Trends in extreme events

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Trends inextreme events

• Neville Nicholls
• Monash University, Australia

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• IPCC Second Assessment of Climate Change, 1995
• Overall, there is no evidence, in a global sense,
  that extreme weather events, or climate
  variability, has increased through the 20th
  century, although data and analyses are poor and
  not comprehensive.
• IPCC Third Assessment , 2001
• Increased frequency of heavy precipitation
  events.
• Decreased frequency of extreme low temperatures
  increased frequency of extreme high temperatures.
• Increased drought frequency intensity (some
  regions).
• No significant global trends in tropical
  extratropical storm intensity and frequency
• No systematic changes evident in frequency of
  small-scale events such as tornadoes.

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IPCC Fourth Assessment (2007)
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USA Climate Change Science Program (Kunkel et
al., 2008)

• there has beenan increase in extreme high
  temperatures and a reduction in extreme low
  temperatures
• heavy downpours have become more frequent and
  more intense in recent decades over most of North
  America and now account for a larger percentage
  of total precipitation
• averaged over the continental U.S.A. and southern
  Canadathere is no indication of an overall trend
  drought
• Atlantic tropical storm and hurricane destructive
  potential has increased
• northward shift in the tracks of strong
  low-pressure systems (storms) in both the North
  Atlantic and North Pacific over the past fifty
  years

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Outstanding questions

• Hot and cold days/nights
• Relationship to changes in mean temperatures are
  extremes changing faster than average
  temperatures?
• Heavy rainfall events
• Are these increasing even where total rainfall is
  declining?
• Drought
• What is the influence of warming circulation
  changes?
• Tropical cyclones
• Spatial variations in trends - links to ocean
  temperature
• Extra tropical storms
• Spatial variations in trends - links to general
  circulation
• El Niño - Southern Oscillation
• Has there been a trend to more or stronger Niño
  events?

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Australia laboratory for trends in extremes -
Spans tropics to mid-latitudes - Flat (few
mountains) - Long meteorological records
quality control - Single country (one data
archive)
Meteorology is perhaps a simpler problem in this
land than anywhere else. (Jevons, 1859)
Spencers Creek
Cape Otway
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Melbourne, 1979-2001
27-30 January 2009 42.3C
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Wildfires, late February 2009
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Cool days
Cold nights
Warm nights
Hot days
Alexander et al., 2007. Colour indicates
significant trend in mean temperature 1957-2005
data.
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Coldest night is warming faster than mean minimum
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Number of very heavy rain days decreased in areas
where total rainfall decreased
1950-2008
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Alexander et al 2007, Trends in Australias
climate means and extremes a global context

• trends in extremes of both temperature and
  precipitation are very highly correlated with
  mean trends.
• most stations have greater absolute trends in
  extremes than means.
• some evidence that the trends of the most
  extreme eventsare changing more rapidly than are
  trends for more moderate extreme events.
• consistent with all other global regions studied.

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Droughts are getting hotter, even where they are
not getting drier
After Nicholls (2003, 2004)
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Spring snow melt is earlier - leading to
streamflow changes
After Nicholls (2005)
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Trends in tropical cyclone activity are not
simply related to SST trends
After Nicholls (1985) and Nicholls et al (1998)
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Numbers of extra-tropical storms declining in
some areas
Alexander and Power (2009) storms diagnosed from
very large sub-daily pressure changes
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It is too simplistic to say that there is a trend
towards more extreme climate weather

• More warm days/nights fewer cool days/nights
• Temperature extremes changing rapidly
• Trend in heavy precipitation frequency matches
  trend in total precipitation
• Droughts becoming warmer (if not drier)
• Drought changes linked to circulation trends
• No universal increase in tropical cyclones (or
  link to warming ocean temperatures)
• Latitudinal shift in mid-latitude synoptic
  systems
• No obvious trend towards stronger/more El Niño
  events

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Summary what important extremes have changed
globally?

• More heat waves fewer cold events
• More extreme sea level events
• Still dont know whether small-scale extremes
  (hail, tornadoes) are changing
• Changes in other extremes are regionally
  dependent
• But they are changing - less predictable