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Module 4
Changes in Climate
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Global Warming?
• Climate change– The pattern(s) of variation in climate
(temperature, precipitation) over various periods of time
• Global warming– The anticipated/apprehended increase in mean
global temperature associated with the increase of greenhouse gases in the contemporary atmosphere
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Historic Seal Level Rise
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Looking back in time
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A longer view
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A much longer view
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Instrumental Records
• The earliest records of temperature measured by thermometers are from western Europe beginning in the late 17th and early 18th centuries.
• The network of temperature collection stations increased over time and by the early 20th century, records were being collected in almost all regions, except for polar regions where collections began in the 1940s and 1950s.
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Glacier ice cores
Proxy Data
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Other historical evidence
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The last 1000 years
• Recent reconstruction of NH air temperature based on tree-ring, ice core, coral and historical evidence.
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Vostok Ice Core (Antarctica)
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So … different
scales of time
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Glacial Cycles
• By ~2.5 myr ago, temperatures began alternating by 4-10oC every 40,000 to 100,000 years (quasi-periodic cycles).– Extensive glacial ice began to occur during the
cold phases– Eventually, massive ice sheets formed over the
land areas of the NH during glacial periods.– During each glaciation, the sea level on Earth
was reduced by 100-130 metres.
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The last Ice Age
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Vostok Ice Core (Antarctica)
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Using oxygen isotope ratios
Isotopes: atoms of different mass number (# neutrons in nucleus)
When water evaporates, O18 is preferentially left behind, and O16 is preferentially stored in glacier ice
Low values of the ratio correspond to a cold climate
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The past 1 myr
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The Ins and Outs of Ice Ages
• The detailed, long term records of oxygen isotope data reveal that the climate tends to drift slowly into glacial conditions, while interglacials tend to develop suddenly in the space of a few hundred years.
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The past 1 myr
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Should read ocean cores
Back 2.5 myr
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Why are there glacial cycles?
• The last 700,000 years are marked by wider swings in temperature that indicate a large shift in the amount of land ice present.– On the geologic time scale, the Earth seems to
be getting cooler.
• There is good evidence that variations in the Earth’s solar orbit are linked to glacial cycles.
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Orbital forcing
• The theory that large scale climate changes (glacials/interglacials) are due to the variations in precession, eccentricity and obliquity of the Earth’s solar orbit that affects the amount of solar radiation received at the surface of the Earth.– Attributed to Milankovitch
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Orbital attributes
• The Earth has three fundamental orbital attributes:– Changes in the tilt of the axis of rotation
(termed the obliquity)– Changes in the shape of the elliptical orbit
around the sun (termed eccentricity)– Changes in the date of the Earth’s closest
approach to the Sun (termed precession of the equinox)
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The tilt of the Earth’s axis varies
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Periods of variation
• The tilt of the Earth’s axis varies over a period of about 41,000 years
• The cycle of orbital eccentricity is 90,000 to 100,000 years
• The precession cycle of the equinox is about 23,000 years
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Milankovitch Cycles
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Should read ocean cores
Back 2.5 myr
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Milankovitch Cycles in the record
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• The previous figure indicates a dominant period in glacial variations of 41,000 years.– There are other significant cycles at 96,000
480,000 (and 23,000) years.
• These correspond to the cycles (and harmonics) in the Earth’s orbital variations.
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Global Annual Temperature Trends:Global Annual Temperature Trends:1901 - 19901901 - 1990
Source: Watson 2000
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•• = increasing= increasing, , •• = decreasing= decreasing
GlobalGlobal Precipitation Trends Precipitation Trends (% per decade) 1900 - 1994(% per decade) 1900 - 1994
Source: Watson 2001
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Extreme Precipitation Events in the U.S.Extreme Precipitation Events in the U.S.