Transcript Lecture 17: Global Change

Global change
Natural short and long term changes
Anthropogenic changes
Natural cycles
• Glacial cycles
• Holocene climate variability
– Orbital cycles
– Milankovitch cycles – regular shifts in earth’s
climate
Glacial-interglacial
• Relationship between glacial-interglacial cycles and CO2
• During glacial periods, older avg global temp (5-6 oC), lower sea level
(~100 m), change in ocean circulation
• Causes
– Changes in Earth’s orbit around the sun – also cause seasons
• Gravitational attraction between Earth and other bodies interact with
orbital factors
• Tilt (obliquity) – changes solar lumination input (41 ky periodicity)
result in seasonality; earth’s spin axis is tilted
• Eccentricity – the degree to which the Earth’s orbit is elliptical (100
ky periodicity); affects seasonality in N hemisphere; affects annual
average solar input
• Precession of spin axis – spin axis moves do to gravitational forces
between bodies (wobble)
• Interaction effects between tilt and eccentricity (positive or negative
interaction)
• All lead to differences in solar input
Natural fluctuations
Figure 14.21 Geometry of the Earth's orbit and axial tilt. A. Precession. The
Earth wobbles on its axis like a spinning top, making one revolution every
26,000 years. The axis of the Earth's elliptical orbit also rotates, though more
slowly, in the opposite direction. These motions together cause a progressive
shift, or precession, of the spring and autumn equinoxes, with each cycle
lasting about 23,000 years. B. Tilt. The tilt of the Earth's axis, which now is
about 23.5 degrees, ranges from 21.5 to 24.5 degrees. Each cycle lasts
about 41,000 years. Increasing the tilt means a greater difference, for each
hemisphere, between the amount of solar radiation received in summer and
that received in winter. C. Eccentricity. The Earth's orbit is an ellipse with the
Sun at one focus. Over 100,000 years, the shape of the orbit changes from
almost circular (low eccentricity) to more elliptical (high eccentricity). The
higher the eccentricity, the greater the seasonal variation in radiation
received at any point on the Earth's surface.
Fig. 14-6 Obliquity, insolation, and seasons.
(but NH is tilted toward from the sun)
(but NH is tilted away from the sun)
Making N hemisphere winters
milder.
Today, tilt and eccentricity (+precession)
oppose one another in the N Hemisphere
Today tilt and eccentricity
(+precession) reinforce one
another in the S Hemisphere
Frequency and amplitude of variability
Not equally
scaled
Astronimical theory of ice ages
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•
•
•
•
Changes in seasonal contrasts over geological
time
Due to 3 dominant factors – precession
(wobble), tilt (obliquity), and eccentricity (shape
of earth’s orbit around the sun)
Driver of ice ages is Milankovitch cycles
Exact causes not well-understood
Needed some major change to initiate cycles
–
–
–
India colliding with Asia (increased weathering)?
Initial cooling due to plate tectonics slowing down?
Feedback loops likely important
Fig. 14-8
Pleistocene glaciations
• Pleistocene glaciation (~ 1mybp) – start of quaternary
– Why did they start? Some perturbation?
• Ice-albedo feedbacks – positive
– What kept them going, but what reversed them?
• Timing of glacial cycles –
– Initially glacials and interglacials equal in length (40-50 ky cycles)
– Increase in length of glacials more recently, why?
Frequency of glaciations
increasing
The Carbonate-Silicate Cycle and Long-Term
Controls on Atmospheric CO
CO
CO
Weathering of
silicate rocks
2
2
2
CO 2
Ions (and silica) carried
by rivers to oceans
Ca 2+ + 2HCO
(+ SiO 2 2 [aq ])
+ SiO
-
Organisms build calcareous
(and siliceous) shells
3
CaCO
(+ SiO
3
+ CO
2
+ H 2O
2 (s)]
CO 2
Subduction
(increased P and T)
CaSiO
3
+ 2CO
2
+ H 2 O  Ca
2+
+ 2HCO
3
+ SiO
CaCO
2
3
+ SiO
2
 CaSiO
3
+ CO
2
Decrease in spreading rates also
decreases subduction
Disturbance to carbonate-silicate cycle via decreased seafloor spreading rates
Controls on Atmospheric CO
CO
Fig. 8-17 Collision of India with Asia.
CO
Weathering of
silicate rocks
2
2
2
CO 2
Ions (and silica) carried
by rivers to oceans
Ca 2+ + 2HCO
(+ SiO 2 2 [aq ])
+ SiO
-
Organisms build calcareous
(and siliceous) shells
3
CaCO
(+ SiO
3
+ CO
2
+ H 2O
2 (s)]
CO 2
Subduction
(increased P and T)
CaSiO
3
+ 2CO
2
+ H 2 O  Ca
2+
+ 2HCO
3
+ SiO
CaCO
2
3
+ SiO
2
 CaSiO
3
+ CO
increase in weathering
Continental collision creating
monsoonal climate with
lots of rain and weathering?
http://www.moraymo.us/uplift_overview.php
2
Lots of Buts….
• Eccentricity cycle (100 ky) is weakest of 3 cycles
but sets frequency of glacial cycles?
• Rate of change during glacial transitions is rapid
relative to astronomical changes?
• N vs. S hemisphere have same schedules for
glaciation but should they?
– Ice core data from both hemisphere similar
Role of the oceans?
• Deep water circulation altered or shut off
during glacials
• CO2 changes – cause or effect/positive
feedback? Unlikely that CO2 itself
triggered glacial/interglacial transitions
Implications
• Global climates and greenhouse gases change over
glacial/interglacial timescales
• Also shorter timescales – Heinrich events (due to FW inputs and
feedback from ocean circulation) and Dansgaard-Oeschger events
(rapid warming in N hemisphere) within glacial cycles
• Due to reorganization of ocean-atm system (multiple steady
states? Glacial and interglacial?)
• Will there be a third, warmer
quasi-stable state
Fig. 3 During a Heinrich event, icebergs
surge into the North Atlantic Ocean. The
lower panel illustrates the entrainment of
debris (black) by icebergs and the
subsequent sedimentation of the debris in
the deep North Atlantic.
Holocene climate variability
• Short-term variability
– 1-2 ky sub-Milankovitch periodicity
– Other cycles
• Important to understanding natural
variability versus anthropogenic
• After last ice age
• Medieval warm period (imp for Europe),
Little Ice age (Greenland)
Causes
• Astronomical
• Early anthropogenic hypothesis – human’s staved
off next glacial; greenhouse gases behaved
differently in initial stages of this interglacial
• Episodic factors – changes in solar activity,
tectonic activity
• Changes in ocean circulation – FW inputs
• Changes in sunspot activity
• Volcanic activity
Little Ice Age
Medieval Warm Period
Holocene climate optimum
maximum seasonal contrast
- tilt and eccentricity reinforce
another
- CO2 at a max.
- “start of interglacial’
LGM
Fig. 14-1 (Ruddiman)
Younger Dryas
- Sudden cooling
Sunspot activity
Correlation =
Causality (?)
High sunspot activity
Wolf Minimum (1282-1342)
Low sunspot activity
Spörer Minimum (1450-1534)
Low sunspot activity
Maunder Minimum
(1645-1715)
Low sunspot activity
Cooling due to volcanic eruption: The global mean temperature changes for 5 years preceding and
following a large volcanic eruption (at year zero). The temperatures are the average changes noted for
five major eruptions: Krakatau, August 1883; Santa Maria, October 1902; Katmai, June 1919; Agung,
March 1963; and El Chichón, April 1982. The effects of ENSO on temperatures have been removed.
(After A. Robock and J. Mao, 1995. The Volcanic Signal in Surface Temperature Observations. Journal of
Climate, 8:1086–1103.)
Fig. 15-6
Present-day climate variability
Present day
• ENSO events
• Sea ice atm-ocean interactions at high
latitudes
Eruption of Mt. Tambora
(1815)
Low volcanic activity
High volcanic activity
• Timing and amplitude of
forcing
• Stochastic resonance –
superimpose random
forcing on low
amplitude periodic
forcing
Global warming
• Recent climate change
– What controls climate and what’s changed?
• Present day forcings
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–
–
–
–
–
Changes in solar input (luminosity)
Changes in albedo (volcanoes, land cover, ice)
Changes in greenhouse gases
Changes in feedbacks
Planetary forcings
Stochastic events
Humans – the new element
• Can’t explain current T trends without it – radiative forcing (changes
in balance of incoming and outgoing radiation)
• Radiative forcing affected by: increases and decreases in solar input,
planetary albedo, and concentrations of greenhouse gases
Archer - Fig. 11.8
(Hadley Centre results)
Albedo
• Aerosols, clouds, ice/water/land distribution
• Feedbacks between temp and changes in things affecting
albedo
• Aerosols – cooling effect (reflect incoming radiation)
– Fine particles
– Also form cloud condensing nuclei
– Produced by natural (volcanoes) and unnatural (fossil fuel burning)
processes
• Relative albedos of ice/water/land
– Ice (0.8) > deserts & unvegetated land (~0.5) > water & vegetated
land (< 0.1)
Earth Surface Albedo
Oceans - < ~0.1
Vegetation - < ~0.25
Non-vegetated land - ~0.5
Ice - ~0.8
Greenhouse effect
• Radiative balance
– Temp controlled by balance between incoming solar flux, amount
of outgoing IR radiated from Earth, redistribution of radiation
before it is reradiated to space (e.g., outgoing IR retained by
greenhouse gases)
• Natural greenhouse
• Unnatural greenhouse
– CO2 – excursions greater than glacial/interglacial
• Correlations with human activities (fossil fuel burning), ocean uptake,
land use changes (deforestation)
• Where does this stuff go? Oceans and atm
• Effects on earth system (ecological, climatological, etc)
– Other greenhouse gases
• Same as above (S, methane, water vapor)
(also see Fig. 16.4 in your book)
Uncertainties
• Clouds
– warming or cooling
– Albedo versus greenhouse
Low albedo (rel. to low clouds)
low temp. (low outgoing IR flux)
Greenhouse effect dominates
High albedo and high sfc.
temp. (large outgoing IR flux)
Albedo effect dominates
Fig. 3-18 The different effects of high and low clouds on the atmospheric radiation
budget.
Evidence of Climate Change
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Temperature records (ground thermometers, proxy records)
Atmospheric temperature records
Ocean warming
Glacier melting
Ecosystem changes
Changes in the hydrologic cycle
IPCC Fourth Assessment Report (2007)
Warming of the climate system is unequivocal, as is now evident from
observations of increases in global average air and ocean
temperatures, widespread melting of snow and ice, and rising global
mean sea level.
The IPCC also finds that it is “very likely” that emissions of heattrapping gases from human activities have caused “most of the
observed increase in globally averaged temperatures since the mid20th century.”
IPCC History: Evolution of
Our Knowledge
• FAR (1990): “The size of the warming is broadly consistent
with predictions of climate models, . . . but the unequivocal
detection of the enhanced greenhouse effect from observations
is not likely for a decade or more.”
• SAR (1996): “The balance of evidence suggests a discernible
human influence on climate.”
• TAR (2001): “There is new and stronger evidence that most of
the warming observed over the last 50 years is attributable to
human activities.”
CO2 removal
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Oceans
Atm accumulation
Reforestation
Problem of not understanding feedbacks or
their direction – how far can the ocean go?
How will changes in temperature affect the
direction of changes, etc.
(Fig. 16-2)
Fossil fuel reserves exhausted
Long term projection
Fig. 11.12
Projections equally depressing
• Stabilizing total emissions or stabilizing
rates of emissions
• Models are simply C cycle models – what
about everything else and their feedbacks?
• Emissions – radiative forcing calculations
are not linear
Figs. 16-3 and 6
Fig. 6-3
2100
If nothing is done to slow
greenhouse gas emissions. . .
• CO2 concentrations will likely
be more than 700 ppm by 2100
• Global average temperatures
projected to increase between
2.5 - 10.4°F
Source: OSTP
Variations of the Earth’s Surface
Temperature - 1000 to 2100
• 1000 to 1861, N.
Hemisphere, proxy
data
• 1861 to 2000,
Global, instrumental
• 2000 to 2100, SRES
projections
Source: IPCC TAR 2001
Main Findings of WG I
• Extensive and wide-spread evidence that the earth is warming; we
are already seeing the first clear signals of a changing climate.
• Human activities are changing the atmospheric concentrations of
greenhouse gases.
• New and stronger evidence of a human influence on climate.
• Global temperature will rise from 2.5 to 10.4°F over this century.
Precipitation patterns will change, sea level will rise and extreme
weather events will increase.
• Human influence will continue to grow during the next century
unless measures are taken to reduce GHG emissions.
Uncertainties in feedbacks
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Clouds
Water vapor
Snow/ice albedo
Ocean circulation
Summary
• The greenhouse effect exists, is natural and we are
perturbing it
• Past climate changes can occur rapidly but not of
the same magnitude
• Rate of increase/change is unprecedented
• Major climate and ecological changes in store
• How do we deal with the change
– Change behavior to stem the rate of increase
– Adapt to what is already in the cards