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Chapter 16
Climate Changes:
Past and Future
Climate change
Climate change can be defined as a change in any
statistical property of the atmosphere,
such as a change in mean temperature.
However, changes in climate may occur
even though the mean values of precipitation,
temperature, and wind remain the same over time.
Changes in extremes are important.
Observerte endringer i
klimasystemet
• Oppvarming av
lufttemperaturen på bakken
og i troposfæren
• Oppvarming av havet
• Økning av havnivå
• Smelting av snø og sjøis
Klimaendringer på alle kontinenter
Eksempler på klimaendringer i Norge
Økning av antall
døgn uten nedbør
Økning av antall
døgn med
sterk nedbør
Eksempler på klimaendringer i Norge
Milde vinterdager og varme sommerdager
CO2,CH4 and estimated
global temperature
(Antarctic ΔT/2
in ice core era)
0 = 1880-1899 mean.
Source: Hansen, Clim.
Change, 68, 269, 2005.
The geologic
column
Glacial/interglacial cycles
For most of its life, Earth has been largely free of
permanent (year-round) ice. It is a warm planet,
punctuated by perhaps seven relatively brief ice ages.
The warm times persist for hundreds of millions of years to
billions of years, whereas the ice ages last on the order of
tens of millions of years to perhaps a hundred million years.
Oscillations in temperature and ice cover are called
glacial/interglacial cycles.
Glacial/interglacial cycles
•Earliest known ice age: 2300 MYA (million yr ago)
•Last 2500 MY 10-20% (50%?) have been ice ages
•Extreme warm period 120-90 MYA (mid-Creataceous
period)
•Extreme cold period 700 MYA (“snowball Earth”)
•Ice age today
Current ice age
•Origin ~55 MYA
•Slow ice accumulation in Antarctica/Greenland
•Antarctica started 34 MYA
•Antarctica still forest 20 MYA
•Antarctica as today 10 MYA
•Greenland glaciated 5 MYA
•Large oscillations within the ice age
•Glacial/interglacial cycles
•Somewhat irregular
•Last 750 kY dominated by ~100.000 yr cycles
•Shorter term oscillations superimposed
•Last glacial maximum ~20.000 YA
CO2, CH4 and temperature records from Antarctic ice core data
Source: Vimeux, F., K.M. Cuffey, and Jouzel, J., 2002, "New insights into Southern Hemisphere
temperature changes from Vostok ice cores using deuterium excess correction", Earth and Planetary
Science Letters, 203, 829-843.
In the depths of the last glaciation, around 20,000 years ago,
land ice covered much more area as seen in the map above.
Sea level was about 120 m lower than it is now,
so that a land bridge existed between Siberia and Alaska.
Warming began about 15,000 years ago, interrupted about
2,000 years later by the Younger Dryas, a time when
colder conditions returned for about 1,200 years.
11,800 years ago another period of abrupt warming
began bringing climate into the present interglacial.
There is evidence that the period a.d. 900–1200 was warm
in the North Atlantic. This Medieval Warm Period,
coincides with the Viking settlement of Greenland.
The so-called Little Ice Age, from 1450 to 1850,
was a cold period for western Europe as alpine glaciers
advanced and temperatures fell by about 0.5 to 1°C.
The Maunder Minimum was a period of minimal sunspot
activity between about 1645 and 1715, which coincided
with one of the coldest periods of the Little Ice Age.
However, there have been episodes in which variations
in sunspot activity did not coincide with climate change.
Milankovitch cycles
Three astronomical factors influence the
timing and intensity of the seasons:
•eccentricity in the orbit
•Earth’s axial tilt off the perpendicular to the
plane of the orbit
•timing of aphelion and perihelion relative to the
timing of the equinoxes
.
The eccentricity of Earth’s orbit changes cyclically with a
cycle of about 100,000 years being especially prominent.
Over about the last 15,000 years, there has been a steady
decrease in eccentricity, which will continue for 35,000 years.
Obliquity is the tilt of Earth’s axis, which also varies cyclically
with a dominant period of about 41,000 years during which
it varies between 22.1° and 24.5° off the perpendicular.
The most recent peak in obliquity occurred 10,000 years ago,
thus, we are about midway in the half cycle
from maximum to minimum obliquity.
Precession is the change in the orientation of the Earth’s axis.
Combined with changes in the orientation of the elliptical orbit,
the result is a 23,000-year cycle in radiation.
The breakup of Pangaea (the early supercontinent) and the
slow movement of the resultant continents undoubtedly
caused major climatic changes because all the factors that
affect climate variables were themselves greatly affected
by the movement of the continents.
Houghton, Ch 14, Fig 14.11 p
259
Greenhouse gases
Since the middle of the nineteenth century, there has been
an exponential increase in the input of carbon dioxide
to the atmosphere by fossil fuel consumption.
However, carbon dioxide is only one of several
anthropogenic greenhouse gases that absorb outgoing
longwave radiation. Methane, nitrous oxide, and
chlorofluorocarbons are also effective absorbers
whose contents are currently increasing in the atmosphere.
Carbon cycle
There is a constant exchange of carbon dioxide between
the ocean and atmosphere with the ocean
acting as a net sink for the greenhouse gas.
Photosynthesis by marine plants removes carbon and settling
plant and animal remains transfer carbon downward.
If the removal rate were to decrease, there would be an
accelerated increase in atmospheric carbon dioxide levels.
Climatically active constituents
SPECIES
Water
Carbon dioxide
Methane
CFCs & HCFCs
Nitrous oxide
Ozone (surface)
Particles
CHANGES per year
Most important greenhouse gas
+ 0.6 %
+1% until 1990. Less now.
0 – 100%
+
Regional increase
Regional increase
To Stabilize Concentrations
Decrease necessary
Carbon dioxide
> 60 %
Methane
15 – 20 %
Nitrous oxide
70 – 80 %
CFC – 11
70 – 75 %
CFC – 12
75 – 85 %
HCFC - 22
40 – 50 %
Aerosol climate effects
Atmospheric turbidity refers to the amount of suspended
solid and liquid material (aerosols) contained in the air.
Aerosols directly affect the transmission and absorption of
both solar and infrared radiation. Aerosols can also affect
climate indirectly as cloud condensation nuclei.
Aerosoler i atmosfæren sett fra
satellittinstrumentet MODIS
Radiative forcing from aerosols
•
Direct effect
– Aerosols scatter and absorb solar and infrared
•
Indirect effects
– Warm (liquid-water) clouds
• First is associated with the change in droplet
concentration caused by increases in aerosol cloud
condensation nuclei
• Second is associated with the change in precipitation
efficiency that results from a change in droplet number
concentration
– Ice clouds may also be affected by aerosols
To estimate radiative forcing from aerosols
•
Spatially and temporally resolved information on
the atmospheric burden and radiative properties
of aerosols is needed.
•
Important parameters are size distribution,
change in size with relative humidity, refractive
index, and solubility of aerosol particles.
•
An ability to distinguish natural and
anthropogenic aerosols.
Evidence
for
indirect
effect!
Ship tracks appear as white streaks embedded in a low-level
cloud deck of speckled light gray clouds. The image shows an area
just offshore of the U.S. with small amounts of cloud-free ocean.
Tropospheric stratospheric aerosols
Numerical models indicate that increased
tropospheric aerosol contents have the net effect
of reducing surface temperatures globally.
Stratospheric aerosols tend to be smaller
and have lower terminal velocities.
Because they are small, the reduction in solar radiation
reaching the surface exceeds the gain in longwave radiation.
Feedbacks
Feedbacks refer to the interconnections of different
components of the atmospheric systems.
In a simple system that consists of only two variables,
changes in either one affect the state of the other.
If there is a change in the first variable,
it will produce a change in the second,
which will in turn affect the first variable.
Negative feedbacks inhibit further change and are
self-regulating. When the second variable responds to
initial change in the first variable, its response
will suppress further change in the first.
Positive feedbacks amplify change in the initial variable.
The response of the second variable causes the
initial change to grow in a snowball effect.
Among the feedback mechanisms affecting climate are:
Ice-Albedo Feedback
Evaporation of Water Vapor
Ocean-Atmosphere Interactions
Atmosphere-Biota Interactions
Surface Melt on Greenland
Melt descending
into a moulin,
a vertical shaft
carrying water
to ice sheet base.
Source: Roger Braithwaite,
University of Manchester (UK)
Increasing Melt Area on Greenland
• 2002
all-time record melt area
• Melting up to elevation of 2000 m
• 16% increase from 1979 to 2002
70 meters thinning in 5 years
Satellite-era record melt of 2002 was exceeded in 2005.
Source: Waleed Abdalati, Goddard Space Flight Center
Greenland Mass Loss – From Gravity Satellite
Uncertainties in calculated future
climate change
• Magnitude of human emissions
• How the emissions alter atmospheric
concentrations
• How climate changes with the changed
concentrations
Climate models
• Mathematical models built on physics
Noteworthy (unavoidable) simplifications:
• Clouds (formation and distribution)
• Energy exchanges between
– Atmosphere and ocean
– Atmosphere and land
– Surface and deep ocean
Fremtidig klimautvikling
•Fortsatte klimaendringer kan ikke unngås
•Tilpasning er helt nødvendig
•Tiltak trengs for å begrense klimaendringene
•Involverer politikkutforming og teknologianvendelse