Is climate close to a "tipping point"?

Question

James Hansen has long used the idea of a climate tipping point in his public arguments for urgent and drastic action to combat climate change (For an extended exposition of his idea, see a pdf of one of his papers here.)

Note: in response to some comments about the term "tipping point" being vague, I have added some quotes below to illustrate what Hansen himself seems to believe. These are from a Huffington Post article by Hansen himself in 2008. I think it is fair to say he is talking about an irreversible catastrophe involving runaway change (my emphasis in the quotes to highlight this idea).

...we have used up all slack in the schedule for actions needed to defuse the global warming time bomb. The next president and Congress must define a course next year in which the United States exerts leadership commensurate with our responsibility for the present dangerous situation.

Otherwise it will become impractical to constrain atmospheric carbon dioxide, the greenhouse gas produced in burning fossil fuels, to a level that prevents the climate system from passing tipping points that lead to disastrous climate changes that spiral dynamically out of humanity's control.

...climate is nearing dangerous tipping points. Elements of a "perfect storm," a global cataclysm, are assembled.

Climate can reach points such that amplifying feedbacks spur large rapid changes.

Not all climate scientists agree. This New York Times report, for example, points out that:

But other scientists say there is little hard evidence to back up specific predictions of catastrophe. They worry that the use of the term “tipping point” can be misleading and could backfire, fueling criticism of alarmism and threatening public support for reducing greenhouse gas emissions.

Moreover, the last interglacial didn't tip the world into irreversible warming despite passing the threshold 2°C warmer than pre-industrial temperatures (it was followed by the ice age that ended ~10,000 years ago).

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Update Tipping points are back in the news. The BBC recently reported this:

Researchers believe we could soon cross a threshold leading to boiling hot temperatures and towering seas in the centuries to come.

Even if countries succeed in meeting their CO2 targets, we could still lurch on to this "irreversible pathway"...

An international team of climate researchers, writing in the journal, Proceedings of the National Academy of Sciences, says the warming expected in the next few decades could turn some of the Earth's natural forces - that currently protect us - into our enemies.

So are warnings that the earth's climate is close to a tipping point just alarmism designed to stir public action? Or is there concrete evidence from modelling or climate history that clearly signals an imminent tipping point?

Answer 1

Yes, climate is close to a tipping point: the increase in global heat content, brought about predominantly by increased concentrations of greenhouse gases, is now triggering positive feedbacks.

Note that "positive feedbacks" is used here in the way it is used in control systems: the "positive" refers to an amplification effect: the feedback reinforces / strengthens what's already going on. The article by James Hansen sets out some of what's going on with the tipping points, and is a clear and accessible read.

Let's break the opening statement into components, and look at each one.

Has global heat content increased? Has this warming been brought about predominantly by increased concentrations of greenhouse gases? Are there potential positive feedbacks in the climate system? Are these positive feedbacks being triggered?

Has global heat content increased?

Yes. We've covered this previously:

• Has global mean temperature at sea level increased since 1900?
• See also How are Temperatures on Earth Changing?

Has this warming been brought about predominantly by increased concentrations of greenhouse gases?

Yes. We've covered this previously too:

• Is CO₂ the cause for Global Warming?
• Do human activities contribute to climate change?

Are there potential positive feedbacks in the climate system?

Yes. For example:

• methane, e.g. as in methane clathrates, is stored under ice in very large quantities. Methane is a powerful greenhouse gas. So if that ice were to melt, releasing methane clathrates, that would be a positive feedback effect - a tipping point.

Are positive feedbacks being triggered?

Yes.

• We have already observed methane releases in the Arctic:

Could there be even worse to come?

Yes.

See: How Likely are Major or Abrupt Climate Changes, such as Loss of Ice Sheets or Changes in Global Ocean Circulation, which says:

Abrupt climate changes, such as the collapse of the West Antarctic Ice Sheet, the rapid loss of the Greenland Ice Sheet or large-scale changes of ocean circulation systems, are not considered likely to occur in the 21st century, based on currently available model results. However, the occurrence of such changes becomes increasingly more likely as the perturbation of the climate system progresses.

Answer 2

There is little hard evidence for planetary tipping points and plenty of reasons they are unlikely to exist

There is a good summary of the case against tipping points in a recent New Scientist (by the author of a more technical paper on the topic available here).

The argument is summarised by New Scientist:

..no theoretical or empirical evidence exists for such a claim, and that a widespread belief in the existence of such a point of no return threatens to push ecological science and its application in the wrong direction.

or, in more technical terms in the academic article (my highlights):

Tipping points – where systems shift radically and potentially irreversibly into a different state – have received considerable attention in ecology. Although there is convincing evidence that human drivers can cause regime shifts at local and regional scales, the increasingly invoked concept of planetary scale tipping points in the terrestrial biosphere remains unconfirmed. By evaluating potential mechanisms and drivers, we conclude that spatial heterogeneity in drivers and responses, and lack of strong continental interconnectivity, probably induce relatively smooth changes at the global scale, without an expectation of marked tipping patterns. This implies that identifying critical points along global continua of drivers might be unfeasible and that characterizing global biotic change with single aggregates is inapt.

His argument admits that ecological tipping points can exist in many ecosystems but that the characteristics necessary for a tipping point to exist don't apply to the whole planet:

Tipping points happen when the components of a system respond gradually to an external force until a level of change is reached at which the response becomes non-linear and synergistic. This amplifies the effect of the force and rapidly drives the system into a new state.

To respond in this way, systems must meet certain requirements. Either external forces are applied uniformly and each part of the system responds in the same way, or the system must be highly interconnected to allow synergistic responses to emerge. Or both.

Do these criteria apply to the biosphere as a whole? I think not. For planetary tipping points to exist, the forces of humanity would need to act uniformly across the planet, all ecosystems would need to respond to them in the same way, and the response would need to be transmitted rapidly across Earth's many ecosystems and continents.

This even applies to climate change:

Even the force of human-induced climate change, so evident across the planet, does not meet these requirements. For example, it warms and dries some regions while cooling and moistening others...

...So there is little chance of anthropogenic climate change leading to a global tipping point in the biosphere. When it comes to other changes, including land use, habitat fragmentation and extinction, the case for a global tipping point is even weaker.

It isn't that the author doesn't think there is a world problem. He does think man is changing the world and often for the worse. But talking in terms of tipping points is neither helpful to the debate not backed by reliable evidence. He argues:

To deny the likelihood of an impending global tipping point is not to deny that we are transforming the biosphere profoundly and permanently in ways that are likely to disgrace us in the eyes of future generations.

We should try to avoid ecological damage by our actions. We should feel as guilty for allowing local ecosystems to degrade as we should about the whole planet. But we should not feel compelled to create poorly evidenced myths of global catastrophe to drive our actions.

Answer 3

The scientific review community responded to Barry Brook et al (2013) "Does the terrestrial biosphere have planetary tipping points?" with over one hundred and fifty citations.

One citation appeared in "On the origin of planetary-scale tipping points" by Timothy M. Lenton and Hywel T.P. Williams (2013):

Barnosky et al. recently proposed that human activities could cause ‘a planetary-scale “tipping point”’ in the ‘global ecosystem’. In response, Brook et al. argued persuasively against planetary-scale tipping points in the terrestrial biosphere, despite the existence of local- and regional-scale tipping points.

...the terrestrial biosphere, in isolation, is not the right place to be looking for a planetary-scale tipping point; one must consider the coupled dynamics of the Earth system as a whole, including evolution.

Their criticism of Barnosky et al is direct:

Barnosky et al.’s central proposal is that human land use will pass a tipping point that triggers greatly increased biodiversity loss. This is inferred from smaller-scale models of population collapse due to habitat fragmentation. However, it is assumed that projected human population growth will be directly correlated with increased land use, when actual per capita land use has steadily diminished over time [13]. Furthermore, a global ‘fold’ bifurcation is invoked without a clear mechanism, making it pure supposition.

In the same issue as Lenton & Williams (2013), we find this article:

Multiscale regime shifts and planetary boundaries

Hughes, Carpenter, Rockstroem, Scheffer, & Walker

Life on Earth has repeatedly displayed abrupt and massive changes in the past, and there is no reason to expect that comparable planetary-scale regime shifts will not continue in the future. Different lines of evidence indicate that regime shifts occur when the climate or biosphere transgresses a tipping point. Whether human activities will trigger such a global event in the near future is uncertain, due to critical knowledge gaps. In particular, we lack understanding of how regime shifts propagate across scales, and whether local or regional tipping points can lead to global transitions.