The Science of Climate Change
Dr. Douglas Allen Dept. of Physics and Astronomy
Dordt College, Sioux Center, Iowa
April 19, 2007
Why is the Debate so Confusing?Why is the Debate so Confusing?Scientific ReasonsPhilosophical Reasons
Climate Change ScienceIndicators Attribution Projections Consequences
Summary
Climate vs. Weather
Climate and weather are both described in terms of physical properties of the atmosphere (temperature, wind, pressure, humidity, precipitation, etc.)
Weather: refers to what is happening at any given time (example: today’s high temperature is …)
Climate: refers to average weather over time and/or geographic area. (example: the average temperature in Iowa in 2005 was …)
“Climate change”: refers to a change in the average weather over a certain geographic area.
“Global climate change”: refers to the change in the average climate over the whole Earth.
Why is the Climate Change Debate so Confusing? Scientific Reasons
Climate changes occur on long time-scales need long data records to discern trends
Climate change trends are smaller than normal weather fluctuations need good statistics
The atmosphere-ocean system is very complicated
need computer models
Projections depend on uncertain human actions
need to develop plausible scenarios
No one can be an expert on all areas of the debate need to decide who you will trust
Why is the Climate Change Debate so Confusing?Philosophical Reasons
Worldviews influence how we filter scientific “data”.“Virtually all the major disagreements between rival theories in the sciences and in philosophy can be ultimately traced to the differences between the religious beliefs that guide them.” (R. Clouser, The Myth of Religious Neutrality).
(Unwritten) rules of policy argument tend to be (Unwritten) rules of policy argument tend to be more lenient than rules of scientific argument.
In policy argument, scientific “facts” may be poorly presented in order to support a particular position.
Policy debate demands fast answers and can be unsympathetic to scientific caution.
For further discussion see “The Science and Politics of Global Climate
Change: A Guide to the Debate,” by A. Dessler and E. Parson, 2006.
The Responsibility of the Scientist in the
Climate Change Debate
Provide accurate information about the likely magnitude, causes, and projections of global climate change.
Be up front about all assumptions made.
Provide levels of uncertainty and statistical significance.
Use the peer-review process (an effective filter).
Provide results that are reproducible and refutable.
Provide quantitative results, not anecdotal evidence.
Reference claims to peer-reviewed publications.
Climate Change Research• Information sources
– Observations of Climate Variables• Direct: weather stations, ships, planes, buoys, satellites
• Indirect: tree rings, ice cores, corals, ocean sediments, boreholes, glacier records, etc.
– Models that Simulate Past and Future Climate• Global circulation models (GCMs)
• Regional climate models (RCMs)• Regional climate models (RCMs)
• Coupled Atmosphere Ocean GCMs (AOGCMs)
• Dissemination of information– Peer-reviewed journal articles
– Scientific Assessments• Intergovernmental Panel on Climate Change (IPCC)
• Reports published in 1990, 1995, 2001, 2007
Warming is
larger over land
Warming is
larger during
winter
Is the Global Surface Temperature Rising?
Increase of 0.74oC or 1.3oF from 1906-2005Global Average
Temperature
Deviation
(1850-2005)
IPCC 2007 Figure
SPM-3 Data from thermometers
Data from tree rings, ice cores, and other proxies
See Mann et al., Geophys. Res. Lett., 26 (6), 759-762, 1999.
Warming is
larger at high latitudesN. Hemisphere
Temperature (1000-2000)
The “Hockey Stick” Diagram
Other Indicators of 20th Century Warming
• 17 centimeter (±5) sea level rise
• Decline in NH snow cover
• Retreat of mountain glaciers
• Global ocean heat content increase since the late 1950’s (when we started collecting good data)
• 10 -15% drop in Arctic spring and • 10 -15% drop in Arctic spring and summer sea-ice extent
• Warming of lower- and middle troposphere
• Average atmospheric water vapor content increase
From Climate Change
2007: The Physical
Science Basis, IPCC.
See IPCC Climate Change 2001: Working Group I Technical Summary
Potential Causes of 20th Century Warming
• Earth’s orbital variations Large timescales
• Tectonic activity Large timescales
• Volcanoes Irregular, short-lived impact
• Internal variability Magnitude of internal variability too small to account for observed global-scale changessmall to account for observed global-scale changes
• Solar variability Solar variations play a role in climate, but sun’s output has been relatively steady over last 25 years
• Human activities Enhanced greenhouse effect Enhanced aerosols
See “The Science and Politics of Global Climate Change: A
Guide to the Debate,” by A. Dessler and E. Parson, 2006.
Total Solar Irradiance (1600-2000)
Maunder Minimum
~0.25% increase
(~3 W/m2)
Little Ice AgeData from Lean (2000)
Data from Solanki and Krivova (2003)
Solar Variability
Solar Variations
• The Sun has a
0.54 W/m2 increase (1980-2005)
0.06 W/m2 decrease (1980-2005)
0.62 W/m2 increase (1980-2005)
0.10 W/m2 increase (1980-2005)
Potential Causes of 20th Century Warming
• Earth’s orbital variations Large timescales
• Tectonic activity Large timescales
• Volcanoes Irregular, short-lived impact
• Internal variability Magnitude of internal variability too small to account for observed global-scale changessmall to account for observed global-scale changes
• Solar variability Solar variations play a role in climate, but sun’s output has been relatively steady over last 25 years
• Human activities Enhanced greenhouse effect Enhanced aerosols
See “The Science and Politics of Global Climate Change: A
Guide to the Debate,” by A. Dessler and E. Parson, 2006.
The Greenhouse Effect
Average temperature without greenhouse gasses = -6°C (21°F)
Average temperature with greenhouse gasses = +15°C (59°F)
Characteristics of Greenhouse GasesGas Natural Sources Anthropogenic Sources Average Atmospheric
Residence Time
CO2 - Carbon
Dioxide
Respiration, plant
decomposition
Fossil fuel burning,
biomass burning (clearing
forests)
About 125 yrs.
(multiple time scales)
CH4 - Methane Ruminant animals,
wetlands
Rice paddies, biomass
burning, landfills, natural
gas exploitation
12 yrs.
N2O - Nitrous Oxide Natural soil Fertilizer app., fossil fuel 120 yrs.N2O - Nitrous Oxide Natural soil
processes
Fertilizer app., fossil fuel
burning
120 yrs.
CFC’s –
Chloroflouro-
carbons
None Aerosol prop.,
refrigerants, foam packing
65-130 yrs.
Trace gases -
tropospheric O3, CF4,
and others
Internal comb. engines,
aluminum production,
other
50,000 yrs. for CF4
Note: Water vapor is the dominant greenhouse gas, but water vapor
respond quickly to the atmospheric temperature and so is treated as a
part of the climate system that responds to external forcing.
CO2 has risen by 35 %
Trends of Greenhouse Gas Concentrations
N2O has risen by 18 %
280 parts
per million
379 parts per
million in 2005
CH4 has risen by 148 %
Correlation vs. Attribution
Carbon dioxide increase
appears to correlate well with
recent increase in temperature.
But a correlation alone doesn’t
necessarily indicate cause and necessarily indicate cause and
effect. [Examples]
In order to attribute causal
relationship scientists
construct models to test the
relative effects of various
physical processes.
Attribution of Climate Change Using Models
• To establish cause-effect relationship, need to use computer models that
simulate the known “laws” of physics, chemistry, etc.
• Climate models are similar to weather models, but focus on large-scale, long-term changes rather than short forecasts.
• Models attempt to reconstruct past climate as well as project future climate.
The global mean radiative forcing of the climate system for 2005 relative to 1750
IPCC 4th Assessment Report Figure SPM-2
Simple Climate Model ResultsSun
Solar Energy IR Energy
Absorbed by Earth = Emitted by
Earth Earth
Earth
Temperature without GG = -6°C (21°F)
Temperature with GG = +15°C (59°F)
Doubling carbon dioxide (all else constant) should result in an increase of 1.2°C.
With feedbacks (water vapor, ice-albedo, clouds), expected increase of 2 to 4.5°C
Earth Earth
(1 – A) E = σ T4
E = 343 W/m2 (Average Solar Flux at Earth)
A = 0.16 (Earth’s Albedo, without clouds)
T = 267° K = -6° C (Earth’s average temperature)
See “Global Warming:
The Complete Briefing”
by John Houghton
Including
Solar +
Volcanic
Activity
Including
Greenhouse
Gas increases
GCM reconstructions of global average temperature (1860-2000)
See Stott et al., Science, 290, 15 Dec. 2000, 2133-2137, 2000.
Model
Observations
Including
All Factors
Model Projections of Global Average Temperature
See Stott et al., Science, 290, 15 Dec. 2000, 2133-2137, 2000.
Projection based on
IPCC B2 Scenario
Multiple Model Projections
Global Average Temperature Global Average Precipitation
Average temperature increase of 1.8 C over 70 years.
Models generally predict increased precipitation
Experiments with 1% per year increase in carbon dioxide. Doubling of carbon dioxide would occur in year 70.
All 20 models show increased warming, and most models show increased precipitation.
From Climate Change 2001: The Scientific Basis, IPCC.
Projected temperature and sea levels based on various carbon emission scenarios
Increase from 1.7 to
4.2°C (3 to 7.5°F) Increase from
Global Average Temperature Global Average Sea Level
From Climate Change 2001: The Scientific Basis, IPCC.
Increase from
10 to 90 cm
Projections from 4th Assessment Report (graphs not yet available):
Temperature increase of 1.1 – 6.4°C, Sea Level Rise of 18-59 cm
Likely Global Warming Consequences
Increasing GG Concentrations? Virtually certain
Rising Temperatures? Virtually certain
Melting ice? Virtually certain
Rising Sea Levels? Very likely
Eroding coastlines? Very likely
J. Knox, Living in a Globally Warmed World, Phi Kappa Phi Forum, Vol. 86, 11-16.
Possible Global Warming Consequences
Strengthening Hurricanes? The jury is out
Intensifying heat waves? Possible
Worsening droughts and floods? Possible
Invading tropical diseases? Possible
Proliferating tornadoes? Unlikely
J. Knox, Living in a Globally Warmed World, Phi Kappa Phi Forum, Vol. 86, 11-16.
Summary
Climate is influenced by many complex factors, some of which we understand well while others are poorly understood.
Observational evidence supports global average surface warming of ~0.74°C, sea level rise of ~17 cm over the last century, and widespread melting of snow and ice.
Most of the observed increase in globally averaged Most of the observed increase in globally averaged temperatures since the mid-20th century is very likelydue to the observed increase in anthropogenic greenhouse gas concentrations.
Models project additional warming of 1.1 – 6.4°C and sea level rise of 18 - 59 cm by end of the 21st century.
Regional impacts are also likely, but specific projections are more uncertain than large-scale projections.
Short List of Recommended Resources
• IPCC Assessment Reports– www.ipcc.ch
• Web pages– www.realclimate.org
• Books– Global Warming: The Complete Briefing – Global Warming: The Complete Briefing
• John Houghton
– The Science and Politics of Global Climate Change • Andrew Dessler and Edward Parson
– Surface Temperature Reconstructions for the Last 2000 Years • National Research Council, 2006
• Data– World Data Center for Paleoclimatology
• www.ncdc.noaa.gov/paleo/data.html
Extras
Winter Temperature
Trends (1976-2000)
Summer Temperature
Trends (1976-2000)
IPCC TAR Fig 2.10
Jones et al. (2001)
Around 10 million
people in Bangladesh
live less than 1 meter
above sea level.
Milankovich Cycles
Quinn, T.R. et al. "A Three Million Year Integration of the Earth's
Orbit." The Astronomical Journal 101 pp. 2287-2305 (June 1991).
The global mean radiative forcing of the climate system for 2005 relative to 1750
IPCC WGI Fourth Assessment Report Figure SPM-2
Geographic Distribution of Trends (1976 – 2000)
IPCC TAR Figure 2.9. Adapted from Jones, P.D., T.J. Osborn, K.R. Briffa, C.K. Folland, E.B.
Horton, L.V. Alexander, D.E. Parker and N.A. Rayner, 2001: Adjusting for sampling density in grid
box land and ocean surface temperature time series. J. Geophys. Res., 106, 3371-3380.
Atmospheric “Governing Equations”
Conservation of momentum
Conservation of mass
Equation of state
)(
2
dpdT
RTp
t
pdt
d
=
⋅−∇=∂
∂
×Ω−+∇−∇−=
α
ρρ
φα
v
vFv
Conservation of energy
Conservation of moisture)()( CEqt
q
Qdt
dp
dt
dTCp
−+⋅−∇=∂
∂
=−
ρρρ
α
v
Climate models attempt to solve this system of equations
numerically (i.e., with computers). Models attempt to
reconstruct past climate and predict the future climate.
Earth’s AtmosphereComposition:
78% nitrogen 21% oxygenother “trace” gases (water vapor, carbon dioxide, ozone, methane, etc.)
Density at surface: 1.275 kg/m3Density at surface: 1.275 kg/mPressure at surface: 1 atm (14.7 lb/in2)
Troposphere (0-10 km)Stratosphere (10-50 km)Mesosphere (50-80 km)Thermosphere (above 80 km)
Average Surface Temp (15°C, 59°F)
Earth’s Orbital Variations (Milankovitch Cycles)
Precession (23,000 year cycle)
Current Distance of Earth from Sun
January: Earth at 147 million km
July: Earth at 152 million km
Total Solar Irradiance at Top of
Earth’s Atmosphere
7% diff in TSI between Jan/July
Data from SORCE/TIM Mission:
http://lasp.colorado.edu/sorce
Jan
July
Earth’s Orbital Variations (Milankovitch Cycles)Eccentricity (100,000 year cycle)
http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm
Obliquity (41,000 year cycle)
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