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.