On record-breaking events

NASA’s Earth Observatory reports that there was a record low Arctic sea ice concentration in June 2005. There was a record-number of typhoons over Japan in 2004. In June, there were reports of a number of record-breaking events in the US. And on July 28, the British News paper The Independent reported on record-breaking rainfall (~1 m) in India, claiming hundreds of lives. These are just a few examples of recent observations. So, what is happening?

Whenever there is a new record-breaking weather event, such as record-high temperatures, it is natural to ask whether the occurrence of such an event is due to a climate change. Before we proceed, it may be useful to define the term ‘statistically stationary’, the meaning here being that statistical aspects of the weather (means, standard deviation etc.) aren’t changing. In statistics, there is a large volume of literature on record-breaking behaviour, and statistically stationary systems will produce new record-breaking events from time to time. On the other hand, one would expect to see more new record-breaking events in a changing climate: when the mean temperature level rises new temperatures will surpass past record-highs. This is illustrated in Fig. 1.

Fig. 1. An example showing two different cases,/>studies that have found regional trends in temperature extremes (particularly low temperatures).  There have also been <a href=some reports on trends of more extreme precipitation, although The International Ad Hoc Detection and Attribution Group (IDAG, 2005) did not manage to attribute trends in precipitation to anthropogenic greenhouse gases (G) – a quote from their review article is: “For diurnal temperature range (DTR) and precipitation, detection is not achieved”, here ‘detection’ implying the signal of G. A high occurrence of new record-events is an indication of a change in the ‘tails’ of the frequency distribution and thus that values that in the past were considered extreme are becoming more common.

But how does one distinguish between the behaviour of a stable system to one that is undergoing a change in terms of record-events? This kind of question has traditionally not been discussed much in the climate research literature (e.g. record-events are not discussed in DIAG, 2005 or the IPCC TAR), perhaps because it has been perceived that analysis on record-breaking events is difficult if not impossible. There are many different types of record-events related to climatic and weather phenomena. One hurdle is that the number of record-/extreme-events is very low and there is not enough data for the analysis. Another problem is the data quality: does the sensor give a good reading near the ranges of their calibrated scales?

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