Climate Forcing Factors in Earth History

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List of Nominations List of Nominations

Climate Forcing Factors in Earth History

Eric J. Barron

March 14, 2009



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What is the Nature of the Challenge?



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An Enormous Spectrum of Climate Change and Variability

Rob Rohde; wikipedia



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Rich Spatial Record of Climate Change



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The Concept of “Forcing Factors”

• Complex interaction of the components of the climate system (atmosphere, ocean, land, ice, biota)

• Separate the “interaction” from the “causes” of climate change– External to the climate system as a “forcing”

vs,– Internal dynamics and feedbacks



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Distinguish between External Forcing Factors and Responses or

Feedbacks

• Forcing? Biological innovation that covers the land with plants and alters the energy balance (yes)

• Forcing? redistribution of plants in response to climate that alters the surface energy balance (no)

• Forcing? Injection of massive amounts of carbon dioxide in the formation of flood basalts (yes)

• Forcing? Higher water vapor content in the atmosphere in response to warming (no)



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Organize our Thinking with aGlobal Energy Balance Model

Energy must Balance

So/4 (1-α) = ЄσTp4

So/4 – solar constant divided by 4 since the area of the Earth’s surface is 4x the area of the disk that intercepts sunlight

(1-α) – α is the albedo of the planet

Є – emissivity

Tp – planetary temperature

σ - constant



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First Order Climate Forcing Factors

• Three categories

–Solar input

–Atmospheric opacity

–Albedo



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Not sufficient characterization of forcing

• Add a spatial element – so, for example, the distribution of solar energy matters not just the solar constant

• Add a temporal element – so, for example, we need to add heat capacity

• Combined - Include forcing factors that change the structure of the circulation – heat transport



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Classification of Forcing Factors

• Solar energy at the top of the Atmosphere

• Changes in the Composition of the Atmosphere

• Changes in the solid Earth interface with the ocean and the atmosphere



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First Major Class of Climate Forcing Factors

• The magnitude, character and distribution of solar energy at the top of the atmosphere– Solar evolution and variability– Changes in the Earth’s orbit– Change in the transmission of energy

between the Sun and the Earth



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• Main Sequence Evolution•Hydrogen burned to Helium•Number of particles per unit mass changes•Rebalance of pressure and temperature•T and Solar luminosity increases through time•100 My ago – 0.9% less than today (about 10% per billion years)



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• Other Time Scales of Variability• Mixing perturbations• Decreased luminosity, slow recovery, overshoot• Potential cause of glaciation?• A few million years +• A few % changes in luminosity

•What else on dynamical, thermal and convection time scales?



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• The Solar Cycle• Sunspot record• 11 year • Observed correlations• cosmic ray flux• Potential



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Changes in the Earth’s Orbit modify the Distribution and Amount of Solar Energyat the Top of the Atmosphere



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Differences in Distance: +/- 3 % with current orbitLittle annual difference



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•Changes in the latitudinal distribution in solar input

•21.5 to 24.5o

•Few W/M2 at high latitudes



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Amplitude of Seasonal Cycle

Closest to Sun in N. Summer or Winter

20 W/M2 seasonally



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Potential for changes in dust between the Sun and the Earth

Current increases (?)

Time scale; impact ????



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Second Major Class of Climate Forcing Factors

• Changes in the Composition of the Atmosphere– Selective absorbers (greenhouse gases)

– Aerosols



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List of Nominations List of Nominations Wavelength specific absorption(Selective Absorbers)

Molecule specific characteristicsvibration, rotation modes

Reradiates energy in all directions



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Human Forcing on Short Time Scales

• Human source term that is measured and larger than the increase in the atmospheric reservoir



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Barnola et al. Vostok



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The Vostok core demonstrating the close link between carbon dioxide and temperature

through the last glacial cycle.

From J.M. Barnola et al., Nature 329 (1987): 408-414, p. 410.

CO2 ppm

Temperature from Deuterium isotopes



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Intersection of the Geologic and Biologic Carbon Cycle –Not all

correlations imply the nature of the forcing

• Can temperature (ice age) perturb (lead) the carbon balance– More ice – lower sea level – sediment flux with nutrients

changes ocean productivity and CO2 draw down– More ice – more wind transported dust – more ocean

fertilization – greater CO2 draw down– More wind – more intense circulation – greater upwelling

(nutrients) – greater productivity and CO2 draw down– Cold temperatures of the ocean – increased gas solubility

– deep water carbon dioxide reservoir grows– Cold temperatures and more ice – decreased plant and

soil reservoir for carbon – atmospheric reservoir source declines



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Perturbing the Geologic Carbon Cycle (very long time scales)

• Alter the balance of volcanism (carbon dioxide input) and the rates of weathering (carbon dioxide removal) - yields changes in atmospheric levels – Rate of sea floor spreading and subduction– Sea level – area of continent to weather– Topography and/or silicate exposure – rates of weathering

• Analysis – – mass balance models based on weathering assessments (Sr

isotopes), volcanism, carbonate deposition, sea level etc., – carbonate character– stomatal density



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List of Nominations List of Nominations Potential for Abrupt Changes in Greenhouse Gases

Flood basaltsClathrates (forcing or feedback)



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Dust loading – Stratosphere – alters reflection and absorption

Changes in Stratospheric chemistry

Tropospheric aerosols



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Transmission effectsEl Chichon – 78%Pinatubo – 82%

Period 1-3 yearsCan the forcing be sustained to create a Climate forcing in the geologic record?



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T difference Pinatubo (Robock)

Plate 8. Winter (December-January-February (DJF)) lower tropospheric temperature anomalies (with the nonvolcanic periodof 1984–1990 used to calculate the mean) for the 1991–1992 Northern Hemisphere (NH) winter following the 1991 MountPinatubo eruption. This pattern is typical of that following all large tropical eruptions, with warming over North America,Europe, and Siberia and cooling over Alaska, Greenland, the Middle East, and China. Data are from microwave sounding unitchannel 2R [Spencer et al., 1990], updated courtesy of J. Christy and now called channel 2LT.



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Multiple sources of aerosols

Light blue lowest; purple highestSource: NASA global aerosol project

Can continental configuration yield an aerosol forcing?



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Third Major Class of Climate Forcing Factors

• Changes in the Earth’s Surface

–Distribution of land and sea

–Orography

–Land cover



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Plate Tectonic Forcing Changes Many Factors

• Distribution of Land and Sea– Surface energy balance; heat capacity;

potential for snow cover; mechanisms of heat transport (ocean gateways); position of planetary waves; storm tracks

• Orography– Potential for snow cover, position of planetary

waves; distribution of precipitation; major circulation features such as the monsoons



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Plate Tectonic Forcing changes many components

6 to 12o

TemperatureDifference

Plate tectonicsDirect and CO2



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Sea Level Change

Red Hallam, Blue Exxon



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Orography and Continental Configuration

Single featureOcean gatewaysContinental elevation Low latitude snowIndian Monsoon



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Biologic Innovation – Land Plants

Changes in surface albedoSurface Energy Fluxes



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Rich Spectrum of Forcing Factors

• Solar energy at the top of the Atmosphere

• Changes in the Composition of the Atmosphere

• Changes in the solid Earth interface with the ocean and the atmosphere

How important are these factors in governing climate?



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Definition of Climate Sensitivity

Equilibrium change in global mean temperature in response to a change in

global mean radiative forcing

oC change per W/m2 change in forcing



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Climate Sensitivity with no feedbacks

• The direct temperature effect for a change in forcing– Fix the albedo (ice doesn’t change; clouds don’t

change, vegetation doesn’t change, etc)– Fix the emissivity (no change in water vapor or any

other character of the atmosphere)

• Change the solar insolation by 1 Wm-2

• Complete the calculation = .21oC per 1 Wm-2

• Solar constant 1 % change = 3.4 Wm-2



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Ice Age Climate

• Distribution of temperature data (such as CLIMAP) yields from 3 to 5oC globally average surface temperature difference

• Driving Forcing Factor (orbit driven changes in solar insolation – 1% change – 3.4 Wm-2)

• Ice age record demands that climate be sensitive to external forcing.

• Importantly, a great deal changed in response to the orbit– Atmospheric composition (CO2 and CH4 from ice cores)– Dust levels (from ice cores)– Ice cover (most significant element - CLIMAP)– Water vapor and cloud cover (?)– Vegetation – CLIMAP and others



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Mid-Cretaceous

• Data – warmer by 7o C +/- 2 (Barron et al., 1995)• Carbon Dioxide – 2 to 6 X present day (wide

variety of estimates – Berner)• Assume direct forcing is linear with growth in

carbon dioxide – 4.2 to 12.6 Wm-2

• Use the greatest possible range – Translate to CO2 doubling yields 1.7 to 9 oC – (remember climate models predict 1.5 to 4.5oC)

• But, sun was also weaker ~1%• How important is the geography?



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Sensitivity: Doubling of Carbon Dioxide

• Direct forcing for a doubling would equal 4.2 Watts/m-2

• Using energy balance with no feedbacks would yield less than 1oC (.88)

• Modern Climate Models Predict ~1.5-4.5oC• The key is to be able to determine the nature of

the feedbacks– e.g. Ice-albedo feedback

• The geologic record demands a high level of climate senstivity



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Tool Set

• Characterize the forcing (not all are well known)– Magnitude– Time scale– Spatial character

• Correlation with geologic phenomena (not always indicative in the confusion of forcing vs feedback)



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Tool Set

• Climate Model Application (increasingly prominent – many examples not shown)– Comparison with independent geologic data

becomes crucial



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The Greatest Challenge

• The planet is not the product of a single cause and its effect

• Many forcings are operating simultaneously (consider only the complexity of the ice age forcing)

• A forcing at one time scale can be a feedback at another time scale

• A geologist senses the fully integrated forcing and response of the Earth system

Climate Forcing Factors in Earth History

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