LAST week a team of scientists published their review of the evidence that has accumulated in the past few years about the properties and dangers of climate tipping points.

They compared this evidence with the 1.5-2°C rise in global temperature that will occur under the Paris Agreement. They found that we may still reach several climate tipping points or maybe have already crossed them.

A ”tipping point” is when a variable in a system, such as the global temperature in our climate, crosses a threshold which causes feedback.

From that point, significant self-perpetuating changes start to occur that are irreversible and often abrupt and dramatic.

The self-sustaining feedback loop means that even if we could magically reduce the temperature after crossing a tipping point, the changes would continue to happen and we cannot return to the previous state.

One of the tipping points identified in the review was the dieback of the Amazon rainforest.

The presence of the rainforest recycles rainfall in the Amazon basin, allowing it to persist through the dry season.

As some of the forest is lost due to both deforestation and high temperatures causing drying out (and eventually fires), this reduces its ability to promote and  retain rainfall and leads to further drying.

After the tipping point of forest death, the forest can no longer sustain itself and the entire forest is condemned.

The Amazon rainforest is a huge sink for human carbon emissions. Upon its death these emissions will no longer be absorbed and will therefore contribute towards more climate change.

Another large store of carbon is permanently frozen soil in the northern boreal forests that lie in a ring around the north of the planet through Canada, Finland and Russia.

It is not fully known how these permafrosts will react to rising temperatures. The best-case scenario is that they thaw gradually, slowly releasing their carbon into the atmosphere over many years.

However, there is some evidence that a partial thaw would lead to an increase in microbial activity in the previously frozen carbon-rich organic matter.

This bacterial activity would generate its own heat and melt the permafrost further, creating a feedback loop leading to permafrost collapse and the release of all the stored carbon very quickly.

Four of the tipping points identified are the collapse of ice sheets around Greenland, West and East Antarctica, and the Arctic Sea.

It’s easy to grasp that less sea ice will form in a warmer sea as temperatures rise, but sea ice melting may unfortunately also face a tipping point at which a feedback loop begins so that sea ice loss accelerates further sea ice loss.

Open seas with less ice at the beginning of winter seem to lead to less sea ice growth. Ice is very reflective, so if less ice is formed then less heat is reflected and is instead absorbed by the sea in spring and summer.

The warmer sea melts more ice, so even less ice is present for the beginning of next winter. When the sea ice is gone there will be no heat reflected and global temperatures will rise.

While each of these tipping points would represent the terminal decline of a major earth ecosystem, what makes them particularly important is their capacity for interaction. Each tipping point has a roughly estimated temperature range at which it is predicted to occur.

Reaching one tipping point may push temperatures up enough to reach another tipping point, leading to a cascade of devastating, irreversible changes.

The self-sustaining nature of these tipping points means that once we are on the conveyor belt, we cannot get back off.

The uncertain ranges of temperature at which these tipping points may occur is also concerning. We have already reached just over one degree Celsius of warming since pre-industrial times.

This may not seem like much, but it already puts us in the lower ranges for several tipping points where they are considered “possible.”

One of these tipping points that we may have already passed is coral bleaching. At a high enough sea temperature coral ejects the symbiotic algae that live inside their tissue.

The coral turns white and brittle and the incredibly rich ecosystems of other marine life that live inside and around the coral also dies. 

Reef health often depends on specialised fish that live in the coral; declines in the population of these fish due to mass coral bleaching would cause further reef deterioration.

More broadly bleaching can cause population crashes of fish that feed on animals in the reef, impacting humans that rely on fishing as a food source and ultimately the ocean carbon cycle.

Coral bleaching has already happened in many coral reefs, including large portions of the Great Barrier Reef off the coast of Australia.

As sea temperatures have risen, mass bleaching events have become more common. It’s impossible to tell right now whether we have already crossed the threshold of reef decline but many believe it is very likely.

Single warming events have catastrophic effects which mean the time-scale for coral loss is years, not decades. It has been predicted that 99 per cent of coral reefs will have died within 10 years.

Current warming has brought us into the plausible range for several tipping points. Within these ranges, the likelihood of passing tipping points becomes ever more likely the higher the temperature rises. 

The Paris Agreement aims to limit the temperature increase to “well below 2, preferably to 1.5°C.” 

Even this seemingly small amount of extra warming pushes us into the “possible” ranges of several more tipping points, and changes the status of lower tipping points to “likely.”

The uncertainty in how soon we will reach these tipping points, how bad they will be for further warming, and how they will interact with each other demands that we take the strongest measures to prevent as much future warming as possible.

In the words of the scientists conducting the review: “The Earth may have left a safe climate state beyond 1°C global warming.”