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What Does “Abrupt” Mean?

A couple of years ago, I was asked to help in the writing of a report from the U.S. Climate Change Science Program on Abrupt Climate Change. The 460-page report appeared some months ago, but mercifully a short summary was also provided. I was pleased to see that the lead authors defined “abrupt” carefully and clearly – I will come to that definition later – but working out what “abrupt” means caused me a good deal of trouble when I was just getting started.

Abrupt climate change is the name of a new branch of climatology, or rather of palaeoclimatology. I kept asking the palaeoclimatologists what the word meant, and got a variety of answers that were not entirely satisfactory. Eventually one of them said, laconically, “faster than the forcing”, and I decided that that was the answer I was looking for.

You need to understand here that “forcing” is scientists’ jargon for “cause”, a word that we don’t like because it is philosophically very shaky. Loosely, the forcing is the input to the system and, in the case of the atmosphere, “climatic change” is the output. Carbon dioxide from fossil fuels is an input. Warming is one of the expected outputs, and so is sea-level rise. In their present-day configuration, glaciers (excluding the ice sheets) are transferring about an additional half a gigatonne of water to the ocean, per year, for every gigatonne of carbon dioxide added to the atmosphere.

The current rate of addition of carbon dioxide to the atmosphere is about 28 Gt/yr. You are free to think either that this is or is not rapid. It is consistent with the calculation that the rate of transfer of meltwater, currently about 400 Gt/yr, is growing at about 12 Gt/yr. Again, you may or may not consider this rapid.

My original problem was that I think “abrupt” has to be more serious than “rapid”. If you don’t think the present-day changes are rapid, just wait a bit. If you want abrupt change, you may have to wait a bit longer, and could well be disappointed, but you can find lots of examples by looking back rather than ahead. Dansgaard-Oeschger transitions are examples of abrupt warming, well-documented over the course of the last ice age. There was a disconcerting cold snap at about 6250 BC – disconcerting to us, although our Mesolithic forebears had so little capital invested that they may have shrugged it off or even failed to notice it. There is evidence of still bigger abrupt changes further back in the past, up to tens of millions of years ago.

The definition settled on for our report on abrupt change was “a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems”. In other words, “inconveniently rapid for the next generation or two of human beings”. You may prefer this to “faster than the forcing” because of its greater immediacy, but it doesn’t tell you as much about how things work.

Whichever definition you choose, it is important to realize that “rapid” does not merge smoothly into “abrupt” as the forcing grows more intense. The point about “abrupt” is that it is not what the forcing would lead you to expect. Could it happen to us, or to our grandchildren? There doesn’t seem to be any reason why not, even though we can’t assign a probability to it with any confidence.

Posted by Graham Cogley on August 3, 2009 3:00 PM |

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Comments (14)

  • 1 dan.hatton August 6, 2009 1:02 PM

    I'm more than a little surprised by the suggestion that having a lot of capital invested increases one's vulnerability to climate change. Would you care to expand on it, please?

  • 2 Alan Page August 6, 2009 1:49 PM

    Dan,
    A grocery store is basically a very deeply capitalized system of making sure that you can eat. A low capital and low vulnerability situation is hunting for the next meal with a long stick. If the game is moving south because they can't find food where they were you move with them, no house, no roads, no trucks, just bare feet and a few tools (sticks, and stones). If you are used to following the food you would not even notice the changes that were happening where the food no longer grew.

  • 3 Graham Cogley Author Profile Page August 7, 2009 3:17 AM

    Dear Dan and Alan – I think Alan has expanded on my meaning very well, but I was really just echoing an idea that has been current for quite a while. Indeed there is a proverb that says it: The bigger they are, the harder they fall. The idea is usually an implicit criticism of our complicated western lifestyle, but on reflection I am no longer so sure it is apposite. It all depends on how hard you fall and on what resources you have left with which to pick yourself up again.

    Those who experienced the 6200 BC cold snap certainly did not understand what was happening. After all, we only figured it out about a decade ago, and we needed mass spectrometers and a wealth of knowledge about other parts of the picture. But if the cold snap was the cause of human suffering, perhaps we ought not to belittle the suffering by saying things like “Oh well, they were able to shrug it off”.

    Both sides of this question seem to be arguable. But on the whole I am happier living in the Anthropocene than I think I would have been in the Mesolithic.

  • 4 Alan Page August 7, 2009 10:36 AM

    Dan,
    I agree with your sentiments and the difficulties of the heights from which one can fall. I too work at living in this time, and wonder what it will be like to have been part of the cause of an abrupt change? Watching this change happening and talking about how it is going to affect us seem to be very different activities that can not be shared for very long or very well. As you say the human preference is to live as we are rather than as we have in the past or may in the future. The real question is how to understand the forcings and limit them before things happen at a rate faster than the forcing.