Is runaway climate change possible?


in Science, Security, The environment

One aspect about the possibility of runaway climate change needs to be clarified. The basic mechanism through which it could take place is akin to a feedback loop in a sound system: a small initial warming gets amplified through a feedback, producing more warming in a manner that itself generates even more warming. For such a loop to occur, the feedback effect needs to be quite strong.

Stefan–Boltzmann’s law expresses this mathematically. For an intuitive appreciation, consider the difference between bank lending and a nuclear chain reaction. In an idealized case, a bank would draw from the savings of customers to make a loan. The recipient of that loan might then put part of it in the bank, and the bank may then make additional loans on the basis of that. The total lending of the bank becomes larger than the original loan, but to a non-infinite extent. By contrast, each time an atom of uranium splits in a runaway chain reaction, it releases neutrons that cause more than one other atom to split as well. The result is a reaction occurring at an ever-increasing rate.

It is quite possible that genuine runaway climate change is not possible on Earth – that the existing feedbacks are of the bank lending rather than the nuclear blast variety. That being said, the possibility of warming itself producing further warming remains extremely worrisome. It wouldn’t require ever-escalating temperatures for climate change to be globally devastating. Quite probably, any warming of more than 5˚C would deserve the adjective. The most credible climatic models project approximately that level of warming by 2100, if emissions continue to increase at the present rate.

{ 18 comments… read them below or add one }

Tristan October 20, 2008 at 9:47 am

I think what people mean by runaway climate change is a feedback loup that makes human C02 production a small part of the excessive C02 emissions.

“Runaway” can just refer to any positive feedback loop at all, not necessarily infinite ones. For example, I think we’d all agree that the rainforests burning are a runaway example of climate change, even if there aren’t infinite rainforests.

Milan October 20, 2008 at 10:35 am

I think the defining element of runaway climate change is that humanity loses the ability to return to a stable climate, except possibly through geoengineering.

Right now, most scientists think that cutting emissions to zero would eventually stabilize the climate at a temperature level not enormously above the one that exists now. A runaway scenario would leave the planet more like Venus: fundamentally transformed in terms of temperature and physical processes:

“Venus succumbed early to a “runaway water vapor greenhouse,” in which the increased water vapor content arising from increased temperature reached an end state with much of the ocean evaporated into the atmosphere. Once this happens, it is easy for the water vapor to decompose in the upper atmosphere, whereafter the light hydrogen escapes and oxygen either escapes or reacts with rocks. One hypothesis is that the weak magnetic field at Venus, which otherwise would protect the planet from the solar wind, is one reason for why the oxygen and hydrogen escaped faster into space. Once water is lost, the reaction that turns carbon dioxide into limestone can no longer take place, so CO2 outgassing from volcanoes accumulates in the atmosphere instead of staying bound up in the rocks. The end state of this process is the current atmosphere of Venus, with essentially no water in the atmosphere and essentially the planet’s whole inventory of carbon in the form of atmospheric CO2. Earth, in contrast, kept its water, which allowed the planet to keep most of its carbon inventory safely bound up in the crust. The amount of CO2 in the atmosphere of Venus is approximately the same as the amount of CO2 bound up in the form of carbonate rocks on Earth today.”

tristan October 20, 2008 at 2:38 pm

I see. So, do you think that geo-engineering Venus would be good practice for here on earth?

Milan October 20, 2008 at 3:27 pm

What happened to Venus probably cannot be reversed without god-like powers. You would need to cool it down, change the composition of the atmosphere and lithosphere, regenerate the hydrosphere, and (if the magnetic field theory is right) alter the behaviour of the planet’s internal components.

. October 20, 2008 at 3:32 pm

Terraforming Venus

Terraforming of Venus is the (theoretical) process of engineering the global environment of the planet Venus in such a way as to make it suitable for human habitation. There is considerable debate as to whether or not terraforming Venus is possible. The existing environment of Venus would require three major changes to the planet:

* Reducing Venus’s 450°C (850°F) surface temperature.
* Eliminating most of the planet’s dense 9 MPa (~90 atm) carbon dioxide atmosphere, via removal or conversion to some other form.
* Addition of breathable oxygen to the atmosphere

R.K. October 20, 2008 at 9:16 pm

What happened to Venus probably cannot be reversed without god-like powers

We have ‘god-like powers’ compared with people who lived 500 years ago.

Neal October 20, 2008 at 9:48 pm

I would have thought that runaway climate change describes the kind of thermal maximum hypothesized to have caused the end-Permian extinction. Rapidly rising CO2 or another comparable forcing leads to the biosphere releasing vast amounts of additional CO2 and perhaps methane from permafrost, clathrates, rotting organic matter, etc. which causes the chemistry of the oceans to change to the point where hydrogen sulfide producing bacteria dominate, oxygen levels plummet and toxic H2S skyrockets and virtually all aerobic life suffocates.

Milan October 21, 2008 at 11:19 am


It seems that there are various conceptions of just how far climate needs to run in order to qualify as ‘runaway.’

This video uses the term to describe a temperature rise consistent with the projection for business-as-usual emissions from the Hadley Centre (5.5 to 7.1 degrees by 2100).

The Permian-Triassic Extinction event definition is intermediate. It would certainly be horrific, and almost certainly mean the end of humans. We also know it is possible on Earth, though the original may have been kicked off by volcanic activity or an extraterrestrial collision.

The Venusian example is about as extreme as you can get: permanent, massive, probably irreversible recomposition of the atmosphere. The article linked above saying runaway climate change is impossible on Earth is referring to this definition.

Milan October 21, 2008 at 11:25 am

I actually have a Post-It on my monitor that says “Permian-Triassic Extinction Event,” to remind me of the worst that can happen.

Sarah October 21, 2008 at 4:06 pm

Interesting post, yay for terminological rigour & consistency. Also, that sounds like a cool sort of Post It note to have – perhaps you should produce a range of similar ones and stick them onto the monitors of other people? A sort of stealth green campaign within the civil service…

Milan October 21, 2008 at 4:09 pm

On my P-T Event note, one of my co-workers has written:

“Looking on the bright side?”

Milan October 21, 2008 at 4:12 pm

This chart is also worth pinning to one’s wall, as a reminder of how much faster some things are progressing than expected.

DMG November 26, 2008 at 3:17 pm

Another question is how quickly a runaway scenario would play out. If it would take thousands of years, like the melting of Greenland and the WAIS, then it might be something that could be stopped using air capture or geoengineering.

Milan November 27, 2008 at 11:20 am

It certainly seems preferable to avoid kicking off runaway change, rather than initiating runaway change that starts slowly enough for us to have a chance at stopping it later.

That being said, slow onset runaway change is clearly better than a fast onset variety. It would give us a chance to see if anyone’s geoengineering idea actually works.

. December 3, 2008 at 11:15 pm

Stephen Hawking warns about warming
Celebrity cosmologist says Earth could end up as hot as Venus

Hawking said he was “very worried about global warming.” He said he was afraid Earth “might end up like Venus, at 250 degrees centigrade [482 degrees Fahrenheit] and raining sulfuric acid.”

. June 26, 2009 at 2:06 pm

Ice sheets can retreat ‘in a geologic instant,’ study of prehistoric glacier shows
Published: Sunday, June 21, 2009 – 12:56 in Earth & Climate

Modern glaciers, such as those making up the Greenland and Antarctic ice sheets, are capable of undergoing periods of rapid shrinkage or retreat, according to new findings by paleoclimatologists at the University at Buffalo. The paper, published on June 21 in Nature Geoscience, describes fieldwork demonstrating that a prehistoric glacier in the Canadian Arctic rapidly retreated in just a few hundred years.

The proof of such rapid retreat of ice sheets provides one of the few explicit confirmations that this phenomenon occurs.

Should the same conditions recur today, which the UB scientists say is very possible, they would result in sharply rising global sea levels, which would threaten coastal populations.

“A lot of glaciers in Antarctica and Greenland are characteristic of the one we studied in the Canadian Arctic,” said Jason Briner, Ph.D., assistant professor of geology in the UB College of Arts and Sciences and lead author on the paper. “Based on our findings, they, too, could retreat in a geologic instant.”

. November 30, 2009 at 9:49 am

Climate Tipping Points of No Return

Climate change won’t be a smooth transition to a warmer world, warns the Tipping Points Report by Allianz and WWF. Twelve regions around the world will be especially affected by abrupt changes, among them the North Pole, the Amazon rainforest, and California.

Sea level rises, unpredictable monsoons in India, Amazon die-back, and the desertification of Southwest North America (California and neighboring states) are the most significant climate change catastrophes we face, according to the report’s authors from the Tyndall Centre for Climate Change Research.

In the Southwestern U.S. the tipping point has probably already been passed. The scientists now predict that levels of aridity last seen in the 1930s Dust Bowl will have become the norm by mid-century.

However, in some cases the report offers reassurance. Permafrost melt in Eastern Siberia could release greenhouse gases CO2 and methane. But it would require an extreme 9 degrees Celsius surface warming for the system to tip. Claims that the release of greenhouse gases trapped in the permafrost will lead to runaway global warming are “grossly exaggerated,” the authors conclude.

peer-reviewed science December 21, 2018 at 10:48 pm

Domino-effect of climate events could move Earth into a ‘hothouse’ state

Leading scientists warn that passing such a point would make efforts to reduce emissions increasingly futile

Trajectories of the Earth System in the Anthropocene

We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System—biosphere, climate, and societies—and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.

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