Module 4

Changes in Climate

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

Historic Seal Level Rise

Looking back in time

A longer view

A much longer view

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.

Glacier ice cores

Proxy Data

Other historical evidence

The last 1000 years

• Recent reconstruction of NH air temperature based on tree-ring, ice core, coral and historical evidence.

Vostok Ice Core (Antarctica)

So … different

scales of time

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.

The last Ice Age

Vostok Ice Core (Antarctica)

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

The past 1 myr

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.

The past 1 myr

Should read ocean cores

Back 2.5 myr

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.

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

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)

The tilt of the Earth’s axis varies

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

Milankovitch Cycles

Should read ocean cores

Back 2.5 myr

Milankovitch Cycles in the record

• 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.

Global Annual Temperature Trends:Global Annual Temperature Trends:1901 - 19901901 - 1990

Source: Watson 2000

•• = increasing= increasing, , •• = decreasing= decreasing

GlobalGlobal Precipitation Trends Precipitation Trends (% per decade) 1900 - 1994(% per decade) 1900 - 1994

Source: Watson 2001

Extreme Precipitation Events in the U.S.Extreme Precipitation Events in the U.S.