Presentation on theme: "3. Natural Climate Variability. SPM 1b Variations of the Earth’s surface temperature for the past 1,000 years Approx. climate range over the 900 years."— Presentation transcript:
SPM 1b Variations of the Earth’s surface temperature for the past 1,000 years Approx. climate range over the 900 years up to 1900 Last century appears unusual But the climate variations before the industrial revolution were almost certainly natural – what caused them? And could 20 th 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
Solar forcing of climate Sunspots The energy output of the Sun shows slight fluctuations as the number of sunspots varies
Sunspots discovered around 1610 by Harriot / Galileo Typically the diameter of 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
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 & seabirds Coral bleaching due to high sea temperatures South American rainfall Global teleconnections – e.g. linked to droughts in Africa Modulates strength of tropical storms
Sea surface temperature anomalies This 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/
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, wet winter NAO – Cold, dry winter 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 by volcanoes, fluctuations in the sun, and internal variability. Natural variability can’t explain 20 th 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