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).


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?

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    Don't mind down votes, but it's nice to know why so questions can be improved. – matt_black Oct 19 '12 at 21:33
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    It's almost a given that when "some scientists say one thing, some say another" that the right answer is going to be "we don't know". What is absolutely certain is that if a large community of expert scientists can't agree on something, a bunch of random guys on the internet isn't going to give you a definitive answer. – DJClayworth Oct 20 '12 at 14:52
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    How do you define "tipping point?" How do you define "close?" In a sufficiently delicate system, the system is always "close to the tipping point". That isn't always the same as "imminent disaster", however. I think there are too many fuzzy terms in the claim for it to be meaningful. – Flimzy Oct 20 '12 at 20:56
  • Interesting, how same people who talk about "being a the tipping point", at same time what they say about geoenigneering is that "it's too early for such a drastic measure" – vartec Oct 21 '12 at 10:04
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    The question is so vague as to prevent a true skeptic from forming an opinion. "Imminent" is certainly a relative term when we are talking about the possible destruction of life or at least civilization. Some hyperbole and vagueness might be expected in the face of continuing apathy. In your quotes above he steers clear of any concrete timeline or specific date of no return. It hardly matters given the stakes. My interpretation of "tipping point" and what makes it valuable as a rhetorical device is that it would be invisible at the moment it passed. If the tipping point is the point after whic – timquinn Dec 31 '12 at 5:07

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

Yes. We've covered this previously too:

Are there potential positive feedbacks in the climate system?

Yes. For example:

Are positive feedbacks being triggered?

Yes.

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.

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    I have no problem with the logic that tipping points are possible, but that is different to demonstrating their likelihood or proximity. If methane clathrates, for example, are going to cause catastrophic positive feedback soon, then why didn't they do it in the last interglacial (warmer than now!) and prevent the ice age we are only just out of? By the way the observed methane releases are apparently not new. – matt_black Oct 20 '12 at 9:58
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    @matt_black: We may have a definition problem (which Henry alludes to). I didn't think tipping point necessarily meant "exceeds the parameters by which it could ever return", just a point at which positive feedback causes small changes to snowball into more rapid change in the short-term. If you think Hansen is arguing for the former, I think you should extract his definition into the question. – Oddthinking Oct 20 '12 at 22:45
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    @Oddthinking I've tried to clarify the definition problem by using Hansen's own words in the question which are fairly doommonger-ish. – matt_black Oct 21 '12 at 17:13
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    @matt_black ok, I'll work on revising the answer, now that the question has been substantially revised. It may take a day or so to pull the sources together. – EnergyNumbers Oct 21 '12 at 18:10
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    The clearest example of a tipping point is sea ice in the arctic: warmer weather means less ice which means the sun shines on the water instead of the ice which absorbs much more heat. And we've been having record low sea ice. Since it's sea ice, it's unlikely to be catastrophic on its own except for polar bears (no sea level increase), but it would be a major change. – Rex Kerr Oct 26 '12 at 15:16

There is little hard evidence for 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.

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    I tried reading this several times, and kept coming up bewildered: "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." What? How does this fit with, for example, the theory of local clathrates emitting methane that affects the entire atmosphere? Neither globally uniform nor synergistic, but provides a potential global tipping point. – Oddthinking Mar 10 '13 at 23:21
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    @Oddthinking True, if you can prove that clathrates will actually do that. Trouble is they haven't in the past, even when it has been a lot warmer than now. So we would need more than scare stories to take the threat seriously. This objection to theoretical climate tipping points is pretty universal. – matt_black Mar 11 '13 at 0:44
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    This is a tricky point to make: the argument he makes is (to my reading) unsound. That doesn't mean his conclusion is false or clathrates will emit methane. But his method of reaching his conclusion can't be trusted. Why? Because his axiom that the only way to have a global tipping point is to meet certain theoretical prerequisites is false. We can posit other mechanisms that could do it without hitting his prerequisites - e.g. clathrates, melting ice-caps, etc. Even if they turn out to be a false alarms, they demonstrate his assumptions are false. – Oddthinking Mar 11 '13 at 4:41
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    @matt_black two things: first, I've added a link to the full paper, see if you find better quotes in there; second, I think you are over-representing your evidence. Yes, there's this article, but it's an opinion piece, it does not represent the scientific consensus on the matter, and you should certainly mention that... reading your answer it seems to imply that this is what most scientists think, but it doesn't correspond to the evidence you present. "There is dissent as to whether these tipping points exist. For example according to..." – Sklivvz Sep 30 '16 at 20:49
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    "Hansen's arguments are also his personal opinion and not backed by strong science and not representative of a consensus." neither is the article that @matt_black cites. Just because something appears in a peer-reviewed journal does not mean it is reliable, and that applies to Hansen's papers as much as any other. To get an idea of reliability you first need to investigate the response of the research community to it (e.g. look at the papers that cite it). – Dikran Marsupial Oct 5 '16 at 7:29

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