Tipping points and the Earth System

The Australian bushfires represent a tipping point. Other subsystems in the Earth System can also undergo abrupt shifts, accelerating the rate of climate change and threatening to make human interventions irrelevant, Will Steffen writes.

The words catastrophic, unprecedented, and other superlatives are being commonly used to describe the bushfires that have raged across Australia in recent months. The country has always had bushfires, but never the number, intensity, or size of the 2019-2020 fires. Why is this so?

Of course, climate change is playing a role. In fact, climate change has played the dominant role in driving this summer’s terrible bushfire conditions. 2019 was both the hottest and driest year on record, creating the conditions for a massive, uncontrollable increase in fire activity. Australia’s fire regime crossed a tipping point, generating a ferocity that overwhelmed firefighters and left Australians stunned and battered.

Tipping points occur when a small change in forcing factors trigger an unexpectedly large and often abrupt response in a system, and the fires are a good example. Often, strongly reinforcing feedbacks within the system are the mechanisms that create a tipping point.

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In the case of the Australian fires, the bush was dry enough and the atmosphere hot enough that more intense fires broke out, creating their own violent weather phenomena that in turn aided the spread of those fires.

Tipping points are more common than many people realise. On a global level, the Amazon rainforest is on the cusp of a tipping point that could turn much of it into an open, much drier savanna. Two factors are rapidly driving the Amazon towards a tipping point.

The first is the increase in deforestation due to the policies of the Bolsonaro government in Brazil. This reduces evapotranspiration from the landscape, important for moisture recycling. The second is a slowdown in the North Atlantic Ocean circulation, which leads to a reduction in rainfall over the Amazon basin.

The Amazon is one of a number of tipping elements in the Earth System that have implications at the global level. Tipping elements come in three types. First, large-scale biomes such as the Amazon rainforest can accelerate global heating by releasing large amounts of carbon to the atmosphere as they tip into less carbon-rich biomes.

Large bodies of ice are more well-known and influence the climate in various ways. Arctic sea ice reduces the reflectivity of Earth’s surface as it melts, and shrinking polar ice sheets raise global sea level. Thawing permafrost (frozen carbon-rich soil) influences the global climate by releasing carbon dioxide and methane to the atmosphere.

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Changes in circulation systems constitute the third type of tipping element. Examples include the North Atlantic circulation or the coupled ocean-atmosphere El Nino system in the Pacific, which has a strong influence on Australia’s climate.

Recent research shows that a number of these tipping points are being activated and show signs of instability. Examples include the North Atlantic circulation, which has slowed by 15 per cent, the Greenland ice sheet, which is melting at an accelerating rate, North American boreal forests, which are experiencing increasing dieback due to climate change-driven insect outbreaks, and coral reefs, which have already suffered extensive bleaching with 99 per cent of reefs projected to be lost at a two degree global temperature rise.

While this in itself represents disaster, even larger risks lie ahead. Tipping points do not operate in isolation from one another, but rather are connected in ways that could form a planet-wide cascade. A useful analogy is a row of dominoes, tipping the first one or two dominoes will knock all of the other dominoes down.

Recent evidence shows that this kind of cascade could already be forming, albeit in its early stages. It could operate like this: melting Arctic sea ice is intensifying warming in the northern high latitudes, which accelerates the melting of Greenland ice.

This, in turn, slows North Atlantic circulation by pouring large amounts of freshwater onto the surface of the denser ocean water. Slowing ocean circulation reduces rainfall over the Amazon basin, pushing the rainforest closer to its tipping point.

To quote the experts, “If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilisation. No amount of cost-benefit analysis is going to help us. We need to change our approach to the climate problem. To err on the side of danger is not a responsible option”.

This is the ultimate risk – the joker in the climate pack.

Without rapid and deep cuts in greenhouse gas emissions, a global tipping cascade could drive the entire Earth System on an accelerating trajectory towards a ‘Hothouse Earth’ state. In essence, such a cascade would take the system out of human control.

In this situation, humanity could rapidly reduce its greenhouse gas emissions to zero and have little or no effect. Triggering a global tipping cascade could lead to an Earth that is five to six degrees hotter, a state that would have disastrous consequences, threatening the viability of civilisation itself.