3. Natural Climate Variability

SPM 1b

Variations of the Earth’s surface temperature for the past 1,000 years

Approx. climaterange over the 900 years up to 1900

Last centuryappearsunusual

But the climate variations before the industrial revolutionwere almost certainly natural – what caused them?And could 20th century warming also be a natural variation?

Causes of natural climate variability

• ‘External’ forcing of the climate system:– Ice ages are caused by changes in Earth’s orbit (Sun-

Earth distance, angle of rotation axis, …) – but these changes occur slowly on long timescales (~10,000-100,000 years) – too slow to explain recent changes

– Large volcanic eruptions– Solar variability

• ‘Internal’ climate system variability (chaos due to non-linear interactions of a complex system)– El Niño, North Atlantic Oscillation

Mt Pinatubo, Philippines, 1991

Satellite measurements of Pinatubo aerosol

Effect of volcanic aerosols

Explosive volcanic eruptionadds SO2 to atmosphere

Sunmorereflected

less absorbed

Solar radiation

Surface cools

SO2 convertedto sulphuric acid aerosollayer

Volcanic climate forcing since 1750

Greenhouse gas forcing since 1750

Solar forcing of climateSunspots

The energy output of the Sun shows slight fluctuations as the number of sunspots varies

Sunspotsdiscoveredaround1610 byHarriot / Galileo

Typically the diameterof the Earth. Slightly cooler regions of the photosphere. Associated with convection in the Sun’s interior. Confusingly, more sunspots correspond to more energy output from the Sun.

Sun-spots show a regular 11-year cycle, but also longer-term cycles

Sunspots since 1700

Solar forcing of climate since 1750

Solarforcing

El Niño-Southern Oscillation (ENSO)

ENSO is one of the main causes of global climate variability. But what is El Nino?

Ocean-Atmosphere interaction in the tropical Pacific Winds, ocean temperatures, cloud and rainfall patterns all change Occurs every 2-10 years, lasts 12-18 months 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 & seabirdsCoral bleaching due to high sea temperaturesSouth American rainfallGlobal teleconnections – e.g. linked to droughts in AfricaModulates strength of tropical storms

Sea surface temperatureanomaliesThis is a snapshot

of sea surface temperature in the tropical Pacific

The anomaly is how it differs from average values.

‘Normal’

vs.

El Nino conditions

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

Normal

El Nino

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

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

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

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.

Pacific SSTs since 1986

El Nino

1997/8 event was largest of the century

Pacific SST anomalies since 1982

El Nino

La Nina

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

Pacific SST anomalies since 1870

Pacific SST anomalies since 1700Some indication that 1982/3 and 1997/8 events were unusually large

El Nino Impacts La Nina Impacts

Current ENSO forecast

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

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

SPM 1b

Variations of the Earth’s surface temperature for the past 1,000 years

Most of the past climate variability can be explained byvolcanoes, fluctuations in the sun, and internal variability.Natural variability can’t explain 20th century warming.

Summary 3 (Natural climate variability)

Main short-term natural influences on climate are volcanoes, solar variability, and internal climate system variability

Natural variability can explain fluctuations of about ±0.4 K

Warming in the last century has exceeded the bounds of natural variability – it must have another explanation – the rise of greenhouse gases from human activities is the obvious candidate.

It is possible that human activities may increase climate variability, as well as mean climate