Dr Mark Cresswell

Natural & Anthropogenic sources of climate variability

69EG6517 – Impacts & Models of Climate Change

Lecture Topics• Introduction – a brief history of time

• Natural – Milankovitch cycles

• Natural – Solar Cycles

• Natural – Volcanic activity

• Anthropogenic – Fossil fuels

• Anthropogenic – Land use change

INTRODUCTION

Is this a new phenomenon?

Introduction #1

It is known that our climate has experienced warmer and cooler phases throughout the past

Sea levels regarded as rising at an “alarming” rate today have been considerably higher in the past

Introduction #2• Proxy reconstructions for the climate of the

Quaternary Period are considerably more abundant and reliable than for earlier periods

• The Quaternary spans the last 2Ma of Earth history and is separated into two Epochs, the Pleistocene (2Ma to 10Ka) and the Holocene (10Ka to present)

Introduction #3• Although deglaciation had been taking place

for at least 4,000 years, a rapid deterioration (cooling) in climate occurred at about 10 to 11Ka

• This event is known as the Younger Dryas Cooling. The North Atlantic polar front readvanced far southward to approximately 45°N (only 5 or 10° north of the glacial maximum position)

Introduction #4

Holocene thermal maximum: 6 to 7 thousand years ago

Introduction #5• Quantitative estimates of mid-Holocene

warmth suggest that the Earth was perhaps 1 or 2°C warmer than today

• Most of this warmth may primarily represent seasonal (summer) warmth rather than year-round warmth

Introduction #6• Beginning about 1450 A.D. there was a

marked return to colder conditions. This interval is often called the Little Ice Age, a term used to describe an epoch of renewed glacial advance

Is this a new phenomenon?

MILANKOVITCH CYCLES

Milankovitch Cycles #1• The distance between the Earth and Sun

changes for a variety of reasons as does the quantity of solar energy reaching Earth

• The Earth follows an elliptical orbit around the Sun. Orbital stretch/shrink ~100,000 yrs

Milankovitch Cycles #2• Milutin Milankovitch (a Serbian

astrophysicist) worked out ways in which the Earth-Sun geometry changed as a function of orbital cycles

Milankovitch Cycles #3

1. Variations in the Earth's orbital eccentricity—the shape of the orbit around the sun.

2. Changes in obliquity—changes in the angle that Earth's axis makes with the plane of Earth's orbit

3. Precession—the change in the direction of the Earth's axis of rotation, i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time

Milankovitch Cycles:

Milankovitch Cycles #4

Illustration of ECCENTRICITY

Milankovitch Cycles #5

Illustration of OBLIQUITY

Milankovitch Cycles #6

Illustration of PRECESSION OF THE EQUINOXES

Milankovitch Cycles #7

Calculated Orbital Variation

SOLAR CYCLES

Solar Cycles #1• There is really no such thing as a “solar

constant”

• We already know that orbital effects can change the quantity of solar radiation reaching the Earth

• The Sun generates variable quantities of energy due to its own internal variability

• Solar activity is know to have cycles – with a periodicity of about 11 years

The 11 year solar cycle

Solar Cycles #2The 11 year solar cycle

Historical overview of solar sunspot cycles

Solar Cycles #3The 11 year solar cycle

Historical overview of solar sunspot area from 1870s to 2000

Solar Cycles #4• As well as sunspot activity, the Sun can

interact with our atmosphere by generating solar flares leading to a powerful solar “storm” (enhanced solar wind)

• Solar flares can damage satellites, and can also affect the Van Allen belts producing Aurora (Northern Lights)

Coronal Mass Ejections (CMEs) are geomagnetic disturbances on the Sun surface that generates the Aurora Borealis.

Solar Cycles #5

VOLCANIC ACTIVITY

Volcanic Activity #1• Active volcanoes generate large quantities

of dust and smoke

• Particulates in the atmosphere block out solar radiation, preventing it from penetrating through to the ground surface

• The main effects of volcanic eruptions is to cool the affected regions (not dissimilar to a “nuclear winter”).

Volcanic Activity #2• When Mount Pinatubo erupted

in the Philippines June 15, 1991, an estimated 20 million tons of sulphur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere

Volcanic Activity #3• A research team ran a

general circulation model developed at the Max Planck Institute with and without Pinatubo aerosols for the two years following the Pinatubo eruption

• The climate model calculated a general cooling of the global troposphere

FOSSIL FUELS

Fossil Fuels #1• The burning of fossil fuels is believed to be the

major source of anthropogenic climate forcing• Burning oil, gas and coal generates a wide variety

of gases and particulates – the most important of which is carbon dioxide (C02)

• The natural Greenhouse effect is enhanced by extra C02 to create the Enhanced Greenhouse Effect. Without the natural greenhouse effect global temperatures would be around 253 K (-20ºC) but is actually 288 K (15 ºC)

Fossil Fuels #2• Other greenhouse trace gases include Methane

(CH4), Nitrous Oxide (N2O) and water vapour.

Earth System

Source

C (X 1015 g)

Sink

C (X 1015 g)

FLUX

C (X 1015 g)

LAND 100 100 0

OCEANS 100 104 -4

HUMANITY 7 0 +7

SUM = +3

Contribution of land, oceans and human activity to carbon flux

LAND USE CHANGE

Land Use Change #1 • Human activity can affect the way in which

the Earth surface responds to solar radiation

• Modifying land surfaces can profoundly affect heating and vulnerability to climate change

• The gradual commercial deforestation of the tropical rainforest regions in South America have removed a valuable carbon sink and released carbon as this timber is burned or decays

Land Use Change #2 • The “slash and burn” policy attributed to

subsistence farmers in Africa and South America have removed tree species

• Removal of trees can lead to landslips, soil erosion and development of dustbowls

• Changes in Sahelian grasslands (removal) may have modified the albedo and soil moisture regime leading to droughts in the region

Other Sources of Variability • Urban heat island

• Ocean circulation

• Geothermal activity

• Tectonic plate movement

Any questions?