3. natural climate variability. spm 1b variations of the earth’s surface temperature for the past...
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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