We evaluate climate using statistical summaries of weather data collected over longer time periods, including means, percentiles, and extremes of various weather parameters such as temperature, precipitation, and wind. Climate can be evaluated on spatial scales ranging from global to regional to local and even micro-scale and for temporal scales ranging from days to months to seasons to years to decades to centuries to millennia and beyond. Weather extremes are part of climate and both weather and climate vary depending on temporal and spatial scales. Weather extremes and climate can also change over time and with longer time periods the magnitude of variation is likely to be larger. This complexity makes evaluating climate trends and associated extreme weather event trends very difficult. Any evaluation is relative – both spatially and temporally.
However, some simple statistics can be applied to examining weather extremes. If we choose a local spatial scale and a millennia time scale, we can look at probabilities of the occurrence of extreme weather events, such as extreme cold and hot temperatures, extreme precipitation, and extreme drought for instance. If we assume extreme weather events are random and select 1,000 weather monitoring locations somewhat evenly distributed around the globe and at least 200 kilometers apart, we can expect that on average each year one location will have a once in 1,000 year extreme weather event (relative to that location), somewhere around the globe. Because of random effects, some years might have none and some might have two or three stations with once in 1,000 year events. Similarly, we can expect that on average each year two locations will have a once in 500 year event, five locations will have a once in 200 year event, ten locations will have a once in 100 year event, 20 locations will have a once in 50 year event, and 50 locations will have a once in 20 year event (for statistics relative to each location). These statistics apply to each type of extreme weather event separately, including extreme cold temperatures, extreme hot temperatures, lengthy heat waves, lengthy cold waves, extreme precipitation amounts, lengthy wet periods, and lengthy droughts.
The point is that with news media today reporting extreme weather events around the globe, there will be many very extreme weather events reported somewhere around the globe every year. This situation is to be expected as a part of normal weather and climate conditions and is nothing unusual.
To evaluate whether there are any significant trends over time for extreme events is difficult and requires long periods of weather data. From a global perspective, the necessary weather data records are insufficient spatially and temporally to determine any significant trends at present. From an individual location perspective, there are a few locations with long periods of record where it may be possible to evaluate changes over time by comparing non-overlapping 30-year periods. Ideally at least 150 years of complete data would be needed from a single location to have five different 30-year periods to compare for trend analysis, but very few locations have enough complete weather data for that length of time. Consequently, we cannot have much confidence in any claimed trends for extreme weather events until weather data are accumulated over much longer time spans, ideally 200 to 300 years or more, for examining trends over time.