Tipping points

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'Warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change' (IPCC, 2007a).

Unfortunately, history shows that the climate system does not always respond in a gradual, linear manner to increased concentrations of heat-trapping greenhouse gases. Rather, the system can respond slowly, resisting change and obscuring the underlying march towards a critical threshold.

Once the threshold is crossed, the system will respond relatively quickly, recalibrating into a new, very different state with massive implications for life (Pearce, 2007).

Furthermore, the emission patterns of today will 'lock in' high global average temperatures for the next 1000 years. Solomon et al (2009) show how:

... climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1000 years after emissions stop. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1000 years.

There is now stronger evidence that the world's oceans and forests are absorbing less of the carbon dioxide released by human activity. Significant weakening of carbon absorption of potential Earth systems compounds increased emissions rates. There is also the potential for the reversal of some carbon absorption systems such that they become net sources of emissions. For example, should drought frequency in the Amazon increase significantly, it is possible that the largest terrestrial sink of carbon dioxide in the world will become a net source of carbon emissions (Phillips et al, 2009).

Lenton et al (2008) assessed 15 potential tipping elements in the Earth system and ranked the likelihood that each may arise in this century. They exclude those tipping elements unlikely to be triggered within the 21st century, leaving eight tipping elements ranked as either:

1 highly sensitive with the smallest uncertainty;

2 intermediate sensitivity with largest uncertainty; and

3 low sensitivity with intermediate uncertainty.

They also assess how much additional global warming would be necessary to trigger the tipping point. The results are summarized in Table 7.1.

Smith et al (2009) summarize the increasing risks associated with climate change by comparing known risks with those assessed in the IPCC's third assessment report of 2001. Figure 7.1 compares these risks using a visual display across five issues of concern originally used in the 2001 IPCC assessment. The transition from white through grey to black indicates increasing levels of risk, which is a function of both the potential severity and likelihood of impacts as global average temperatures increase.

Many have argued that it could be an option to accept some level of risk and warming, and deal with the consequent climate changes through measures that build resilience, allowing us to adapt to climate change. This view has become institutionalized within the negotiating process, with many of the analyses commissioned by the Conference of the Parties under the Bali Action

Plan estimating finance and technology needs according to a 500 to 550 part per million carbon dioxide equivalent stabilization (e.g. UNFCCC, 2007).

Metz et al (2007) give a 50 per cent chance that a 550 part per million carbon dioxide equivalent stabilization will cause a 20 and 30 per cent loss of all species on Earth with almost half of the world's population being at risk of water shortages, 0.25 billion people exposed to potential health problems, and hundreds of millions facing food shortages and coastal inundation.

Decrease in Antarctic sea ice and near complete disappearance of summer sea ice in the Arctic towards the end of this century will devastate ice-dependent ecosystems and extend into the interior of the bordering continental regions of Russia, Canada and Alaska, risking runaway methane emissions released from thawing permafrost, and the release of methane hydrates currently trapped deep in the ocean as temperature increases extend into the ocean.

In some of the temperate zone and in drier tropical areas, water availability will decrease and ecosystems will become stressed. Some ecosystems are already suffering and massive tree deaths are being recorded. In Western Australia, rainfall has declined by 20 per cent in 30 years resulting in a 50 per cent reduction in water availability with a further 20 per cent predicted (Government of Western Australia, 2008). Glaciers in regions such as central Asia and the Himalayan and Tibetan plateau are melting faster than expected

Table 7.1 Tipping elements ranked according to likelihood

Tipping element




Potential impacts

global warming



required (°C


relative to 1990)

Arctic summer sea ice




Possible complete loss of

ecosystem and amplified

warming; accelerated

permafrost loss

Greenland ice sheet




Possible 6m-7m sea-level

West Antarctic ice


Possible 4m-5m sea-level



Atlantic thermohaline




Large-scale climate


changes globally

El Niño Southern




Extensive drought events


Indian summer

Not applicable


Not ranked as

Either potential increased


not comparable

or decreased rainfall

Sahara/Sahel and West




Either potential increased

African monsoon

or decreased rainfall

Amazon rainforest




Massive extinction event,

decreased rainfall and

amplified warming

Boreal forest




Loss of ecosystem

Source: adapted from Lenton et al (2008)

Source: adapted from Lenton et al (2008)

and, as discussed in Chapter 10, the long-term loss of these water resources could affect billions of people.

Impacts upon water regimes are already affecting agriculture and food supply, which will be exacerbated in some regions, mostly the least developed nations, even if warming can be kept within 2°C to 3°C.

Hansen et al (2008) call for stabilization at 350 parts per million of carbon dioxide, significantly below present levels of nearly 390 parts per million of carbon dioxide, which corresponds to limiting global average temperature increases to 1.7°C above pre-industrial levels.

According to Hare (2009), there is a '75 per cent risk that stabilizing greenhouse gas concentrations at 550 parts per million would lead to warming exceeding 2°C'. However, 'for a concentration pathway that peaks at 475 parts per million carbon dioxide equivalent and then drops to stabilize at 400 parts per million carbon dioxide equivalent, there would be about a 20 per cent chance of exceeding 2°C'.

He suggests that a plausible and risk-adverse emissions reduction pathway is to keep global average temperature increases below 2°C and also have a decent chance of holding temperature increases below 1°C. This requires fossil carbon dioxide emissions to approximate zero in 2050 and an 85 per cent reduction in all greenhouse gases from 1990 levels by 2050, peaking in 2020 and 'going negative' by 2075.

TAR (2001) reasons for concern

Risks to many

Risks to some

Large increase

Negative for most

Negative for some regions; positive for others

Net negative in all metrics

Positive or negative market impacts; majority of people affected


Very low

Risks to Risk of Distribution Aggregate Risks of unique extreme of events impacts large scale and weather threatened events systems discontinuities

Updated reasons for concern

Risks to many

Risks to some

Large increase

Risks to unique and threatened systems

Negative for most

Negative for some regions; positive for others

Net negative in all metrics

Positive or negative market impacts; majority of people affected


Risk of Distribution Aggregate Risks of extreme of events impacts large scale weather discont-

events inuities

Source: Smith et al (2009)

Figure 7.1 Risks from climate change: Comparison of estimates of risks for each reason of concern as assessed by the IPCC in 2001 and a revised assessment by Smith et al (2009)

This last point is crucial. A risk-adverse strategy for climate change relies on creating a global economy that actively draws down the concentrations of greenhouse gases - that is, one that absorbs more than it emits. This can only be achieved through massive reforestation and new energy generation technologies such as biomass energy with carbon capture and storage and technologies that directly remove carbon dioxide (or other greenhouse gases) from the atmosphere. It suggests that the shared vision for the convention should be a carbon-neutral global economy soon after 2050.

The marginal costs of such a strategy are likely to be high; but relative to the residual costs and risks of a 2°C to 3°C outcome, it is, in our judgement, a cost worth bearing. Indeed, the cost is expected to be less than 1 to 2 per cent of gross domestic product (GDP) (see Chapters 9 and 24). Risk is a function of both the likelihood of an event occurring and the severity of the event. What is an acceptable level of risk is a matter for judgement - it is fundamentally a moral question. Should we accept the additional cost and maintain a safe climate, or should we shoot for 2°C to 3°C and accept the loss of much of the world's coral reefs, extinction of many species, displacement of millions of people, significant health risks, and loss of or disruption to some entire nations? Add to this the more recent evidence that global warming of 2°C to 3°C may not avoid catastrophic tipping points over the longer term and I am left with no doubt which is the most rational global strategy.

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