Transcript [02] Natural & Anthropogenic sources of climate change

Natural & Anthropogenic sources of
climate variability
Dr Mark Cresswell
69EG6517 – Impacts & Models of Climate Change
Lecture Topics
• Introduction – a brief history of time
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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
Milankovitch Cycles:
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 #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 #2
The 11 year solar cycle
Historical overview of solar sunspot cycles
Solar Cycles #3
The 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)
Solar Cycles #5
Coronal Mass Ejections (CMEs) are
geomagnetic disturbances on the Sun
surface that generates the Aurora
Borealis.
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
LAND
Source
Sink
FLUX
C (X 1015 g) C (X 1015 g) C (X 1015 g)
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
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Urban heat island
Ocean circulation
Geothermal activity
Tectonic plate movement
Any questions
?