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)