Download Climate change I: causes

Climate change I:
climate trends
Bio 415/615
Questions
1. How much colder was the Earth during
the last glacial maximum?
2. What are Milankovitch cycles?
3. How do we know the Earth is warming?
4. Why do scientists believe the warming
is anthropogenic?
5. How does the greenhouse effect work?
BE SKEPTICAL
• Don’t take anything at face value!
• Earth science is complicated
• Believe good data and well constructed
empirical models
• Remember Nero’s Dilemma: ‘science not
yet good enough’ is usually not a
legitimate policy position
Last 500 my: delta O18 in fossils
Veizer et al. (1999)
Plate tectonics? Solar system properties?
Why delta O18?
• O18 is a stable isotope of O16, created in supernovae
• water that evaporates and forms clouds is enriched
in the lighter form of oxygen (O16)
• glaciers are thus made out of relatively more O16,
while the oceans are enriched in O18
• oxygen in sea shells in marine cores (e.g., CaCO3)
can be sampled in the lab for relative abundance of
each isotope, compared to a standard
• tells how much water globally was in ice versus
oceans
• can perform same technique with hydrogen isotopes
Last 65 my: delta O18 in benthic
marine fossils
Zachos et al. (2001)
Last 3 mya: closing of isthmus of Panama
Last 5 my: delta O18 in benthic
marine fossils
Shows Milankovitch cycles of 100 and 41 kyr
Lisiecki and Raymo 2005
Milankovitch cycles
• Earth’s movement goes through cycles
that affect its climate
• Named after one of its discoverers,
Milutin Milankovic (Serbian engineer)
• Explain 100,000-yr ice ages (glaciation
cycles)
• Not accepted until data were available
to test it (late 1970s)
Milankovitch cycles
100,000 yr cycle: the earth’s
orbit gets more or less circular
41,000 yr cycle: the earth’s orbit
gets more or less tilted
~20,000 yr cycle: the earth’s tilt
changes WHEN hemispheres ‘look
away’ from sun
Last 450,000 yrs: deltaD samples
in 2 Antarctic ice cores
Shows glaciation cycles of about 100,000 yrs
Petit et al. 1999
Northern Hemisphere was about 5 C colder
(between 3 and 7 C, depending on dataset and
region) at the last glacial maximum
Last 12,000 yrs: multiple sources
Sources include deltaD from polar ice cores,
tropical ocean sediment cores, mountain glacier
ice core (Kilimanjaro), extrapolations from
pollen records
Rohde 2008 (online)
Last 150 yrs: instrumental records
UK met office
Recent compared to 1940-1980 mean
NASA
Pleistocene: ice ages
• The Pleistocene describes the first
epoch of the Quaternary Period (our
glacial age): 2 mya-10,000 years ago
• The Holocene describes the last 10,000
years (human expansion), a relatively
warm period (interglacial)
• An ice age cycle last about 100,000
years, with relatively short interglacials
(~ 20,000 years)
What happened as the climate
warmed?
• Why is this a significant question for
conservation biology?
How do we know what happened
to plants and animals?
• Plants: fossils (e.g.,
cones, wood) and
pollen (palynology)
Scots pine
• Animals: fossils
• All: genetics!
Pollen diagrams
Shows relative abundance of different pollen
types in a lake sediment core. Core depths are
aged by radiocarbon dating of macrofossils.
Vegetation shifts
Margaret Davis, 1983
Species moved
independently of one
another. Moreover,
the character of
communities was
constantly changing:
many of the these
communities have no
modern analogue.
New evidence for periglacial survival?
from McLachlan et al. 2005
Is the world warming?
Figure 3.1
IPCC 2007
Figure 3.1. Annual anomalies of global land-surface air temperature (°C),
1850 to 2005, relative to the 1961 to 1990 mean for CRUTEM3 updated
from Brohan et al. (2006). The smooth curves show decadal variations
(see Appendix 3.A). The black curve from CRUTEM3 is compared with
those from NCDC (Smithand Reynolds, 2005; blue), GISS (Hansen et al.,
2001; red) and Lugina et al. (2005; green).
Is the world warming?
“Comparison of measured sea
surface temperatures
in the Western Pacific with
paleoclimate data suggests
that this critical ocean
region, and probably the
planet as a whole, is
approximately as warm now as
at the Holocene maximum and
within 1°C of the maximum
temperature of the past
million years.”
Hansen et al. 2006, PNAS
Hansen et al. 2006
“Global warming was 0.7°C between the late 19th century (the
earliest time at which global mean temperature can be
accurately defined) and 2000, and continued warming in the
first half decade of the 21st century is consistent with the
recent rate of 0.2°C per decade.”
Are humans causing warming?
Intergovernmental Panel
on Climate Change,
Fourth Assessment
Report, Summary for
Policymakers, 2007.
Are humans causing warming?
YES. THE ARGUMENT:
IPCC 2007
1. Carbon dioxide and methane are
the most powerful anthropogenic
greenhouse gases and significantly
influence global temperatures.
2. CO2 and CH4 concentrations in the
atmosphere are higher than at any
time over the last 650,000 yrs.
3. CO2 and CH4 concentrations have
increased largely due to fossil fuel
burning (and agriculture), with
rates of increase unprecedented
for over 10,000 yrs.
BUT…
Humans also cool
the atmosphere, by
putting aerosols in
the air (small
particles that block
incoming radiation)
and changing
surface albedo
(reflectance of
solar radiation).
But cooling effects
are strongly
outweighed by
warming factors.
IPCC 2007
Keeling curve: CO2 on the rise
Before 1900, CO2 concentration was 275-285 ppm
CO2 during the recent Pleistocene
From ice cores
Future of CO2 depends on
emissions
Marland et al. 2003
Greenhouse effect
Think in terms of input and output of the Earth’s energy
(radiation):
• Solar radiation heats the Earth.
• Earth loses heat through longwave (infrared)
radiation (so do you by standing in front of a window…
every body not at absolute zero emits this radiation).
• Some gases in Earth’s atmosphere BLOCK longwave
radiation due to their chemical properties. This
energy thus gets ‘trapped’ inside the atmosphere, and
changes the Earth’s radiation balance.
• (This is NOT what happens in a greenhouse.)
Greenhouse effect
Greenhouse gases
without them, Earth would be uninhabitable (think Mars);
too much of them also a problem (think Venus)
• Water vapor: accounts for 1/3 to 2/3 of the greenhouse effect,
but is relatively constant and not generally anthropogenic
• Carbon dioxide: accounts for 1/10 to ¼ of greenhouse effect; is
sequestered by biomass, oceans, soils, and within the Earth’s
crust, and released by burning biomass, soils, and fossil fuels
• Methane: a potent greenhouse gas but relatively rare, emitted
by anaerobic microbes in wetlands, less than 1/10 of greenhouse
effect
• Ozone, nitrous oxide, CFCs: produced mostly as products or
byproducts of industry
• Most of the atmosphere is N2 and O2; these are NOT
greenhouse gases (they neither emit nor absorb longwave
radiation)
The amount of
carbon (CO2) in the
atmosphere
depends on how
much is
sequestered in
plants, soils, the
ocean, and rocks.
Sometimes dead
plant material does
not decompose and
is buried and
transformed by
heat and pressure
underground,
forming carbonrich ‘fields’ of coal,
oil, and gas (fossil
fuels). Humans
burns these fuels
and put C back into
the atmosphere.
Carbon cycle
No end in sight to fossil fuel
burning?
globalwarmingart.com
Have humans influenced climate
since before industrialization?
Methane: 5,000 yr anomaly?
Ruddiman 2003
CO2: 8,000 yr anomaly?
How fast will the world warm?
• General Circulation Models (GCMs) achieved
widespread acceptance in 1990s for the ocean
(OGCMs) and atmosphere (AGCMs); a ‘full’ climate
model is now an ‘AOGCM’
• Model fluids in 3D across ‘cells’ over the Earth
• Includes physical properties like pressure, velocity,
temperature
• Land surface properties like albedo and hydrology
• Atmospheric properties like cloud cover and water
vapor, air chemistry
• Significant sources of uncertainty remain: e.g., clouds
(can both cool and warm)
Models are
stochastic!
Figure 10.1
IPCC 2007
Figure 10.1. Several steps from emissions to climate response
contribute to the overall uncertainty of a climate model projection.
These uncertainties can be quantified through a combined effort of
observation, process understanding, a hierarchy of climate
models, and ensemble simulations. In a comprehensive climate
model, physical and chemical representations of processes permit
a consistent quantification of uncertainty. Note that the uncertainty
associated with the future emission path is of an entirely different
nature and not addressed in Chapter 10. Bottom row adapted from
Figure 10.26, A1B scenario, for illustration only.
Models must
predict what the
future world will
look like, in terms
of population size,
economic growth,
and what energy
economies will use.
These 3 emissions
scenarios vary in
energy efficiency,
rate of tech
advancement,
extent of
globalization, etc.
Figure 10.8
Figure 10.8. Multi-model mean of annual mean surface warming (surface air temperature change, °C) for
the scenarios B1 (top), A1B (middle) and A2 (bottom), and three time periods, 2011 to 2030 (left),
2046 to 2065 (middle) and 2080 to 2099 (right). Stippling is omitted for clarity (see text). Anomalies
are relative to the average of the period 1980 to 1999. Results for individual models can be seen in
the Supplementary Material for this chapter.
Clearly, warming
over the next
century depends
on how the global
economy develops
in relation to new
technologies that
do not involve
fossil fuels. Most
agree at least 3 C
warming is
inevitable, and
perhaps as high as
7 C.
Figure 10.29
IPCC 2007
Figure 10.29. Projections and uncertainties for global mean temperature increase in 2090 to 2099 (relative to
the 1980 to 1999 average) for the six SRES marker scenarios. The AOGCM means and the uncertainty
ranges of the mean –40% to +60% are shown as black horizontal solid lines and grey bars, respectively.
For comparison, results are shown for the individual models (red dots) of the multi-model AOGCM
ensemble for B1, A1B and A2, with a mean and 5 to 95% range (red line and circle) from a fitted normal
distribution.
Water predictions
Changes in
precipitation are
more difficult to
predict. A warmer
atmosphere holds
more water, but this
means different
things in different
places.
IPCC 2007
Figure 10.12
Figure 10.12. Multi-model mean changes in (a) precipitation (mm day–1), (b) soil moisture content (%), (c) runoff (mm
day–1) and (d) evaporation (mm day–1). To indicate consistency in the sign of change, regions are stippled where
at least 80% of models agree on the sign of the mean change. Changes are annual means for the SRES A1B
scenario for the period 2080 to 2099 relative to 1980 to 1999. Soil moisture and runoff changes are shown at land
points with valid data from at least 10 models.
Water predictions
Models agree that
more water in the
atmosphere means
larger precipitation
events—this makes
rainstorms stronger,
and days between
rain longer.
Figure 10.18
IPCC 2007
Warming side-effects: drought
FAQ 3.2, Figure 1. The most important spatial pattern
(top) of the monthly Palmer Drought Severity
Index (PDSI) for 1900 to 2002. The PDSI is a
prominent index of drought and measures the
cumulative deficit (relative to local mean
conditions) in surface land moisture by
incorporating previous precipitation and estimates
of moisture drawn into the atmosphere (based on
atmospheric temperatures) into a hydrological
accounting system. The lower panel shows
how the sign and strength of this pattern has
changed since 1900. Red and orange areas are
drier (wetter) than average and blue and green
areas are wetter (drier) than average when the
values shown in the lower plot are positive
(negative). The smooth black curve shows
decadal variations. The time series approximately
corresponds to a trend, and this pattern and its
variations account for 67% of the linear trend of
PDSI from 1900 to 2002 over the global land
area. It therefore features widespread increasing
African drought, especially in the Sahel, for
instance. Note also the wetter areas, especially
in eastern North and South America and
northern Eurasia. Adapted from Dai et al.
(2004b).
IPCC 2007
FAQ 3.2, Figure 1
Changes that will directly
influence plants and animals
Mean temperature
itself may influence
plants and animals in
the future, but more
relevant might be the
length of growing
seasons and high
temperature
extremes. Both are
on the increase.
Figure 10.19
IPCC 2007
Warming side-effects: sea level
Sea level has risen about 18 cm
since 1880, as measured by tidal
gauges in 23 sites around the
world (Douglas 1997). Sea level
rises from land glacier melting
AND thermal expansion.
These projections are from the
IPCC 2001 report; newer data
suggest the higher end curves are
more reasonable.
globalwarmingart.com
Warming side-effects: glaciers
Mountain glaciers are generally
thinning out at a rapid pace, many
over 1 m/yr. 83% of all glaciers
are thinning. Those in Scandinavia
are stable or increasing.
These are two glaciers in Glacier
Bay National Park, Alaska.
Dyurgerov and Meier 2005; globalwarmingart.com