climate variability, change and extreme events greg holland

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?

3Holland, Climate Extremes

Feb 2010

Impact of Anthropogenic Changes


Feb 2010

lwf.ncdc.noaa.gov/paleo/globalwarming/images/

We are entering an era unprecedented in modern human existence!

Predicting Climate Variability and Change


(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


Feb 2010

Most sunlight

reflectedMost sunlight

absorbed

Ice Sheet Example

Climate Shifts Tend to be Rapid

Tipping Point Example: Major Atlantic Hurricanes

(Holland and Webster 2007)


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?


An Example: Atlantic Hurricanes

0 20 40 60 80 1000

5

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.


• 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


Feb 2010

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.


Equatorward Movement of Hurricanes


(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?


Summary


• 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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climate variability, change and extreme events greg holland

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