approaching the boundaries for freshwater use, land use changes, ocean acidification, and interference with the global phosphorus cycle.

Planetary Boundary Parameters Boundary Current Status Pre-Industrial
Climate change CO2 parts per million (ppm) 350 392 280
Radiative forcing: watts per meter squared 1 1.5 0
Ocean acidification Saturation state of aragonite in water 2.75 2.90 3.44
Stratospheric ozone Dobson Units 276 283 290
Nitrogen cycle Millions of tonnes/yr removed from atmosphere 35 121 0
Phosphorus cycle Millions of tones/yr entering ocean 11 8.5-9.5 -1
Freshwater use Km3/yr human consumption 4,000 2,600 415
Land use change % of global land converted to cropland 15 11.7 low
Biodiversity loss Species per million/yr extinct 10 >100 0.1-1
Aerosol loading Particulate concentration in atmosphere To be determined
Chemical pollution Several possibilities To be determined

Climate Change

The effects of climate change are already evident in the loss of summer sea ice in the Arctic Ocean, the melting of glaciers, the shrinking of the Greenland and West Antarctic ice sheets, and sea level rise. Scientists measure climate change by the amount of CO2 in the atmosphere (parts per million) and the level of radiative forcing, i.e. change in the input or output of radiation to or from the atmosphere. Using pre-Industrial Revolution era measurements as a baseline or background measure, the background levels for CO2 are 280 parts per million (ppm), and for radiative forcing, 0 watts per square meter (a positive number indicates warming). The boundary is 350ppm for CO2 and 1 watt per square meter above background levels; however, radiative forcing already measures 1.5 watts per square meter and CO2 levels reached 391.92 ppm in April 2011. The scientists believe crossing these boundaries can increase the risk of triggering irreversible changes, such as the loss of major ice sheets, accelerated sea level rise and sudden shifts in forest and agricultural systems.

Ocean Acidification

25% of all manmade CO2 emissions is removed from the atmosphere and taken up by oceans. But the more CO2 the oceans absorb, the more seawater becomes acidic and less saturated with aragonite, a form of calcium carbonate used by marine organisms such as corals to make their shells. It’s estimated that by 2050 only 15% of coral reefs will have enough aragonite for adequate growth. The background measurement of the saturation state of aragonite is 3.44, and the boundary is set at 2.74. Today, at a level of 2.90, ocean acidification is happening over 100 times faster than at any time in the last 20 million years. We are in danger of losing the coral reefs and their ecosystems, as well as certain marine plankton that could affect the entire food chain.

Stratospheric Ozone Layer

The stratospheric ozone layer filters out the sun’s ultraviolet (UV) rays that can cause skin cancer in humans and damage plants and plankton. When chlorofluorocarbons and other halocarbon refrigerants rise into the stratosphere, they release chlorine and bromine atoms, which break down ozone molecules and deplete the ozone layer. Ozone depletion is also exacerbated by the presence of polar stratospheric clouds over the Antarctic, which enhance the chemical reactions that destroy ozone. The 1989 Montreal Protocol, banning ozone-depleting chemicals, has helped shrink the hole in the ozone layer over the Antarctic and keep us within the planetary boundary of 276 Dobson Units (a measure of ozone density) against a background level of 290 Dobson Units.

Fertilizer runoff

Interference with the Nitrogen and Phosphorus Cycles

Human activities convert around 120 million tonnes (one tonne =2,204.62 lb.) of naturally occurring nitrogen from the atmosphere into reactive nitrogen, mainly as fertilizer to help feed the world. But reactive nitrogen pollutes waterways and coasts, and in nitrous oxide form, exacerbates global warming. Synthetic fertilizer, leguminous crops (soybeans, peanuts, alfalfa), many types of manufacturing, and fossil fuel burning industries and vehicles all produce reactive nitrogen. The planetary boundary for the nitrogen cycle is figured in millions of tonnes per year removed from the atmosphere; the background level is 0, the boundary is set at 35, and we are already at 121. Phosphorus is a mineral that is mined for use in fertilizers, detergents, pesticides, steel production, and even toothpaste. It is measured in millions of tonnes per year entering the ocean. The background level is -1, the boundary is 11, and currently 8.5 to 9.5 million tonnes end up each year in the ocean where it depletes oxygen levels, harming marine life.

Global Freshwater Use

Around the world, freshwater is becoming increasingly scarce because of human impacts on global resources. Humans have altered the flow of rivers so that 25% of the world’s river basins run dry before the rivers reach the ocean, and have manipulated the water cycle through land use changes. Moreover, the freshwater cycle is greatly affected by climate change. Before the Industrial Revolution, humans consumed 415 cubic kilometers (km3) of freshwater per year. The boundary of 4,000 km3 allows for sufficient water in the system to regenerate precipitation and provide for ecoservices and aquatic habitats, and we now consume 2,600 km3.

Photo credit: Jami Dwyer

Land System Change

The conversion of forests and other ecosystems mainly for agriculture has had significant impacts on biodiversity, water flow and CO2 emissions. It’s estimated that 12% of global land is currently being used as cropland. A boundary of 15% is set because beyond that the most productive land on the planet will have been used up, which would necessitate clearing much more marginal land for incremental gains in agriculture. To keep within the 15% boundary, the scientists recommend that crops only be grown in the most productive areas, agriculture be based on natural processes, and the demand for food be better managed.

Rate of Biodiversity Loss

Changes in land use, the introduction of invasive species, and climate change are critical factors in the rate of biodiversity loss, which today is 100 to 1,000 times more than the background extinction rate.

Photo credit: Brad Polzin

And while extinctions of particular species occur locally, they can leave large-scale land and marine systems more vulnerable to changes in climate and ocean acidification since biodiversity plays a key role in keeping ecosystems resilient. With the global extinction rate expected to increase 10-fold this century, the scientists concluded that humanity is already in the danger zone for this planetary boundary. And just recently, a study by United Nations University found that the strategy of creating land and marine preserves to protect vulnerable species is not preventing global biodiversity loss. The authors of the study stressed that approaches that tackle the root causes of biodiversity loss—human population growth and resource conservation—are what is needed.

Aerosol Loading

Aerosol particles in the atmosphere influence climate globally and impact human health. Some particles, like sulfur dioxide from coal-fired power plants and volcanoes, reflect the sun and cool the earth. Others, like soot or thin high clouds, behave like greenhouse gasses to warm the planet. Aerosol particles can also influence the hydrological cycle and monsoon circulation, cause forest degradation and acid rain, and they are responsible for respiratory illnesses that result in 800,000 human deaths each year. The scientists have not yet been able to define a boundary, however, because of the variety of aerosol particles and their complex interplay.  A new study by the National Oceanic and Atmospheric Administration found that aerosol particles slowed global warming, preventing .07˚C of warming since 2000.

Chemical Pollution

Chemical pollution of the Earth from radioactive compounds, heavy metals and organic compounds produced by humans, affect human health, ecosystems, biodiversity, and global warming. It can also cause reduced fertility and genetic damage, which can result in species loss. But as with aerosols, there are too many diverse chemicals in the environment and not yet enough understanding about their combined effects to establish a boundary.

The 28 scientists acknowledge that the 9 planetary boundaries are rough estimates because many uncertainties and knowledge gaps remain. Moreover the established boundaries are based on the assumption that no other boundaries are being transgressed. And it is not known how much time we have after crossing the boundaries before we reach tipping points that could produce irreversible changes.

But we do know that the 9 boundaries are inextricably linked—breaching one makes it more likely that others will be breached. Land use changes in the Amazon rainforest can affect weather systems, and alter precipitation and freshwater resources. Exceeding the nitrogen and phosphorus boundary can damage the resilience of the ocean and reduce its ability to absorb CO2. Global climate change is hastening glacial melt, which will also affect freshwater supplies.

The 9 planetary boundaries are meant to offer a new, albeit incomplete, global approach to sustainable development that can help guide future policy and governance. Within these boundaries, we still have options for our future development and well-being.  Crossing them could be distrastrous for humanity.

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Christopher Watts
Christopher Watts
12 years ago

9 should surely be 10. Exponential population growth should surely be no.1.

Reply to  Christopher Watts
7 months ago

Population growth peaked in the 1960s and all projections put us as capped global pop around 9-10Billion. We already waste 40% of food produced in the US and there is a history of population being a straw man for real issues and typically leads to genocide type movements. Capitalism is not built on efficiency. Resource use is the problem, and that means a single US person is worse than like 9 third world adults. Said differently, if the US and other developed countries change how we live then we could sustain huge populations safely. We need to change how we live.

Kristopher R. Carambelas
12 years ago

The Holocene, at least in the Great Basin portion of North America, was anything but stable.