Climate change, climate variability and natural hazards · Climate & Weather-related Natural Hazards - Lecture 4 David Stevenson 13 Summary of climate variability and its influence

Climate & Weather-related Natural

Hazards - Lecture 4

David Stevenson 1

Climate change,climate variability

and natural hazards

(continued)

El Niño-Southern Oscillation (ENSO)�

The major mode of global climate variability.

It is naturally occurring (evidence back >125,000 yrs ago)�

Ocean-Atmosphere interaction in the tropical Pacific

Winds, ocean temperatures, cloud and rainfall patterns all change

Occurs every 2-7 years, lasts 9-12 months (up to 2 years)�

Irregular – initiation not understood

During a strong El Nino (e.g. 1997/98), global temperatures can rise, by up to about 0.3 °C

Impacts:

Peruvian fishing & seabirds

Coral bleaching due to high sea temperatures

South American rainfall

Droughts in Africa/Indonesia – also promotes fires

Modulates strength of tropical storms


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Sea surface temperatureanomalies

This is a snapshot

of sea surface temperature in the

tropical Pacific

The anomaly is

how it differs from

average values.

Current SST anomaly


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‘Normal’

vs.

El Nino

conditions

www.pmel.noaa.gov/tao/elnino/

Normal

El Nino


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A: Cold water off Peru – not El Nino

B: Warmer water than usual at

150m depth in W. Pacific

January 1997

February 1997

C: Warm water spreads across

the Pacific beneath the surface


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D: Warm water reaches surface in the central Pacific – this starts an interaction with the winds: Easterly winds weaken, and this tends

to make the ocean warmer: positive feedback

April 1997

May 1997

Warm water reaches the surface

off Peru, and warm water in

the central Pacific expands


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September 1997

Strong El Nino is underway –

sea surface is 2-4°C warmer

across half the Pacific

January 1998

El Nino fully underway.

Note the cold water at depth

spreading from the W. Pacific


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March 1998

El Nino has started to shrink – peak temperature anomaly 5°C, compared to 11°C in January

May 1998

El Nino almost over, after about 1 year of elevated sea-surface temperatures.

Note the expanding cold anomaly: this heralds

La Nina, the opposite phase of El Nino.


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Pacific SST anomalies since 1982

El Nino

La Nina

The NINO3.4 Index uses SSTs from a particular region of the tropical Pacific

Current ENSO forecast

http://iri.columbia.edu/climate/ENSO/


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Does ENSO affect hurricanes?

Yes – less Atlantic hurricanes in El Nino years.

Thought to be mainly because high level winds increase, making it harder for hurricanes to form.

ENSO impacts

The most famous impacts of El Niño events are:

Damages from floods and landslides caused by very high rainfall in Peru and southern California

Collapse of the Peruvian anchovetta fisheries because of warmer coastal waters.

Forest fires in Indonesia that have caused serious air pollution problems (1997/8 event see later)�

Crop failures and sometimes famine from droughts in southern Africa

http://iri.columbia.edu/climate/ENSO/societal/impact/index.html


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Summary of 1997/8 El Niño impacts

NB More hurricanes in N. Pacific, less in N. Atlantic

El Niño 1997/8 wildfires

Wildfires in Borneo and Sumatra

Anthropogenic - fires for land clearing intensified due to dry conditions associated with an El Niño event that commenced in June 1997

Area burned: 70-100,000 km2

Fires out of control in Sept 1997, halted during monsoon rains in November

Commenced again in Feb and lasted until rains in April 1998


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ENSO and climate variability

ENSO impacts are only a subset of the

impacts of year-to-year global climate

variability.

At most ENSO may be responsible for

about 50% of seasonal climate variability

in some regions. In most regions of the

world its influence is small or non-existent.

North Atlantic Oscillation (NAO)�

Main expression of climate variability in NW Europe

Reflects strength of westerly winds off the Atlantic

NAO +Mild, wetwinter

NAO –Cold, drywinter

www.met.rdg.ac.uk/cag/NAO/


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North Atlantic Oscillation (NAO)�

Main expression of climate variability in NW Europe

Reflects strength of westerly winds off the Atlantic

NAO +Mild, wetwinter

NAO –Cold, drywinter

www.met.rdg.ac.uk/cag/NAO/

Winter-time (JFM) NAO index 1950-2010

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao_index.html


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Summary of climate variability and its influence on natural hazardsClimate always shows some level of natural variation –even without man-made climate change

ENSO is the major mode of global climate variability

ENSO clearly influences the distribution of drought, floods and hurricane activity

Understanding and predicting ENSO can be important for warnings of some hazards, e.g. drought in S. Africa

NAO is a more important index of climate variability in NW Europe:

+ve NAO: Mild, wet winters over UK

-ve NAO: Cold, dry winters over UK (e.g. 2010)

Impacts of Climate Change on Natural Hazards

IPCC2007


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Most of the increase is due to thermal expansion as the oceans warm


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IPCC 2007 – Sea level rise20th century rise was +17 cm (range 12-22 cm)

Rate of rise shows some evidence of increasing:1961-2003: +1.8 mm/yr

1993-2003: +3.1 mm/yr

Future forecasts (2100):Low emissions scenario: 18-38 cm

High emissions scenario: 26-59 cm

NB don’t include some key factors, because there isn’t enough knowledge (carbon-cycle; ice sheet flow); likely these are underestimates

Sea-levels take many centuries to reach equilibrium, so there is a commitment to >1 m (likely several metres) in most cases.

Most of the world’s major cities are close to sea-level

Most of the observed increase of sea

level is due to thermal expansionTemperatures in the Atlantic:

Heat content increases in the Atlantic (last 50 years):

[units: 1022 Joules]


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NewOrleans,on the

Mississippi

delta


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Many

Pacific/IndianOceanIslands

Future curve

Present curve

(extrapolated)

Quite small rises in mean sea level can dramatically increase the frequency of coastal flooding from storm surges


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Projections of global surface warmingunder different emission scenarios

IPCC2007


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Summer 2003 European heatwave

All Swisssummers1864-2003

Schar et al,

2004

Nature About 4°°°°C

Black contours

are number ofstandard deviationsaway from the mean.

Summer 2003 heatwave becomes commonplace by ~2050

Stott et al, 2004 (Nature)


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It is estimated thatthe 2003 Europeanheatwave had areturn period ofseveral thousand

years, for presentclimate.

By the 2nd half ofthe 21st century, aheatwave of thismagnitude may occur every otheryear.

IPCC 2007 – Changes in future rainfall


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IPCC 2007 Climate change & tropical cyclones

There is observational evidence for an increase of intense

tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface

temperatures.

There are also suggestions of increased intense tropical

cyclone activity in some other regions where concerns over

data quality are greater.

Multi-decadal variability and the quality of the tropical cyclone

records prior to routine satellite observations in about 1970

complicate the detection of long-term trends in tropical

cyclone activity.

There is no clear trend in the annual numbers of tropical

cyclones.


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Mid-latitude storms –future modelled trends (IPCC 2007)

A poleward shift in the storm tracks is simulated by a number of climate models in the 21st century (as compared to the 1981-2000 average)

Some models also suggest mid-latitude storm intensification

Implications for flooding

Summary of major climate change impacts on natural hazards

Sea-level rise – coastal flooding from

storm surge events becomes much easier

Higher temperatures – increase in heat-wave events

Changes in precipitation – more floods in

some areas; others more drought-prone

Possible increases in severe storm

magnitudes

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Climate change, climate variability and natural hazards · Climate & Weather-related Natural Hazards - Lecture 4 David Stevenson 13 Summary of climate variability and its influence