climate variability, change and extreme events greg holland
TRANSCRIPT
Climate Variability, Change and Extreme Events
NCAR Earth System LaboratoryNational Center for Atmospheric Research
NCAR is Sponsored by NSF and Partial support for this research was provided by the Willis Research Network, the Department of Energy,
and the Offshore Energy Industry
Greg Holland
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Summary
• Climate Variability and Change;
• Impact on Extremes
– Tipping Points
– Extreme Weather
• General Discussion, using Hurricanes as an example
• Droughts and Flooding Rains.
Holland, Climate Extremes Feb 2010
• Variability:
– Variations in solar output and wobbles in the tilt of the earth (range of time scales);
– Direct Impacts on the climate system (volcanoes, asteroid impact, internal variability; years to decades);
• Change
– Continental drift (forever)
– Ice age variations (millenia)
– Solar decline (eons)
– Human influence (centuries at least).
Why Does the Climate Vary and Change?
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Impact of Anthropogenic Changes
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lwf.ncdc.noaa.gov/paleo/globalwarming/images/
We are entering an era unprecedented in modern human existence!
Predicting Climate Variability and Change
Holland, Climate Extremes Feb 2010
(Meehl et al 2006)
(Holland 2008)
“Variability” and “change” are hard, indeed often impossible, to logically separate over short periods.
How do we understand and predict the consequences of change?
While history can give us good clues, climate models are the only useful predictive tool.
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Meehl et al 2004
Extreme Changes: Tipping Points
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Most sunlight
reflectedMost sunlight
absorbed
Ice Sheet Example
Climate Shifts Tend to be Rapid
Tipping Point Example: Major Atlantic Hurricanes
(Holland and Webster 2007)
Holland, Climate Extremes Feb 2010
Climate shifts tend to be rapid!
Extreme Weather Variations with Climate Change
• Nearly all projections for greenhouse warming predict an increase in extreme events (IPCC, CCSP, WMO), which are greater than the mean changes;
– Heavy Rain, Droughts, Heat Waves, Hurricanes
• Is this not strange?
• Can we explain it?
Holland, Climate Extremes Feb 2010
An Example: Atlantic Hurricanes
0 20 40 60 80 1000
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10
15
20
25
Maximum Winds (m/s)
Nu
mb
er
Smoothed 3 Times Change 5 m/s
Original
Changeb
Intense
Hurricanes
• Notice how a small shift in the mean makes for a large increase at the tail of the distribution.
• A similar result arises from a small broadening of the distribution.
• We can quantify these effects by application of extreme value theory.
Holland, Climate Extremes Feb 2010
• Assume a mean wind intensity increase of 2-3 m/s per oC global ocean warming, then:
– Cat 5 hurricane frequency increases by ~70% for each oC warming;
• Assume in addition that the variance also increase, say, by 2-3 m/s in standard deviation, then:
– Cat 5 hurricane frequency increases by ~150% for each oC warming!
• These mean and SD changes are below the current observational resolution.
Potential Impact of Climate Change on Atlantic Hurricanes
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For Intense Hurricanes, Climate Change may have already begun!
(Holland 2010, Atlantic Hurricanes) (Elsner et al 2008 global tropical cyclones)
• Bender et al 2010 modeling of 70-y trend: Fewer cyclones, but Cat 4 and 5 hurricane numbers increase by +81% and US damage increase by +28%!
• Holland et al 2010 modeling of multi-decadal trend: More cyclones and >100% increase in intense hurricanes, consistent with trend towards more equatorial developments.
Holland, Climate Extremes Feb 2010
Equatorward Movement of Hurricanes
Holland, Climate Extremes Feb 2010
(Holland et al 2010)
Longer periods over warmer oceans implies more intense tropical cyclones and a substantial increase in the most intense ones.
Note the increase in “Cape Verde Hurricanes”!
Heavy Rainfall and Flooding
• As the globe warms, atmospheric moisture increases (~7% per oC warming, IPCC):
– Leads to more heavy rain events and an acceleration of the hydrological cycle;
– The changes at the extreme will likely be higher than we showed for hurricanes.
• Interestingly, droughts also become more severe, why?
Holland, Climate Extremes Feb 2010
Summary
Holland, Climate Extremes Feb 2010
• Extreme Weather is mostly beyond the capacity of current climate models:• Severe Storms, Floods, Fire, Hurricanes;
• However, there are good theoretical, statistical and physical-understanding reasons to expect that that extremes will increase substantially with global warming;
• Current modeling studies tend to confirm this;
• Next generation models will be able to explicitly predict these extremes.
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