Iceberg 100km east of Dunedin, South Island, New Zealand

1
Lecture 33 Climate Changes Past Future (Ch 16)
Climate change change in any statistical
property of the atmosphere
This chapter will clarify our thinking

• climate change relative to what normal?
• is climate warming happening?
• if so, is it man-induced, or not?

Iceberg 100km east of Dunedin, South Island, New
Zealand
Dunedin 16 November 2006 closest sighting off
New Zealand for 75 years according to NIWA
(National Institute of Water and Atmospheric
Research, NZ)
2
oC
Sec. 16-2 Fig. 1
1880
1980
Logical to consider earths climate to be a
function of these external factors (the text
calls them boundary conditions

• intensity of sunlight (solar output, sun-earth
  geometry)
• arrangement of continents and oceans
• composition of the atmosphere

Some believe earth does not have a unique climate
for fixed values of the above (earths climate
intransitive). Perhaps it would be surprising if
it did that would have to mean these factors
control earths processes down to a surprising
level of detail, eg. evolution of plants
3

• present climate is unusual - earth mostly has
  been considerably warmer than now
• most of its 4.5B yr life, earth free of
  permanent ice
• warmth (perhaps 5 to 15oC warmer global mean!)
  punctuated by seven ice ages
• all human existence has been spent in the last
  of these (last 15 MY)

4
Long-Term Changes The long term (100's million
years) paleoclimate record is characterized by
relatively few, isolated glacial outbreaks - the
great Ice Ages.

• cant be explained by variations in solar
  output
• cant be explained by orbital changes (which are
  too fast)
• best guess today associated with "Plate
  Tectonics" and its influence on the atmospheric
  greenhouse effect.

www.globalchange.umich.edu/
5

• within our ice age, numerous climate
  oscillations (glacial/interglacial cycles)

Fig. 16-3
oC

• planet is now in a warm interglacial

From Antarctic (Vostok) ice core record
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From Antarctic (Vostok) ice core record Both
methane and carbon dioxide correlate with
temperature - i.e., an increase in temperature is
associated with an increase in the abundance of
both these two gases. It is unclear whether the
gas abundance changes are a consequence of the
temperature changes or vice versa.
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Maximum extent of ice, last glaciation
Fig. 16-4

• smaller changes to Antarctic ice sheet
• but timing of major cooling/warming cycles in
  step over past 150KY
• a 5 degree change in global mean temp suffices
  to cause massive change!

Laurentide Ice Sheet
8

• millenial scale oscillations suggest a climate
  flip-flop (two climate states for given
  boundary conditions)
• on this timescale climate entails state of
  oceans too

Fig. 16-7
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So is earths climate warming?
(Yes or no, depending on the time scale on which
we view the record)
So-called hockey stick
Fig. 16-6
We must now ask if this recent rise in
temperature is just part of the natural
variability in climate, or if it marks the onset
of human-induced warming from emission of
greenhouse gases into the atmosphere. At
present, it is impossible to conclude one way or
the other with any certainty. (p484)
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24 November 2005 (New Scientist) The longest
ice-core record of climate history ever obtained
shows that levels of greenhouse gases really do
march in lockstep with changes in
temperature. The frozen record of the Earth's
atmosphere is 3270 metres long and covers the
last 650,000 years. It was obtained from the tiny
air bubbles trapped in a deep ice core from
Antarctica. The bubbles record how the planets
atmosphere changed over six ice ages and the
warmer periods in between. But during all that
time, the atmosphere has never had anywhere near
the levels of greenhouse gases seen today.
Today's level of 380 parts per million of carbon
dioxide is 27 above its previous peaks of about
300 ppm, according to the team led by Thomas
Stocker of the University of Bern.
Since our climate models, albeit imperfect, do
anticipate warming due to rising CO2, it is not
illogical to suggest the recent warming is a
response though we cannot prove this is so.
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What are the paleo-climatological lines of
evidence?
A vast array of techniques has been applied to
extend the instrumental record. Typically, it is
held there is a correlation between the observed
quantity, and some climate statistic. There will
be some form of calibration of the relationship
from a known record. Proxy climate indicators
include

• tree growth rings (going back several KY)
  correlation with temp precip
• oxygen isotopic content of sediments of marine
  organisms
• pollen (whose dating connects vegetation types
  with time)
• ice cores (to 650KYBP)

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Ice cores

• bubbles in the ice give a direct sample of past
  air chemistry

13
Ice cores

• snow that falls during period of warmer climate
  has higher ratio of 18O to 16O the connection
  with temperature is indirect, but experts dont
  doubt its validity

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paleoclimatologists have conducted a number of
tests to calibrate this "paleoclimate
thermometer" in the ice. Figure 1. Ice-core
oxygen isotopic measurements from Greenland
(right hand side) and from Antarctica (left hand
side). The isotope measurements can be
interpreted to yield the global sea surface
temperatures to 160,000 years ago (colder
temperatures to the left). The two traces are
consistent with each other and depict the most
recent glacial period, ending 15,000 years ago.
A decrease of one part per million (ppm) in the
?18O measurement is equivalent to a reduction in
temperature of approximately 1.5oC at the time
that the water evaporated from the oceans.
www.globalchange.umich.edu/
15
Factors involved in climatic change

• varying solar output? inconclusive
  correlations have been found on some timescales
• changes in earths orbit (Milankovitch cycles)
  widely accepted as driving glacial/interglacial
  cycles. Their periods (100, 41, 11) KY are short
  compared to the very long term record.
• changing continent/ocean distribution
• atmospheric composition
• tropospheric aerosols
• stratospheric aerosols
• CO2, methane,

it should not be surprising that there are a
host of processes and conditions that are not
known well enough for us to establish with
certainty the exact outcomes (of the buildup of
CO2) p494
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Global Climate Modelling

• History
• possibility that climate could be affected by
  changing concentrations of greenhouse gases first
  put forward by Arrhenhius (1896 On the
  influence of carbonic acid in the air upon the
  temperature of the ground. Philos. Mag., Vol.
  41, 237276)
• mid C20th attempts were made to estimate the
  equilibrium temperature rise due to doubling of
  atmos. CO2, based largely on radiative
  equilibrium calculations
• 1967 importance of convective processes in
  regulating the surface temperature of the earth
  was taken into account by Manabe and Wetherald
  (J. Atmos. Sci. 24, 241-259).

Mitchell (2004, Can we believe predictions of
climate change? Quart. J. Royal. Meteorol. Soc.,
Vol. 130, pp. 23412360)
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History (ctd)
Zonally averaged atmospheric temperature changes
due to doubling atmospheric CO2. Contours are
every oC, stippled (grey) where negative and
cross-hatched where greater than 4 oC (from
Manabe and Wetherald, 1975, The effects of
doubling the CO2 concentration on the climate of
a general circulation model. J. Atmos. Sci.,
Vol. 32, 315).
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19

• Current estimates using atmos. models coupled to
  a simple ocean give a range of 2 to 6oC for
  global mean temperature response to CO2 doubling

Climatic warming under CO2 doubling (Canadian
Climate Centre model)
(latitude-dependent)
Fig. 16-13a
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How does forecasting climate differ from
forecasting weather?

• many more processes, acting on longer
  timescales, need to be included, eg.
• ocean temperature ( salinity) changes
• ocean circulations influencing CO2 budget
• sun-earth geometry changes
• locations of continents?
• ice sheets and ice packs
• vegetation responses interacting with CO2,
  temperature and humidity
• natural aerosols
• anthropogenic gases and particles
• climate simulation computes the equilibrium
  climate for certain fixed external conditions
  (eg. perhaps fixed ocean temps fixed CO2 fixed
  sun-earth geometry). Thus initial conditions are
  irrelevant (one integrates for long enough to
  forget the initial condition).
• It may be possible to neglect or simplify some
  rapid processes, and even to neglect a spatial
  dimension, eg. zonally-averaged models

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Uncertainties in Climate Modelling using GCMs
The main uncertainties arise with processes for
which we do not have a reliable underlying theory
(including cloud formation and dissipation), and
processes which are not resolved on the model
grid (including transfer of heat, moisture and
momentum from the surface, convection and cloud
processes) There remain model parameters which
cannot be measured or do not correspond to any
measurable quantity, eg. some cloud
parametrizations define a relative-humidity
threshold above which cloud is allowed to form.
Even if there is a single threshold in the real
world, it is unlikely that using it would give
the correct cloud amount... Small errors in cloud
amounts and microphysical properties can produce
large errors in the radiative budget, and hence
large drifts in surface temperature.
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Positive feedbacks (see Sec. 6-3) are those
which reinforce (or act additively with) the
original disturbance, eg. the ice albedo
feedback. Negative feedbacks oppose the root
disturbance. Thus if global warming increases
global cloud coverage, increased solar reflection
is a negative feedback, but increased absorption
of upwelling longwave radiation is a positive
feedback. Overall cloud feedback is a complex sum
of several feedbacks GCMs disagree on overall
sign! There are complex feedbacks whose
parametrization needs to be refined, e.g.
dimethyl sulphide (DMS) gas, released by decay of
ocean biota, forms sulphate aerosols that act as
CCN will warmer ocean temperatures mean greater
ocean productivity and consequently greater
biotic decay rate, causing higher atmospheric
concentrations of CCN and changes to cloud amount
and type?
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Global Climate Modelling

• Four criteria to judge is a climate model
  reliable for predicting climate change?
• physical basis
• simulation of present climate
• simulation of historical climate (period of
  instrumental records, or equilibrium simulation
  of much more distant climates, eg. Last Glacial
  Maximum, 21kBP, ie. 21,000 years ago)
• numerical weather prediction

Only a few fully coupled simulations have been
published to date, but these all show
global-scale cooling broadly consistent with the
paleoclimatic reconstructions... there is still
little or no confidence in the regional
detail predicted by models... most of the range
in climate sensitivity across various GCMs is
associated with differences in cloud feedback
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The second phase of the Paleoclimate Modeling
Intercomparison Project is coordinating
simulations and data syntheses for the Last
Glacial Maximum (LGM 21000 yr before present 21
ka) and mid-Holocene (6000 yr before present 6
ka) to contribute to the assessment of the
ability of current climate models to simulate
climate change. Here the Community Climate
System Model version 3 shows global cooling of
4.5C compared to pre-industrial (PI) conditions
with amplification of this cooling at high
latitudes and over the continental ice sheets
present at LGM.
Otto-Bliesner et al. (2006 J. Climate, Vol. 19)
Change in mean annual surface temperature (C)
LGM minus Pre-Industrial
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The forcings changed for the LGM are reduced
atmospheric greenhouse gases, a 23-km ice sheet
over North America and northern Europe, lowered
sea level resulting in new land areas, and small
Milankovitch anomalies in solar radiation. The
reduced LGM levels of atmospheric CO2 are 66 of
preindustrial levels and 55 of present levels.
Otto-Bliesner et al. (2006 J. Climate, Vol. 19)
Change in mean annual surface temperature (C)
LGM minus Pre-Industrial
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U. Vic Coupled Ocean-Atmosphere Climate Model
(intermediate complexity)

• Ocean
• 3-dimensional
• 3.6o zonal x 1.8o meridional
• 19 vertical levels (?z50 m near surface, ?z
  500 m near ocean bottom)
• dynamic-thermodynamic sea ice (ie. wind- driven
  motion melting)
• inorganic carbon cycle
• Atmosphere
• 2-dimensional, ie. vertically well-mixed (single
  layer), horiz. resolution same as ocean
• horizontal diffusion of energy and moisture,
  present-day climatological surface winds
  determine sea-air transfer coefficient for heat,
  CO2, etc.
• Precipitation when RH gt 85 returns instantly to
  ocean via one of 33 rivers

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U. Vic Coupled Ocean-Atmosphere Climate Model
(intermediate complexity)

• Land
• dynamic vegetation (responds to climate, incl.
  CO2)
• terrestrial carbon cycle
• ice snow albedo feedback
• specified lapse rate used to reduce surface
  temperature over topography

• Performance
• does not need air-sea flux adjustments to keep
  present climate stable
• when forced by historical CO2 emissions
  reproduces historic CO2 trends
• soon to be used for 120,000 year simulation to
  examine glacial-interglacial transitions