‘Tipping point’ is a metaphor, first used by science and the media about climate change from about 2005 as this article explains. The metaphor has become topical now because some of the most senior climate scientists on the planet have used it to warn everyone, just before the nations of the world meet in the Conference of Parties of the UNFCCC meet in the first two weeks of December, as they do every year, this time in Madrid, to plan an international response to what was identified in the Rio Earth Summit as dangerous anthropogenic interference [DAI] with the climate system”.

The article, commentary rather than research – Climate tipping points — too risky to bet against – is freely available at Nature. The authors are Timothy M. Lenton, Johan Rockström, Owen Gaffney, Stefan Rahmstorf, Katherine Richardson, Will Steffen and Hans Joachim Schellnhuber with the message;

The growing threat of abrupt and irreversible climate changes must compel political and economic action on emissions.

Lenton has been writing about tipping points since 2007.

The scientists are telling us four main things:

First, as scientists learn more the climate tipping point, the point at which the whole planetary system spins out of human control, appears possible or indeed likely at lower and lower temperatures.

Second, what had been thought of as separate tipping points are showing that they are linked and having an effect on each other. As this article in the New Scientist says:

Regions of the planet that are thousands of kilometres apart may influence each other, causing the global climate to lurch into a new state.

Third, the combined effect of interacting tipping points means that Earth’s climate may change far more abruptly and dramatically than we thought.

Fourth, the time we have to act may in fact be zero, we really don’t know. However, this is not a reason to wait for better information. The scientists are saying that:

given huge impact and irreversible nature, any serious risk assessment must consider the evidence, however limited our understanding might still be. To err on the side of danger is not a responsible option.

The cascading effects of climate change tipping points, for which the scientists believe there is evidence now, represent an existential threat to civilization. Hence we have a climate emergency.

This graphic is used to show that the IPCC assessment of abrupt and irreversible changes in climate systems have become higher risk at lower temperatures:

In the following graphic they identify some of the ecosystems which may be subject to tipping points:

They say that cascading effects between these systems may be common, citing research last year which:

analysed 30 types of regime shift spanning physical climate and ecological systems, from collapse of the West Antarctic ice sheet to a switch from rainforest to savanna. This indicated that exceeding tipping points in one system can increase the risk of crossing them in others. Such links were found for 45% of possible interactions.

Much the same result was reported in a December 2018 paper Cascading regime shifts within and across scales. In October 2019 a paper Dynamic emergence of domino effects in systems of interacting tipping elements in ecology and climate found domino effects and tipping cascades to emerge under certain conditions.

One often mentioned is the the link between Greenland ice sheet decay and thermohaline ocean circulation. Lenton et al go further:

For example, Arctic sea-ice loss is amplifying regional warming, and Arctic warming and Greenland melting are driving an influx of fresh water into the North Atlantic. This could have contributed to a 15% slowdown15 since the mid-twentieth century of the Atlantic Meridional Overturning Circulation (AMOC) , a key part of global heat and salt transport by the ocean3. Rapid melting of the Greenland ice sheet and further slowdown of the AMOC could destabilize the West African monsoon, triggering drought in Africa’s Sahel region. A slowdown in the AMOC could also dry the Amazon, disrupt the East Asian monsoon and cause heat to build up in the Southern Ocean, which could accelerate Antarctic ice loss.

They point out that atmospheric CO2 is already at levels last seen around four million years ago, in the Pliocene epoch and is rapidly heading towards levels last seen some 50 million years ago — in the Eocene — when temperatures were up to 14 °C higher than they were in pre-industrial times. Existing models can’t adequately cope with what is happening, but the latest runs in preparation for the sixth IPCC assessment report, due in 2021, show much larger climate sensitivity (defined as the temperature response to doubling of atmospheric CO2) than in previous models.

That might put them on a par with what James Hansen found in 2011 (see Reconciling estimates of climate sensitivity), which is about what I would expect.

Speaking of Hansen, in a New Scientist article from July 2007 Huge sea level rises are coming – unless we act now he says:

Models based on the business-as-usual scenarios of the Intergovernmental Panel on Climate Change (IPCC) predict a global warming of at least 3 °C by the end of this century. What many people do not realise is that these models generally include only fast feedback processes: changes in sea ice, clouds, water vapour and aerosols. Actual global warming would be greater as slow feedbacks come into play: increased vegetation at high latitudes, ice sheet shrinkage and further greenhouse gas emissions from the land and sea in response to global warming.

He pointed out that when carbon dioxide levels were perhaps 350 to 450 parts per million about 3 million years ago sea level about 25 metres higher, give or take 10 metres.

He also pointed out that ice sheet decay is not always slow. For example, about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years.

For these and other reasons, five months later in December 2007 Hansen stated that at 387 ppm we had already gone too far. We should aim to get emissions back to 350 ppm in the first instance.

Later Hansen warned that if we reached 400ppm by 2015, as we would under BAU, then dangerous climate change would be unavoidable.

In 2016 Hansen also gave his best estimate of sea level rise as 2 to 5 meters by the end of the century: five to 10 times faster than mainstream science has heretofore predicted.

Increasingly scientists are taking paleoscience seriously in their advice to policy makers. Lenton et al say:

Atmospheric CO2 is already at levels last seen around four million years ago, in the Pliocene epoch. It is rapidly heading towards levels last seen some 50 million years ago — in the Eocene — when temperatures were up to 14 °C higher than they were in pre-industrial times.

Their point is that although climate models are unable to make clear predictions we know more than enough to adopt a precautionary approach to climate risk.

Graham Readfearn reports the article in Scientist’s theory of climate’s Titanic moment the ‘tip of a mathematical iceberg’. He highlights the mathematical formula the article mentions, and the analogy of the reaction time allowed to a captain who suddenly realises his ship is heading for an iceberg, and he’s going to come up short.

I don’t like the analogy, because it assumes everything is fine for the passengers on the ship as they approach the iceberg, apart from fear if they know what is happening, and then it is all over in one cataclysmic event. I’m sorry, but if the human race is to be culled to perhaps 10 per cent of it’s present numbers, that is likely to play out over three or more generations before a new equilibrium is found. It could be a long and nasty journey. Moreover, the climate is dangerous now.

So the analogy leads to some muddled thinking from Will Steffen when he is interviewed. He says:

“… So we have 30 years to decarbonise and to stabilise our pressure on the climate system.”

Extinction Rebellion want net zero emissions by 2025. That is a very different concept of a climate emergency.

David Spratt I think does a better job in Leading climate researchers: we are in a climate emergency, facing existential risks. The bottom line is control, and what we should do because we don’t know whether the climate has tipped:

How close is losing control and how short is the intervention time? There is no precise numerical answer, but the evidence suggests there is an unacceptable risk that it is very low. As the authors say, “we might already have lost control of whether tipping happens”.

If the intervention time has already “shrunk towards zero” as the paper suggests, then consideration of and research into short-term cooling measures such as solar radiation management must be a high priority to determine whether such measures are of net social and environmental benefit to provide some temporary cooling whilst other measures take effect.

Time is not on our side.

Other scientists are calling loudly now for urgent action. For example:

• Joshua S Hill in UNEP says global emissions must be cut by 7.6% every year for next decade.

• Jake Johnson at Common Dreams in ‘The Science Is Screaming’: UN Report Warns Only Rapid and Transformational Action Can Stave Off Global Climate Disaster

Those two were referencing the UNEP Emissions Gap Report 2019.

• Mike Scrafton in Global warming – we’re screwed! writes about the World Meteorological Association Greenhouse Gas Bulletin (GHG Bulletin) – No. 15: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2018

• Andrew Glikson Beyond climate tipping points: greenhouse gas levels exceed the stability limit of the Greenland and Antarctic ice sheets

Here are two on the letter from 11,000 scientists in early November:

• Michael Mazengarb Climate change emergency – 11,000 scientists say crisis accelerating, demand action

• Michael Slezak Climate emergency declared by 11,000 scientists worldwide who warn of ‘catastrophic threat’ to humanity

I’m sure I could find more. David Spratt has started a special series on the climate emergency, with two so far.

• Climate emergency (1): “Something has shifted”

• Climate emergency (2): The scientific case

On tipping points, Spratt says we already have locked in:

a multi-metre sea-level rise from the melt of West Antarctic glaciers, the loss of the world’s coral ecosystems, and conditions in summer where there will be no sea (floating) ice in the Arctic. Some other potential tipping points and big changes in important climate system elements— such as the loss of the Greenland Ice Sheet and the Amazon rainforest — are not that far away.

Since 2007 Spratt has consistently argued for drawing down CO2, and believes stability akin to the Holocene requires a temperature of 0–0.5°C above pre-industrial. This will require 350 ppm, perhaps lower.

It is important to note that the Lenton et al paper does allow for more human agency than just heading for the life boats. However, the time to act on the climate emergency is now, not later when we feel like it. This warning going into the Madrid climate talks relates to the whole world, but is especially is apposite for us:

About 68 nations said before COP25 they will set bolder emissions reduction targets, including Fiji, South Africa and New Zealand. This group is expected to exert pressure on laggard nations.

This pressure has already begun: France has reportedly insisted that a planned free trade deal between Australia and the European Union must include “highly ambitious” action on climate change.

Our homework will be marked by Climate Tracker, who has us as among the worst in the OECD.

Update: See also Tim Radford at Climate News Network Earth nears irreversible tipping points:

In West Antarctica, ice may already be retreating beyond the “grounding line” where ice, ocean and bedrock meet. If so, then the rest of the West Antarctic ice sheet could collapse, and sea levels could rise by three metres.

New evidence suggests the East Antarctic ice sheet could be similarly unstable, and precipitate further sea level rise of up to four metres. Hundreds of millions are already at risk from coastal flooding.

The Greenland ice sheet is melting at an accelerating rate, and once past a critical threshold could lose enough water to raise sea levels by seven metres. Even a 1.5°C warming might condemn Greenland to irreversible melting – and on present form the world could warm by 1.5°C by 2030.