Download Global Warming

Global Warming
(Climate Change)
Outline of Lectures
• Earth’s Energy Balance
– incident solar energy
– outgoing longwave radiation
– detailed energy balance
• Greenhouse Gases
– the greenhouse effect, and GHGs
– increasing GHGs
– biogeochemical cycles; the carbon cycle
• Radiative Forcing: the Enhanced Greenhouse Effect
– explanation of the concept of radiative forcing
– current values of radiative forcings
• Climate Change: Trends and Predictions
–
–
–
–
past global average temperature trends
feedback effects. Factors driving climate change.
computer models
predictions: global warming and other changes in climate
Earth’s Energy Balance
Incoming Solar Energy
emits
26
3.8 x 10 W
in all directions
(approximate)
cross-sectional area
17
1.7 x 10 W
strikes the earth
• to maintain energy balance, Earth must also emit energy at the
same rate of 1.7 x 1017 J/s
• dividing by the surface area, this averages out to 342 W/m2
Earth’s Energy Balance
Incoming and Outgoing Radiation
Earth’s Energy Balance
Incoming Solar Energy
•
Lecture Question
– How much of the sunlight that reaches the earth is:
a) reflected without being absorbed?
b) absorbed by the atmosphere?
c) absorbed by Earth’s surface?
a) 30%
b) 25%
c) 45%
Earth’s Energy Balance
Incoming Solar Energy
•about 30% of
incoming sunlight is
reflected back to
space (clouds,
oceans, snow/ice)
•this is Earth’s albedo
•the amount available
to heat the earth is
235 W/m2
•about 25% is
absorbed by the
atmosphere (see
figure on left)
•the rest (about 45%)
is absorbed by the
surface land and
water
The Greenhouse Effect
• Lecture Questions
– What is the greenhouse effect?
– What are greenhouse gases (GHGs)?
– What are the five main natural GHGs?
– (answers to follow)
– Without the greenhouse effect, the global average surface temperature of the
Earth would be about -19 ºC instead of 15 ºC.
Earth’s Energy Balance
Outgoing Longwave Radiation (OLR)
The major natural
GHGs are
•water (H2O)
•carbon dioxide (CO2)
•ozone (O3)
•methane (CH4)
•nitrous oxide (N2O)
(not shown on left)
•note the IR Window
in the OLR (8-13 mm)
•GHGs absorbing in
this region tend to be
very effective at
trapping OLR
Earth’s Energy Balance: Detailed Balance
107
reflected solar
radiation
342
incident solar
radiation
235
outgoing longwave
radiation (OLR)
Top of the Atmosphere
77
67
Atmosphere (air + clouds)
195
519 input = 519 output
30
168
102
350
non-radiative
heat transfer
Earth's Surface
522 input = 522 output
324
40
The Enhanced Greenhouse Effect
• Lecture Question
– What is the enhanced greenhouse effect?
– The enhanced greenhouse effect is an increase in the amount of
energy trapped by the atmosphere, largely due to increased
concentrations of greenhouse gases.
– What GHGs have been increased in the atmosphere due to
human activities?
– CO2, CH4, N2O, tropospheric O3 (smog), and halocarbons
(CFCs, HCFCs, HFCs, halons, and other cpds)
380
The Global Carbon Cycle
Increase in Atmospheric CO2
CO2 Concentration, ppm
370
What do you see in these data?
360
350
340
330
Monthly Measurements at Mauna Loa
Monthly Measurements at the South Pole
320
310
1960
1970
1980
Year
1990
2000
Increasing GHG Concentrations
Tropospheric Concs
2
GHG Pre-1750 Current Increase Lifetime, yr GWP (100 yr) Forcing, W/m
CO2
280
375
34%
5 - 200
1
1.46
CH4
700
1790
156%
12
23
0.48
N2O
270
318
18%
114
296
0.15
O3
25
34
36%
short
0.35
The Global Carbon Cycle
• Lecture Question
– What is the carbon cycle?
– What are the main processes that contribute to the carbon cycle?
– Global Biogeochemical Cycles
• Most important cycles: carbon (usually taken to be carbon dioxide), nitrogen,
sulfur, phosphorus, water
• Earth is divided up into a number of distinct reservoirs (“boxes”) such as:
atmosphere, water, land, etc.
– Divisions can be finer than this. For example, the atmosphere can be further
subdivided into troposphere and stratosphere; the hydrosphere can be divided
into oceans and freshwater (and these can be further subdivided), etc.
– The amount of the element in each reservoir is esimtated in some manner.
• The processes that allow exchange of the element between the reservoirs
are described and their rates are quantified
– For the carbon cycle, the most important processes are: photosynthesis,
respiration/decay, combustion, dissolution/outgassing, CaCO3 (calcite) formation
and dissolution
OUTER SPACE
escape
Biogeochemical Cycles
meteorites
surface reservoirs
ATMOSPHERE
respiration
photosynthesis
outgassing
BIOSPHERE
(vegetation, animals,
microorganisms)
dissolution
decay
assimilation
decay,
burial
assimilation
HYDROSPHERE
(oceans, lakes,
rivers, groundwater)
runoff
subduction
volcanoes
DEEP EARTH
(mantle, core)
LITHOSPHERE
(soil, sediment, crust)
Main Components of the Natural Carbon Cycle
Source: IPCC
Units: 1015 g C or 1015 g C/yr
Human Activities and
GHG Increases in the Atmosphere
• Lecture Question
– The atmospheric concentration of the following GHGs have been
increasing due to human activities. For each one, state the major
activities that have caused the increase:
•
•
•
•
•
CO2
CH4
N2O
tropospheric O3
halocarbons
• CO2: fossil fuel combustion, deforestation (biomass burning), cement
production
• CH4: coal mining, rice paddies, livestock, landfills, biomass burning, sewage
treatment
• N2O: fertilizer use, producing nutrient N pollution that stimulates nitrification
and denitrification, both of which give N2O as a byproduct
• troposphere O3: main component of photochemical smog, produced by NOx
emissions (cars, power plants) and reactive volatile organic cpds (VOCs)
• halocarbons: used as refrigerants, fire extinguishers, solvents, etc
Human Perturbation of the Global Carbon Cycle
Modeling the Carbon Cycle (Mass Balance)
Anthropogenic sources
Partitioning among reservoirs
fossil fuel combustion,
cement production
5.5 ± 0.5 PgC/yr
changes in land-use
1.6 ± 1.0 PgC/yr
total emissions
7.1 ± 1.1 PgC/yr
net ocean uptake
2.0 ± 0.8 PgC/yr
net land uptake
1.8 ± 1.6 PgC/yr
net storage in atmosphere
(measured)
3.3 ± 0.1 PgC/yr
The Methane Cycle
Natural Sources (mmt/yr)
Wetlands
115 (55 – 150)
Termites
20 (10 – 50)
Ocean
10 (5 – 50)
Other
15 (10 – 40)
Total
160
Anthropogenic Sources, mmt/yr
Coal mining, natural gas,
100 (70 – 120)
petroleum industry
Rice paddies
60 (20 – 100)
85 (65 – 100)
Enteric fermenation
Animal wastes
25 (20 – 70)
Sewage treatment
25 (15 – 80)
Landfills
40 (20 – 70)
Biomass burning
40 (20 – 80)
Total
375
Methane Sinks, mmt/yr
Atmospheric removal
530 (440 – 625)
Removal by soils
30 (15 – 45)
Atmospheric increase
37 (35 – 40)
Radiative Forcing
• Lecture Question
– What is radiative forcing?
– Radiative forcing is a quantitative measure of the imbalance
between incoming (solar) and outgoing (reflected plus infrared)
radiation.
– A positive radiative forcing indicates an increase in the amount
of radiation trapped by the atmosphere (an enhanced
greenhouse effect), eventually leading to global warming. A
negative forcing eventually leads to global cooling.
40
195
390
ATMOSPHERE
168
235
235
235
Radiative Forcing
~ 36
> 390
168
CO2 x 2
390
168
CO2 x 2
~199
235
the greenhouse effect
(radiation trapping)
36
radiation balance with no atmosphere
(but with 31% albedo)
195
EARTH (+15 oC)
235
EARTH (-19 oC)
EARTH (15 oC)
EARTH (>15 oC)
radiative forcing
Increased GHG levels traps additional radiation
(global energy input/output not balanced)
global warming
Trapped radiation warms Earth's
surface and lower atmosphere
Radiative Forcing
• Lecture Questions
– What have been the major (direct) factors that have caused a
positive forcing in the past century?
• increases in concentrations of CO2, CH4, halocarbons, tropospheric
O3, and N2O
• increase in elemental carbon PM (ie, soot)
• increase in solar output
– What factors have caused a negative forcing?
• stratospheric ozone depletion, increases in most PM (esp sulfate),
changes in cloud formation process, increased albedo due to landuse changes
Radiative Forcing
Radiative Forcing
Global Climate Change
• Lecture Question
– The phrase global climate change covers a variety of trends and effects.
List them.
– Changes in global average surface temperatures.
• including daytime max and nighttime min temps
• including number of “very hot” days in summer and number of “very cold”
days in winter
• change in sea level due to liquid expansion and changes in ice
thickness/extent
– Changes in global precipitation
• total amount of precipitation
• frequency and intensity of precipitation events
• changes in spatial patterns (traditionally “wet” and “dry” areas)
– Changes in “extreme” events
• hurricanes, tornados, heavy thunderstorms
– Changes in heat distribution
• changes in atmospheric and ocean circulation of matter and energy
• for example, shutting down the gulf stream
Global Warming
• Lecture Question
– What has been the average global surface temperature increase
in the last century? What is the current rate of increase per
decade?
– IPCC and textbook: the global average surface temperatures
over land increased by about 0.6 ºC (between 0.4 ºC and 0.8 ºC)
– IPCC: the current rate of increase is about 0.13 ºC / decade in
daytime daily max temperatures.
Temperature Data – Direct Measurements
Temperature Data – Indirect Measures
Temperature Data – Long Term Variability
Temperature – CO2 Correlation
Historical Record from Vostok Ice Core
4
320
2
300
0
280
-2
260
-4
240
-6
220
-8
200
-10
180
temperature
carbon dioxide
-12
160
0
100
200
Age, kyr BP
300
400
CO2 concentration, ppm
o
Temperature Deviation from Present, C
Temperature and Carbon Dioxide Data
Temperature – CO2 Correlation
Vostok Historical Data
Correlation of Temperature and CO2 Levels
4
linear correlation coefficient = 0.865
Temperature Deviation, oC
2
0
-2
-4
-6
-8
-10
160
180
200
220
240
260
280
Carbon Dioxide Concentration, ppmv
300
320
Climate Change Controversies
•
The Greenhouse effect
– Mechanism is not controversial
•
Increasing GHG concs
– The increase itself is not controversial
– Nor is it controversial that it is due to human activities
•
Radiative Forcing
– The fact that increasing GHGs will result in a positive forcing is not controversial
– There is some (fairly small) uncertainty in the magnitude of the forcing due to
GHG increases
– There is larger uncertainty on the effect of other changes (especially aerosol
indirect effects) on forcing
•
Temperature Trends
– Generally accepted that global average surface temps are rising
– Some controversy regarding sampling bias, satellite readings; largely resolved
now.
– Fairly broad agreement that the observed warming trend is not due to natural
factors only (although they contribute)
•
Future Predictions
– Firtually certain that increasing GHG concs will eventually lead to warming
– Rate and magnitude of the increase are uncertain; greatest source of uncertainty
is emission rate of CO2 and other GHGs
– The effects of increasing temperature are also controversial (ie, how bad would
warming really be?)
IPCC Statements
• What is the IPCC
– Intergovernmental Panel of Climate Change
– UNEP-WMO
– Authoritative scientific body to assess current status of climate change
science
– Also looks at mitigation (technology, policy, economics)
• Second Assessment Report (SAR), 1996
– “The balance of evidence suggests a discernible human influence on
global climate.”
• Third Assessment Report (TAR), 2001
– “There is new and stronger evidence that most of the warming observed
over the last 50 years is attributable to human activities.”
• Fourth Assessment Report (AR4), 2007
– “Warming of the climate system is unequivocal…[there is] very high
confidence that the globally averaged net effect of human activities
since 1750 has been one of warming.”
• Note: “very high confidence” is specified as “at least 90% probable.”
Statements by US Organizations
• American Meteorology Society (AMS), 2003
– Because greenhouse gases continue to increase, we are, in effect,
conducting a global climate experiment, neither planned nor controlled,
the results of which may present unprecedented challenges to our
wisdom and foresight as well as have significant impacts on our natural
and societal systems.
• American Geophysical Union, 2003
– Human activities are increasingly altering the Earth's climate. These
effects add to natural influences that have been present over Earth's
history. Scientific evidence strongly indicates that natural influences
cannot explain the rapid increase in global near-surface temperatures
observed during the second half of the 20th century.
• Other US organizations who have issued similar statements
– National Academy of Science
– American Association for the Advancement of Science
Some Points Made by Skeptics
• The science is too uncertain
– IPCC statements do not reflect the true degree of uncertainty
• Science is not a democracy
– Although a majority of scientists believe global warming is real
and is due largely to human activities, that does not mean they
are correct
– Minority scientific opinion has been right before
– The science has been politicized
• Past temperatures
– The Earth has been both cooler and warmer than today
• Predictions are too uncertain
– We cannot base policy on them
• Kyoto is a flawed treaty
– More cost-effective to wait before regulating GHG generation
• Warming is not so bad
Predictions for the Future
• Lecture Question
– What does the IPCC predict will happen to the global average
temperature in the next 100 years?
Predictions for the Future
Predictions for the Future
Predictions for the Future
• Questions
– How are future temperatures predicted?
• Sophisticated General Circulation Models (GCMs) are used.
– Commonly called “Global Climate Models”
– Computer-based integration of the exchange of energy and matter in a
3-dimensional spatial grid. They are run forward in time, and future
trends in climate (temperature, precipitation, etc) are predicted for each
element of the grid.
– Very many processes must be considered (see next slide)
– Nonlinear feedback effects must also be considered.
– How are predictions verified?
• Model predictions can be made for past climates and compared to
the actual measurements that were collected.
Factors Driving Climate
bold arrows represent
aspects that may change
Feedback Effects
• What are feedback effects?
– A negative feedback is one that tends to dampen changes
– A positive feedback is one that tends to reinforce changes
• Examples (not inclusive): effects of increasing temperature on
– The carbon cycle
• decreases solubility of CO2 in water, decreasing the ocean uptake rate
• possibly increases the rate of photosynthesis and plant growth
• increases the rate of decomposition
– The hydrologic cycle
• increases water vapor pressure, increasing conc of water in the atmosphere
– Earth’s albedo
• increases cloud cover
• decreases snow cover
– Energy circulation
• atmospheric circulation
• oceanic (thermohaline) circulation
Predictions: GCMs
• What are GCMs?
– Global Climate Models or General Circulation Models
– A GCM “aims to describe geophysical flow by integrating a
variety of fluid-dynamical, chemical, or even biological equations
that are either derived directly from physical laws (e.g. Newton's
law) or constructed by more empirical means.” (Wikipedia)
– There are GCMs for ocean and atmospheric components;
coupled atmospheric-oceanic GCMs (AOGCMs) can be used to
make climate predictions
– Most sophisticated GCMs are 3-dimensional “grids” to model
fluid motion
– Ideally provide both regional and global predictions of climate
– The models are imperfect
• True of any model, but:
• Critics of global warming theory content that the imperfections are
significant
Predictions Verification: Natural Forcings Only
Predictions Verification: Human Forcings Only
Predictions Verification: All Forcings
Other Effects on Global Climate
• Lecture Question
– Other than warming, what other changes are predicted in global
climate in the next century?
– Sea level will rise
• 0.18m – 0.59m (AR4) by 2100 by thermal expansion, which will
continue for centuries
• Potentially much larger increases due to loss/shifts of landlocked ice
sheets such as in Greenland (complete elimination would lead to 7m
rise) and Antarctica
– Precipitation will increase in some areas, decrease in others
• Possible increase in high latitudes and decrease in subtropical regions
– Increase in frequency and intensity of “extreme” climate events
• Heat waves, heavy precipitation events, tornados, hurricanes, stronger
El Nino events, monsoons, etc
– Effects on ecosystems uncertain
• ranges of various organisms will change
• increased spread of infectious disease (esp insect-borne) possible
• decrease in biodiversity feared
– Circulation in the Atlantic Ocean (MOC) will slow