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Extreme weather:
Climate change or
climate variability?
Dr. Nicholas Klingaman
National Centre for Atmospheric Science
Walker Institute for Climate System Research
© University of Reading 2012
www.reading.ac.uk
Introduction and Outline
• Review of the extreme weather events of 2010-11
• The Australian floods of summer 2010-11
• “Is it climate change?”
• La Nina and tropical flooding
• Severe Tropical Cyclone Yasi
• Drought in Southwest Australia
• The Russian heatwave of summer 2010
• Conclusions – Should we blame ourselves?
Extreme weather events
in 2010-2011
Source: MODIS satellite image
- December 2010: Central Queensland flooded
- January 2011: Toowoomba and Brisbane flooded;
also flooding in Sri Lanka, Philippines and Brazil
- August 2010: Flooding in Pakistan
- February 2011: Severe Cyclone Yasi strikes Queensland
- Throughout 2010: Drought in Perth
- July 2010: Heatwave in western and central Russia
Source: telegraph.com.au
Source:
guardian.co.uk
Source:
U.S. Department
ofResearch
Defense Laboratory
Source: NOAA
Earth System
Brief introduction to
Australian climate
• Australia’s coasts receive
the vast majority of the rainfall.
• About 70% of Australia receives
less than 500 mm of rainfall
per year (about half of
SE England’s average rainfall).
• The northern third of Australia
receives nearly all of its rainfall in
summer (December-February).
• The southern third of Australia
receives most of its rainfall in
winter (June-August).
Source: bigthink.com
The Rockhampton Floods
• The wettest December on record
in Queensland; the second-wettest
on record in Australia (since 1900).
• Much of Queensland received
more than twice its average rainfall.
• The wet December followed the
wettest spring on record in Australia.
The Rockhampton Floods
• Exceptionally heavy
rain fell between Christmas
and New Year’s Eve:
• Rockhampton: 320 mm
• Carnarvon: 265 mm
on 27 December alone
• Roughly five times the
average rainfall for
December
• Daily rainfall records fell
along the Carnarvon Hills to
the west of Rockhampton
• Thousands of square
kilometres flooded.
The Rockhampton Floods
• The proximate cause of
the heavy rainfall was
the combination of
• High pressure to the
south, in the Tasman
Sea
• Low pressure to the
north, along the
Queensland coast
• The combined circulation
around these systems
directed warm, moist ocean
air onshore, which
subsequently rose over the
Carnarvon Hills, producing
heavy rainfall
The Brisbane Floods
• Southeastern Queensland received
heavy rain 6-12 January 2011
• Brisbane: 267 mm
• Toowoomba: 345 mm
• Flash flooding struck Toowoomba
on 10 January
• Similar onshore wind pattern to
Rockhampton floods
The Brisbane Floods
2011
• Brisbane City gauge
reached a height of 4.46
metres, well below the
record highest floods.
• The Wivenhoe Dam was
constructed in the wake of
the 1974 floods
(5.45 metres) to protect the
city.
• Preliminary results: Without
the Wivenhoe Dam, the river
level at the City gauge
would have been
6.5 metres (Neville Nichols,
Monash University)
The cost of the
Australian floods
15,000 homes destroyed
35 people killed
Climate change and extremes
• In the wake of the Queensland floods, many asked:
“Is it climate change?”
… or:
“Would the Queensland floods have occurred if humans were
not warming the climate through greenhouse-gas emissions?”
• Climate change will affect the overall number and strength of
extreme weather events.
• Two slightly easier and much more useful questions:
“Will floods in Queensland become more frequent?”
“Will floods in Queensland become more intense?”
Climate change and
extreme rainfall
• For the tropics as a whole,
the frequency of extreme
rainfall is expected to
increase with warming
temperatures.
• Clausius-Clapeyron:
the atmosphere has a
greater capacity for water
vapour at higher
temperatures.
• This does not mean that
global warming will be
every rainfall event
stronger, or that total
rainfall will increase
everywhere.
Changes in frequency of extreme rainfall
in the tropics with warming ocean temperatures
Figure from Allan et al. (2010, Env. Res. Lett.)
Climate change and
Queensland extreme rainfall
• For Queensland,
doubling CO2
in a climate model
increases the
frequency of
100 mm/day rainfall
by 50%:
• Present-day:
1 in 670 days
• Double CO2:
1 in 402 days
• Doubling CO2 also
also increases the
intensity of a 1-in-200
Frequency of different amounts of rainfall in Queensland
day event by 20%:
during summer, for present-day and 2xCO2 climates
• Present-day: 54 mm
• Double CO2: 65 mm
Brief introduction to
El Niño and La Niña
• La Niña and El Niño are naturally
occurring phenomena, in which
ocean temperatures in the equatorial
Pacific cool and warm, respectively.
• Events often develop in
June-September, peak in DecemberFebruary, then decay in March-May.
• La Niña and El Niño are episodic
(irregular). El Niño does not
necessary follow La Niña; neutral
conditions can prevail for years.
• The strength and position of warm
ocean temperatures shifts rainfall
patterns around the world.
Brief introduction to
El Niño and La Niña
El Niño
La Niña
Departures of ocean temperatures in the
central equatorial Pacific from average conditions
(Source: KNMI Climate Explorer from UK Met Office data)
There are insufficient reliable data to determine whether
El Niño and La Niña are changing with global warming.
• On average,
El Niño and La Niña
are responsible
for approximately
25% of the
year-to-year
variations in
Queensland rainfall.
• Strong La Niña
events result in
large floods in
Queensland, but
stronger El Niño
events do not result
in stronger
droughts.
Queensland annual (May-Apr) rain (mm)
Historical relationships between
La Niña and Australian rainfall
Strong
Weak Strong
Weak
La Niña La Niña Neutral El Niño El Niño
1974/75
2010/11
Nino-4 index (degrees Celsius)
Historical relationships between
La Niña and heavy rainfall
• La Niña increases by 20-70% the risk of heavy rainfall in Queensland
• El Niño reduces by 10-40% the risk of heavy rainfall in Queensland
La Niña
minus
neutral
years
Difference
in the
number
of
1-in-30 day
rainfall
events
per
season
El Niño
minus
neutral
years
Climate change or
natural variability?
• Scientists expect warming
temperatures to increase the
frequency and intensity of extreme
rainfall, particularly in the tropics.
Climate
change
More
heavy
rainfall
Strong
La Niña
Heavier
rainfall
• There is a strong historical link
between La Niña and strong
monsoon seasons in Australia,
including with the frequency of
extreme rainfall.
• There is no robust evidence that
global warming has influenced the
frequency or intensity of La Niña; the
2010-11 event was strong, but not
outside the range of past events.
Severe Cyclone Yasi
Yasi was
“Australia’s Katrina”
The track of Severe Cyclone Yasi as it
passed across northern Queensland
Historical relationships between
La Niña and tropical cyclones
Observed tropical-cyclone tracks during the seven
El Niño and La Niña events from 1979-2009.
• Compared to El Niño, La Niña doubles the risk of a tropical cyclone
striking eastern Australia (Callaghan and Power, 2010)
• All years in which more than one tropical cyclone has struck eastern
Australia have been La Niña years (Callaghan and Power, 2010).
Climate change and
tropical cyclones
• In a warmer world,
we expect to see
fewer tropical
cyclones.
• The decrease may
not be evenly
spread across the
globe; some regions
may have more
cyclones.
• The intensity of
tropical cyclones,
particularly the
strongest ones, is
expected to
increase.
Temperature
changes over
1979-1999
Estimates from three
models of the change
in the number of days
with a tropical cyclone:
2081-2100 minus
1981-2000.
From Thorne
From Lavender
et al.
et al. (2010)
(2011).
Climate change or
natural variability?
• Climate change is expected to
reduce the number, but increase
the intensity of tropical cyclones
in the Southwest Pacific.
Climate
change
More
tropical
cyclones
Strong
La Niña
Stronger
tropical
cyclones
• La Niña increases the number of
landfalling tropical cyclones in
eastern Australia, but has no
effect on their overall intensity.
• Was Yasi “enhanced” by global
warming?
Would Yasi have even formed in
a warmer world?
In an El Niño year, would Yasi
have simply have moved out to
sea?
The Southwest Australia Drought
Source: Bureau
of Meteorology
Annual rainfall at
Perth Airport, dating to 1945.
The blue horizontal line is
the median annual rainfall.
Rainfall in Southwest Western
Australia for January-October; the
red bars are the five driest years;
the black line is the 15-year
moving average.
The Southwest Australia Drought
• Western Australia produces
the most wheat of any state
in Australia.
• The 2010 drought likely
reduced wheat yields by
one-third, equivalent to
nearly AU$1 billion
(£650 million).
• Heavy rains in the east of
Australia in SeptemberNovember damaged wheat
crops as they were being
harvested.
The Southwest Australia Drought
Storage in gigalitres
(one million kilolitres)
Total water stored in
Western Australia
dams.
Source:
Western Australia
Water Corporation
Extraction
during dry
season
Recharge
during wet
season
Climate change and
the Southern Annular Mode
• Loss of ozone
and increasing
greenhouse
gases have had
the largest
impact on the
Southern Annular
Mode to date.
• The tension
between the
recovery of the
ozone hole and
increasing
greenhouse
gases is an open
issue.
Simulated changes in surface pressure associated with observed
changes (1958-1999) in each driver. Figure from Arblaster et al. (2006).
Impacts of CO2 doubling
Summer
Autumn
Winter
Spring
Change in annual-total rainfall from
HiGEM 2xCO2 minus HiGEM control
Climate change or
natural variability?
• The ozone hole and
greenhouse-gas emissions have
each contributed to the positive
trend in the Southern Annular
Mode.
• There is little evidence for
natural, decadal variability in the
Southern Annular Mode, but
records are likely not long
enough to robustly detect such
variations.
• Ozone loss and greenhouse-gas
emissions are the most likely
causes of the decline in rainfall
in Southwest Australia.
Greenhouse
gas
emissions
Antarctic
ozone
hole
Natural
variations
(solar, etc.)
Drought
in SW
Australia
?
The Russian Heatwave
• Intense heat and drought
throughout western Russia
resulted in tens of
thousands of deaths.
• Highest temperature ever
recorded in Russia:
44.0°C (111.2°F)
• Grain harvest ruined, at a
cost of $15 billion (£9.4
billion) in GDP
• Russia banned grain
exports, leading to further
worldwide increases in
grain prices.
Departures
of July
20102010
surface
temperatures
Departures
of July
precipitation
from normal July conditions
The Russian Heatwave
Departures of
July 2010 sea-level
pressure values from
normal conditions
The heat and drought in Russia was associated with
a strong blocking pattern in the atmosphere, with
substantial meanders in the jet stream
directing weather systems away from Russia.
Climate change or
natural variability?
• Climate change
is expected to
considerably
increase the risk of
heatwaves in many
parts of the world,
including Europe
and Russia.
• The 2010 heatwave
was likely the result
of natural variations
that were intensified
by man-made
climate change
(Dole et al., 2011).
Frequency of western Russia July temperature
extremes, based on climate-change simulations
from 22 climate models.
Should we blame ourselves?
• Humans are definitely warming the climate through
emissions of greenhouse gases.
• A warming climate is affecting and will affect weather
patterns, especially the number and strength of extreme
weather events.
• The effects of climate change are being seen and
will be seen in the overall behaviour of extreme events.
• Natural variations, like La Niña, still have an important role
in determining when and where extreme weather will occur.
• For individual events, trying to separate natural variations
from climate change is not easy and (in my opinion)
not particularly useful for adapting to climate change.
Shameless self-promotion
• After the violent eruption of Mount
Tambora (Indonesia) in April 1815, 1816
became known as
“The Year Without a Summer”.
• One of the four strongest volcanic
eruptions of the last 10,000 years. A
powerful example of natural climate
change.
• Covers the eruption, how the volcanic ash
cloud affected global weather and climate,
and the subsequent impacts on
agriculture, migration and public health.
• Published late February 2013, but
available for pre-order on amazon.co.uk!
For further information
• E-mail: [email protected]
• Web: http://www.met.rdg.ac.uk/~ss901165
• Twitter: @nick_klingaman
To find out more about Reading’s climate research:
• Walker Institute: http://www.walker-institute.ac.uk
• NCAS-Climate: http://climate.ncas.ac.uk
More information on climate change research:
• Royal Meteorological Society: http://www.rmets.org
• Intergov. Panel on Climate Change: http://www.ipcc.ch