Weeks, Months, Decades, Centuries or Millenniums?

Much care should be given to defining abrupt climate change before we move on to analyse its impacts and study how we should manage it. The climate is defined as 'a broad composite of the average condition of a region, measured by its temperature, amount of rainfall or snowfall, snow and ice cover, wind direction and strength as well as other factors' (Ruddiman 2008). It generally refers to long term fluctuations rather than short term changes.

Palaeoclimatologists generally define abrupt climate change by giving examples of climatic events such as the 8.2 KaBP event, the Younger Dryas or D-O cycles. These events generally last for a millennium. In light of this, as well as the definition of 'climate' offered by Ruddiman, how long is the 'long term'? and how short is the 'short term'? Today's post will set a definition for abrupt climate change that will be used for the rest of the blog. 

Defining the 'abruptness' of climate change is a tricky affair as presented by Professor Mark Hulme. He identifies two definitions that have been used in the past:

- 'abruptness' can be defined in relation to thresholds and nonlinear behaviour of the climate system rather than simply in terms of magnitude or rate of change;

- 'abrupt' climate change can also be viewed as a significant change in climate relative to the accustomed or background climate experienced by the economic or ecological system being subject to the change, having sufficient impacts to make adaptation difficult. 

However, Hulme puts forward the argument that these definitions are inappropriate for analysing the adaptive behaviour in society to abrupt climate change as they are either too broad (as in definition no. 2 which would encompass most scenarios of climate change) or too restrictive (definition no. 1) such that social and ecological impacts are neglected.

He thus proposes a framework to define abrupt climate change by categorising climatic changes by their rate, severity and direction of change. If climate change exceeds predetermined thresholds in rate or severity and if there is a prolonged reversal in the direction of climate change, such climatic changes can then be considered as 'abrupt'. This can be seen as a combination of both definitions mentioned above.

On another note, Hulme also realises that there are limitations to using fixed definitions of abrupt climate change to extend the concept beyond palaeoclimatology. Scientists studying climatic epochs would define abruptness as a matter of decades or centuries whereas abrupt changes in society are usually measured in terms of months or years. Hence, there is a confusion in the time-scales of physical and social processes that are in play when analysing the different impacts of abrupt climate change.

In light of the confusion that might result from cross-disciplinary analysis, this blog will define abrupt climate change using the three categories of rate, severity and direction proposed by Hulme. If the rate of sea level rise (or global warming) for particular regions rise above a certain threshold, or an unprecedented weather/climatic event occurs where there is a sustained change in the direction of climate change, this will be considered as abrupt climate change. Hence, when economic analysis is conducted for the impacts of abrupt climate change, this blog would engage the discussion with reference to the time-scale of such abrupt changes in climate.

One must note that this definition is in no way exhaustive or complete. The literature has revealed that it is very difficult to distinguish small changes in the climate from its natural variability. However, the purpose of this blog is not to strictly define the 'abruptness' of climate change and hence, we will be using the framework presented above while understanding its limitations.

With the definition for abrupt climate change established, I will next embark on looking at the role of economics in managing environmental change in the following post. See you next time!

For more information on the discussion on the definition of abrupt climate change, please refer to Hulme's paper:

Hulme, M (2003) Abrupt climate change: can society cope?, Phil. Trans. R. Soc. Lond. A, 361, 2001-2021. 

For a brief introduction on abrupt climatic events in the past, a simple description is offered in the following book:

Archer, D (2009) The Long Thaw, Princeton, Princeton University Press, Chapter 4.