Planetary Boundaries

Objectives

To give an overview of the nine planetary boundaries that scientists believe we cannot cross. Note that all these topics will be covered in detail later

Definition

Planetary Boundaries

A series of thresholds, crossing of which will cause major changes in the worldwide environment

History

In 2009, a group of scientists, under the leadership of Johan Rockström of the Stockholm Resilience Center, got to together to identify the processes that regulate stability and resilience of the Earth system. . The result was a paper that developed the idea of planetary boundaries.

The planetary boundaries was updated in 2015 In 2022, scientists quantified the novel entities boundary, and stated that humans have crossed that boundary. Also in 2022, scientists expanded the water use boundary to include green water (water available to plants).

List

There have been identified nine planetary boundaries:

• Climate change
• Biodiversity loss
• Land use change
• Flows of nutrients
• Ozone depletion
• Freshwater use
• Ocean Acidification
• Novel entities
• Aerosols loading

Climate Change

Climate change or global warming is the increasing of the temperature of the Earth. The increasing of the Earth's temperature can cause many effects including more extreme weather, melting of land and sea ice, food insecurity, rising of sea levels, and changes to ecosystems.

At the Paris Agreement countries agreed to limit temperature rise to 1.5°C. However, the planetary boundary was set using the concentration of carbon dioxide in the atmosphere and the radiative forcing (change in energy flux of the atmosphere).

Note that although methane, nitrous oxide, and some other gases do have an effect on climate change, carbon dioxide accounts for almost 80% of the effect. Also, nitrous oxide is accounted for under flow of nutrients.

For the planetary boundary, the threshold value was 350 ppm, we are now at over 400 ppm. The temperature rise which we are to limit to 1.5°C, is already at 1°C. And we are already seeing effects of climate change.

Biodiversity Loss

Loss of biodiversity is the loss of species, that is species becoming extinct. Biodiversity provides important ecosystem services for humans and the rest of the environment.

The planetary boundary considers the rate of biodiversity loss. The prevailing view by scientists is that we are currently in the sixth mass extinction event in the history of the Earth.

Land Use Change

Changes of land from forests and other ecosystems to agriculture and other human uses has major effects on biodiversity, water, and climate.

The original boundary was percentage of land surface converted to cropland. That was changed in the 2015 update to percentage of original forest area remaining. Note that even though all ecosystems (more correctly biomes) are important, they now focus on forests as they have the biggest contributions to climate change.

The amount of forest cover varies considerable around the world. Areas with some concern are Europe and the boreal forests of North America. But of high risk are the forests of Africa and Asia (in Asia less than 30% of original forest remains).

Flows of Nutrients

Flows of nutrients, especially nitrogen and phosphorus, play an important role in ecosystems. These are often discussed with relation to material cycles. But increasing of amounts of nitrogen and phosphorus from human sources (especially artificial fertilizers) can have serious effects on the environment. Eutrophication caused from excessive nutrients in water systems can cause serious damage including major fish kills.

The planetary boundaries considered are (1) the amount of phosphorous from fertilizers released to erodible soil (for freshwater systems), (2) the amount of phosphorus entering the oceans from freshwater systems, and (3) the amount of nitrogen from industrial and intentional biological fixation.

The main contributors to these flows is from a few agricultural regions with large inputs of phosphorus and nitrogen -- especially, North America, Europe, the Ganges River basin, and Eastern China.

Ozone Depletion

Ozone depletion is caused by certain chemicals which destroy the ozone layer in the stratosphere. The ozone layer acts as a shield against dangerous UV light.

Due to the Montreal Protocol the ozone depletion boundary will probably not be crossed.

Freshwater Use

Humans put tremendous pressure on freshwater systems. And in fact, the largest human use of water is for agriculture (70%). This water usage greatly affects many parts of the environment including biodiversity, food security, ecosystem services, climate regulation, and human water supply.

There are two types of water considered here: (1) Blue water - water from rivers, lakes, runoff, and renewable groundwater; and (2) Green water - precipitation, evaporation, and soil moisture.

The original boundary considered only blue water - in terms of total amount of water withdrawn (volume/year). As mentioned above, a new study in 2022 extended the boundary to green water. This is the percentage of ice-free land which is different than historical value.

The world is within the boundary for blue water, but has transgressed the green water boundary.

Ocean Acidification

Carbon dioxide dissolving in water decreasing its pH (that is, it becomes more acidic). Therefore, with the increased carbon dioxide in our atmosphere, more carbon dioxide is being dissolved in the ocean, decreasing pH. This acidification of the ocean has an effect on marine organisms, especially those with calcium carbonate in their structure - especially, shellfish, hard corals, and diatoms (a type of plankton). This is due to the fact that the lower the pH, the more soluble calcium carbonate becomes.

One important point, ocean acidification is not due to global warming, but directly due to the carbon dioxide.

The planetary boundary is the saturation state of aragonite. Aragonite is the main form of calcium carbonate found in corals and main shellfish.

Novel Entities

This boundary was originally called "Chemical Pollution". It was later expanded to include new substances that are not necessarily chemicals, such as nanomaterials, microplastics, and modified living organisms. These can have disruptive effects on the environment, especially as many of these are persistent. There are over 100,000 chemicals (not counting nanomaterials) produced today, but very few have been tested. Even with testing, other long-term effects are often discovered later (for example, CFCs).

Originally, there was no boundary value for chemical pollution/novel entities. However, in 2022 they quantified the novel entity boundary. They consider the following set of variables: production of chemicals and plastics, percentage of chemicals on the market that have been assessed for environmental and human health, amount of hazardous chemicals and plastics released to the environment, toxicity of chemicals, and ecological disruption from plastic pollution.

Based on these variables, the authors have concluded that the planetary boundary for novel entities has been exceeded.

Aerosols Loading

Aerosols, or particulate matter, are solid or liquid particles suspended in air. They are one of the major forms of air pollution, having a serious effect on human health. They also affect the water cycle (precipitation, evaporation, etc.) and can affect the climate system.

The variable which is used is the aerosol optical density, that is visibility. However, no value as been set yet - except for a regional boundary in the South Asia monsoon region.

Current Status

In summary, we have crossed six of the nine planetary boundaries. They are : climate change, biodiversity loss, land use change, flow of nutrients (both P and N), freshwater use (green water), and novel entities. Note that the boundary for aerosol loading has not been set.