Transcript Climate change and the probability of extreme events

Department of Meteorology
Climate Change
&
The Probability of Extreme Events
Brian Hoskins
Royal Society Research Professor & Professor of Meteorology
University of Reading
My position: a Sceptical Alarmist
•Overwhelming evidence based on theory, observations and models that we
are perturbing the climate system in a very significant way
•There may be a joker in the pack but nearly all the ones we can think of
would make things worse
•Natural variability could lead to a cooler decade, less tropical cyclones…
but the problem would not have gone away
•Decisions for the next decades have to be made now
•Uncertainty in knowledge, construction of models, observation of initial
states, specified fields (e.g. greenhouse gases) and inherent in the
chaotic nature of the atmosphere-ocean system
•Consensus does not mean correct
•My confidence in the current ability to give most regional projections is low
•Many quote/use the (extreme) regional results & likely impacts with
unwarranted confidence
•Sceptical questioning & probing is essential in climate science as in all
science
IPCC (2007)
Changes in
Greenhouse gases
in the atmosphere
from Ice-Core and
Modern Data
IPCC: “Global Warming is unequivocal”
Since 1970, rise in:
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Global surface temperatures
Tropospheric temperatures
Global ocean temperatures
Global sea level
Water vapour
Rainfall intensity
Precipitation in extratropics
Hurricane intensity
Drought
Extreme high temperatures
Heat waves
Decrease in:
NH Snow extent
Arctic sea ice
Glaciers
Cold temperatures
Estimates of Northern Hemisphere
Temperature for the Past 1000 Years
Drawn by Tim Osborn using published data
Understanding and Attributing Climate Change
Atmospheric Water Vapour
Clausius-Clapeyron; the water holding capacity of the atmosphere
goes up at about 7% per degree Celsius increase in temperature.
Models all suggest that this will occur with greenhouse gas
warming.
Observations show that it is happening at the surface and in lower
atmosphere: 0.55C since 1970 over global oceans and 4% more
water vapor.
This means more moisture available to enhance the greenhouse
warming, enhance the growth of storms and produce heavier rain.
Proportion of heavy rainfalls:
more land areas with increases than decreases
Regions of disproportionate changes in heavy
(95th) and very heavy (99th) precipitation
Changes in the track density of midlatitude storms; 1979/03-1958/78
Medium strength
(20-40hPa)
Intense
(>40hPa)
Hoskins & Hodges (2006)
North Atlantic hurricanes have increased with SSTs
N. Atlantic
hurricane
record best
after 1944
with aircraft
surveillance.
(1944-2005)
SST
Global number
and
percentage of
intense
hurricanes
is increasing
Reasons for Confidence in Model Projections
•Models built on basic physics
•General consistency of globally averaged T response
from simplest to most complex
•Success in forecast/hindcast of weather, seasonal
climate, impact of Pinatubo, past century
•Simulation of phenomena such as El Niňo, storms
Reasons for Lack of Confidence in Model Projections
•Underestimation of natural variability? E.g. 1940s
•Uncertainty in forcing used for past century, e.g. solar, aerosols
•Just starting to have interactive atmospheric chemistry & carbon cycle
•Uncertainty in cloud behaviour, aerosol effects, solar variability,…
•Poor representation of some phenomena particularly on smaller scales
IPCC (2007) Surface Temperature Projections
2020s & 2090s relative to 1980-99
Global mean
2020s
2090s
Changes in Temperature Extremes
Climate Extremes 2081-2100
annual global mean 2m temperature
95%
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Weisheimer, A. & T.N.Palmer (2005)
19
°C
Extremely Warm Dec-Feb
Probability of exceeding 95% ile DJF temperature
2081-2100 (A1B, A2, B1)
Daily Doubled
maximumCO
temperature
change due to CO2 doubling
2: Changes in probability distributions
for summer day temperatures in Southern England
Hottest day
of summer
Average day
of summer
Hadley Centre
Projected patterns of end of 21st century
change (%) in precipitation for one scenario
Dec-Feb
June-Aug
Fig. SPM-6
Stippled areas are where more than 90% of the models
agree in the sign of the change
Precipitation increases very likely in high latitudes
Decreases likely in most subtropical land regions
This continues the observed patterns in recent trends
Extremely Wet Dec-Feb
Frequency of exceeding the 95%ile precipitation in DJF
2081-2100
Extremely Dry June-Aug
Frequency of falling below the 5%ile precipitation in JJA
2081-2100
DailyDoubled
rainfall change
due to CO2
CO2: Changes
in doubling
probability distributions
for winter wet days in Southern England
Wettest day
of winter
Average wet
day of winter
Hadley Centre
DailyDoubled
rainfall change
due to CO2
CO2: Changes
in doubling
probability distributions
for summer wet days in Southern England
Wettest day of
summer
Average wet
day of summer
Hadley Centre
IPCC 2007 projections of 21st
century sea level rise
Surface Melt on Greenland
Melt
descending
into a moulin,
a vertical shaft
carrying water
to ice sheet
base.
Source: Roger Braithwaite,
University of Manchester (UK)
Climate change & Midlatitude Cyclones/Storm-Tracks
• Reduced pole-equator temperature
contrast
• More moisture in warmer air
• Same storms will transport more energy
polewards
Greenland
N America
W
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W
N Atlantic
W
N Africa
Eurasia
Conclusions on midlatitude cyclones?
• Less in general?
• More in the NE Atlantic?
• Less in the Mediterranean?
• More strong ones?
IPCC Projections for some extreme weather events
Working Group 1
Heat waves
Very likely
Heavy rain events
Very likely
Drought areas
Likely
Intense tropical cyclones
Likely
Extreme sea level
Likely
Working Group 2
Impacts on
Agriculture
Water resources
Human health
Industry/settlement/society
Mechanisms for extreme changes?
•Large dynamical ice sheet loss: Greenland & West Antarctic
•Reduced carbon absorption/emission: soil, vegetation, ocean
•Methane emission from melting tundra, peat, clathrates
•Rapid change in the circulation of the atmosphere/ocean:
reduction in the Atlantic northward heat transport
frequency or nature of ENSO
Asian monsoon circulation
summer European blocking
nature or location of winter storm-track
nature or location of tropical cyclones
•Complex dynamical system behaviour