Defining climate change terms

It is important to avoid carelessly lumping together things that are very different, especially in the language we use to discuss climate change. Likewise, it is important to avoid using the same term to mean different things. How then should we understand the terms ‘dangerous,’ ‘catastrophic,’ and ‘runaway’ in relation to climate change? Each term should have a definition which is clear and comprehensible, and which does not obviously overlap with the definition of the others. That allows people to discuss the different levels without excess confusion about what people mean by things.

As human beings burn fossil fuels and otherwise tinker with the planet’s carbon cycle, we produce an increasing amount of climate change. The extent of this change can be described as happening at different levels, either numerically or descriptively. This can be done in several technical ways.* While these are useful, I think there are three descriptive terms that are among the most useful, for distinguishing between future scenarios. They are defined not in relation to one number or another, but in terms of their overall effect on humanity and the Earth:

Dangerous climate change

Bad as it would be, this is the least serious level of warming I will define. Of course, your definition of ‘dangerous’ depends on your interests and situation. Climate change is already dangerous for polar bears, Bangladesh, and small island states.

For this term, I will adopt the definition that has become common among scientists and policy-makers: a mean temperature increase of more than 2°C, compared with pre-industrial temperatures. Basically, a world 2°C hotter than the one that existed before we started seriously burning fossil fuels.

Note that we have already created 0.74°C of warming, according to the Fourth Assessment Report of the IPCC, and that some additional warming is inevitable on account of the greenhouse gases we have already emitted. This term already has importance in international law. The objective of the United Nations Framework Convention on Climate Change (UNFCCC), which the Kyoto Protocol extended, is to “prevent dangerous anthropogenic interference with the climate system.”

Catastrophic climate change

I define this as warming at such a level that it threatens the continued existence of human civilization, for instance by radically altering regional weather patterns and putting massive strain on agricultural systems.

The exact temperature level at which this would occur is impossible to know for certain in advance, but I don’t think it is implausible to put warming of more than 4°C to 5°C in this category. There is a plausible case that the relatively stable climate of the Holocene was one major reason for the emergence of agriculture and civilization as we know it. A sufficient level of climatic disruption could put that in jeopardy.

Runaway climate change

This requires careful definition, since there are always feedback effects in the climate system. One of the most important is water vapour; when greenhouse gas concentrations rise and increase the quantity of solar radiation retained by the Earth system, that warming increases the amount of water vapour held in the atmosphere. Since water vapour is itself a greenhouse gas, that feedback causes further warming. Because of feedbacks of this kind, every time the climate system gets ‘pushed’ a little bit, it ‘runs away’ a bit further before coming to a new equilibrium. When I refer to ‘runaway’ change, I am not referring to this ordinary situation. Rather, I am talking about when change begins to beget ever-more change, like a screeching microphone too close to a speaker.

Runaway climate change would be a cascading amplification of warming in which positive feedbacks build upon one another and the climate system is ultimately put into a radically different state. This can be understood by analogy. When human activities alter the climate, it is a bit like rocking a vending machine back and forth. You can tilt it a certain distance forward or back and, when released, it rocks back to its original vertical state. Pushing it into a runaway circumstance is like tilting it so far that it begins falling over under its own weight. In the climate system, this could be caused by things like melting ice vanishing, meaning less solar radiation gets reflected into space. It could also arise partly from powerful greenhouse gases trapped under the ground and sea in the Arctic escaping because the air and water around them warm up.

In a worst-case scenario, this could make the entire planet permanently hostile to life. We know that planets can move from a state potentially hospitable to life to one that is utterly hostile. At one point, there was liquid water on the surface of Venus. A brightening sun caused runaway climate change on that planet, and now the mean surface temperature is over 400°C. It is not known with certainty whether runaway climate change is possible on Earth, or could be induced by human activity.

I think I have generally used these terms with these meanings in past posts on this site and a sibilant intake of breath. I shall certainly endeavour to use them consistently in the future.

Avoiding dangerous climate change is an ethical necessity and highly desirable practically, though the political will to achieve it is entirely lacking globally. Avoiding catastrophic or runaway climate change is absolutely necessary if any of the other ambitions of humanity are to be achieved. If we fail to achieve that, future generations will be correct in cursing us for our selfishness and lack of vision.

* For instance, in terms of parts per million of carbon dioxide equivalent in the atmosphere, in terms of changes in radiative forcing measured in watts per square metre, or in terms of mean surface temperature change.

20 thoughts on “Defining climate change terms

  1. Byron Smith

    Thanks, this is a helpful clarification of terms. Is there an adjective you would use to qualitatively describe presently experienced climate change?

  2. Milan Post author

    ‘Worsening’ and ‘accelerating’ seem like the obvious choices.

    The rate at which emissions are accumulating in the atmosphere continues to increase.

  3. Milan Post author

    ‘Accelerating’ is really the best word for it.

    As long as emissions continue to rise, the rate at which climate is changing will continue to increase. When emissions start to fall, climate change will still be worsening (because concentrations will still be rising), but it will be worsening at a decreasing rate.

    Finally, when we reach global carbon neutrality, climate change will cease – once the lag effects associated with past emissions play through.


    Mathematically, we can think of three different functions:

    • a concentration function, f(x)
    • a net emissions function, which is the first derivative of that, f'(x)
    • and a function for the rate of change of net emissions, f”(x)

    Now, take a look at the dotted line here, which is one possible path for global emissions – f'(x).

    As long as the line is above zero, climate change will continue to worsen. Out to the point where the line peaks, climate will worsen at an increasing rate (with f”(x) > 0). Afterward, it will worsen at a decreasing rate (with f”(x) < 0).

    In short, when emissions peak, climate change will no longer be accelerating, but it will still be worsening. It will only stop worsening when net global emissions reach zero and the full effect of any lags has manifested itself.

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  5. Byron Smith

    Yes, I don’t think we’ll be arguing about what to call 6.4ºC warming once it comes – there will be too many other things happening.

    Thanks, “accelerating” is good. “Worsening” if and when carbon emissions are falling.

  6. Milan Post author

    It may also be important to think about the rate at which temperature is changing and the rate at which adaptation activities are being undertaken.

    0.1ËšC of warming, spread over 1000 years, seems like something we would hardly notice. 2ËšC or more over a century would be enormously more taxing.

    That said, even with maximum adaptation effort, it seems that there are some things we could not save, once a certain level of warming has taken place. For instance, there is a level where summer sea ice in the Arctic will basically be gone, with potentially grave effects for some species. Likewise, there is a level at which the Greenland and Antarctic ice sheets become doomed to disappear, though it could happen more or less quickly.

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  8. .

    ‘Paltry’ Copenhagen carbon pledges point to 3C world
    By Richard Black
    Environment correspondent, BBC News

    Pledges made at December’s UN summit in Copenhagen are unlikely to keep global warming below 2C, a study concludes.

    Writing in the journal Nature, analysts at the Potsdam Institute for Climate Impacts Research in Germany say a rise of at least 3C by 2100 is likely.

    The team also says many countries, including EU members and China, have pledged slower carbon curbs than they have been achieving anyway.

    They say a new global deal is needed if deeper cuts are to materialise.

    “There’s a big mismatch between the ambitious goal, which is 2C… and the emissions reductions,” said Potsdam’s Malte Meinshausen.

    “The pledged emissions reductions are in most cases very unambitious,” he told BBC News.

    In their Nature article, the team uses stronger language, describing the pledges as “paltry”.

    “The prospects for limiting global warming to 2C – or even to 1.5C, as more than 100 nations demand – are in dire peril,” they conclude.

    Between now and 2020, global emissions are likely to rise by 10-20%, they calculate, and the chances of passing 3C by 2100 are greater than 50%.

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  10. .

    Global Warming: Future Temperatures Could Exceed Livable Limits, Researchers Find

    ScienceDaily (May 4, 2010) — Reasonable worst-case scenarios for global warming could lead to deadly temperatures for humans in coming centuries, according to research findings from Purdue University and the University of New South Wales, Australia.

    While the Intergovernmental Panel on Climate Change central estimates of business-as-usual warming by 2100 are seven degrees Fahrenheit, eventual warming of 25 degrees is feasible, he said.

    “We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world’s population in an uninhabitable environment,” Huber said. “When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It’s the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing.”

    Steven Sherwood, the professor at the Climate Change Research Centre at the University of New South Wales, Australia, who is the paper’s lead author, said prolonged wet-bulb temperatures above 95 degrees would be intolerable after a matter of hours.

    “The wet-bulb limit is basically the point at which one would overheat even if they were naked in the shade, soaking wet and standing in front of a large fan,” Sherwood said. “Although we are very unlikely to reach such temperatures this century, they could happen in the next.”

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  19. .

    The IEA also looked at what it might take to hit a two-degree target; the answer, says the agency’s chief economist, Fatih Birol, is “too good to be believed”. Every signatory of the Copenhagen accord would have to hit the top of its range of commitments. That would provide a worldwide rate of decarbonisation (reduction in carbon emitted per unit of GDP) twice as large in the decade to come as in the one just past: 2.8% a year, not 1.4%. Mr Birol notes that the highest annual rate on record is 2.5%, in the wake of the first oil shock.

    But for the two-degree scenario 2.8% is just the beginning; from 2020 to 2035 the rate of decarbonisation needs to double again, to 5.5%. Though they are unwilling to say it in public, the sheer improbability of such success has led many climate scientists, campaigners and policymakers to conclude that, in the words of Bob Watson, once the head of the IPCC and now the chief scientist at Britain’s Department for Environment, Food and Rural Affairs, “Two degrees is a wishful dream.”

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