Defining Extremes - FAQs

In order to explore the subject of extreme weather and climate change, there's acouple of FAQ's that I need to clarify. If you have any further burning questions, please comment and I will follow them up in future posts.

What justifies a weather event to be regarded as extreme?

In short, a weather event can be described as extreme when it lies far beyond (on either side) of the mean and median of that weather event. The easiest way to address this question is using statistics with the aid of some simple graphs:

Any event or climate variable (such as temperature or precipitation) can be described in terms of its distribution, and the solid central curve in graph (a) could represent the the present day frequency distribution of this. Shading indicates the extreme parts of the distribution (events which occur infrequently, that are far from the mean or median value).

If there is a shift of the distribution in a future climate, there will be an increase in extreme events on one end, and a decrease at the other (a). For example, if the temperature at a certain location warms by an x amount, this will almost certainly produce an increase in the number of extreme hot days and a decrease in the number of extreme cold days. It is also important to note that there is a non-linear relationship between change in mean of the distribution and extreme changes. A small change in mean can result in a large change in the frequency of extremes (Read this paper for more info on this).

The standard deviation in a future climate may change, producing changes in extreme events at both ends of the frequency distribution (b). A change in variance of a distribution has a larger effect on the frequency of extremes than a change in the mean (but has to be more than one standard deviation from the mean in order to be extreme and hold true). For example a 1 degree Celsius change in the standard deviation of the distribution will have a larger impact on the extremes than a 1 degree change in the mean of the distribution. A good paper explaining this further can be found here.

To complicate things further, you can end up with something such as that in graph (c), whereby both the mean and variance change, altering the occurrence of extremes in several ways. An example of this would be precipitation, which is not normally distributed, and a change in mean also causes a change in variance. 

Are the effects of extreme weather always negative?

No, although the negatives tend to far outweigh the positives. Despite extreme weather events often being associated with negative effects on society and the natural environment, through events such as storms, droughts, extreme colds and floods, there are also some advantages. For example, energy production and reduction in energy costs (e.g. from power generated through wind turbines in windier conditions), surplus of water in some areas of Australia (following cyclones), which feeds longer term storage. Arctic, Antarctic and Siberia may experience more plant growth and greater biodiversity with a warming climate. Also, changes in precipitation patterns may result in longer growing seasons in some areas. However, with each of these positives come many negatives, including loss of power, damage to infrastructure, reduction of growing seasons, loss of biodiversity... the list goes on.

As a short and sweet introductory reading to this subject, I can suggest this free-to-access paper which gives a general overview of trends in extreme weather and climate events. It also touches upon the effects of such changes on the human and natural environment.