University of Exeter › aosta › LecturesSeminars › Lenton_1.pdf · 2016-07-05 · Glaciation Plate tectonics Soil erosion Abiotic processes. 1s 1h 1yr 103yr 10 6yr 10 9yr Timescale

Origins of a habitable planet

Tim Lenton University of Exeter

How has Earth remained habitable?

• Pure luck: Life on Earth has survived by chance alone

• Lucky Gaia: Earth regulates in a habitable state by chance

• Probable Gaia: The presence of life makes regulation in a habitable state a more probable outcome

Habitable extra-solar planets

Silicate weathering feedback

CO2Silicate

WeatheringRate

Global temperature

-

Walker, Hays and Kasting (1981)

+

+

Solarluminosity

+

How do you detect life on a planet?

Free energy

Matter Low internalentropy

Life

High entropywaste products

Environment

E. Schrodinger (1944) What is life?


Free energy


Life


Thermodynamic Disequilibrium

Entropy/Free energyGradient

Environment

J. E. Lovelock (1965) Nature 207: 568-570E. Schrodinger (1944) What is life?


Free energy


Life


Thermodynamic Disequilibrium

Entropy/Free energyGradient

Environment

Observation

J. E. Lovelock (1965) Nature 207: 568-570E. Schrodinger (1944) What is life?

Atmospheric compositions

10-1

10-2

10-3

10-4

10-5

10-6

10-7 0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

Earth Mars Venus

Mix

ing

Rat

io

HydrogenMethaneNitrogenNitrous OxideCarbon MonoxideCarbon DioxideOxygen

Fluxes of gases

0.01

0.1

1

10

100

1000

10000

Earth Without Life

Surfa

ce F

lux

(1012

mol

es y

r-1)

HydrogenMethaneIsopreneDimethyl SulphideAmmoniaNitrogenNitrous OxideCarbon MonoxideCarbon DioxideOxygen

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

-4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Time (Gyr from present)

Sola

r lu

min

osity

(nor

mal

ised

)The faint young Sun puzzle

A. I. Boothroyd (1992)Photo credit: ESA-NASA

Formula for luminosity:

S(t) = S0 (1 – 0.38t/τ0)-1

S0 = 1368 W m-2

Time t expressed in Gyr from the present- 4.55 < t < 4.77 Gyrτ0 = 4.55 Gyr

Environmental regulation?


Organisms contribute to self-regulating feedback mechanisms that have kept the surface of the Earth habitable for life.“Symbiosis as seen from space”.


Organisms contribute to self-regulating feedback mechanisms that have kept the surface of the Earth habitable for life.“Symbiosis as seen from space”.

The Earth is not a unit of selection! Why should the organisms that leave the most descendants be ones that contribute to regulating their planetary environment?

Origin of the Earth (4.6 Ga)

The Hadean (4.6-3.8 Ga)

The Archean (3.8-2.5 Ga)

Origin of life (3.8-3.3 Ga)

3.5 Ga stromatolite from S. Africa

Dividing cells (3.26 Ga)

Thanks to Andy Knoll (Harvard) for this photograph

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m


Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Oceancirculation

Rockweathering

Glaciation Platetectonics

Soilerosion

Abiotic processes


Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Feedbacks


Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Oceancirculation

Rockweathering



Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Selection mechanisms

Emergence of nutrient recyclingThe ‘Flask’ model

Williams & Lenton (2007) Oikos 116: 1087-1105


Nutrient input

Nutrient output



Nutrient input

Nutrient output

Abiotic variables



Nutrient input

Nutrient output

Seeded with clonal population of microbes

Abiotic variables



Nutrient input

Nutrient output

Population diversifies

Abiotic variables

Rec

yclin

g R

atio

Time



Nutrient input

Nutrient output

Recycling population expands

Abiotic variables

Rec

yclin

g R

atio

Po

pula

tion

Time



Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Groupselection

Oceancirculation

Rockweathering



Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle



Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Groupselection

Oceancirculation

Rockweathering



Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction

Ecosystemengineering


Communityassembly

Emergence of environmental regulation

Spatial system of ‘flasks’ connected in a ring

Measure the ‘Error ’ = Mismatch between the state of the abiotic environment and the organisms’ preference

Williams & Lenton (2008) PNAS 105(30), 10432-10437




Envi

ronm

enta

l Err

or

Time





Envi

ronm

enta

l Err

or

Time

Mea

n Er

ror

Mixing rate (log scale)

Extin

ctio

ns


Mechanism of regulation


Mechanism of regulation

Net transferof organisms

Environment-improvingecosystem

Environment-degrading ecosystem

Large population Small population


Spatial structure and time delays• Mechanism only works for heterogeneous environmental variables

• Some key variables are well mixed e.g. O2, CO2, therefore the world is a single ‘flask’ and a different mechanism of regulation is needed

• Time delays due to long residence times of geochemical reservoirs can disable negative feedback and promote instability

Regulated variable Timescale Mechanism

Marine Nutrients 104 yr ‘Biotic plunder’ (R* Tilman 1982, Tyrell 2004)

Temperature 106 yr Silicate weathering with biotic enhancement(local competition for nutrients)

Atmospheric Oxygen

107 yr Biota overproduces, fire/toxicity upper limit

Could life wipe itself out?• Detrimental effects of life on the environment should

become self-limiting before they render the planet uninhabitable

• But in ‘Flaskworld’ sometimes a new life form drives everyone extinct (an ‘over-virulent parasite’).

• On Earth, if a geophysical positive feedback regime is entered, this could conceivably cause disaster:– e.g. ‘Snowball Earth’

Ice-albedo feedback

Absorptionof sunlight

Ice and snow cover

Global temperature

-

Budyko (1968), Sellers (1969)

+Solar

luminosity

+

-

Gain = 0.12 (0.03-0.21) about present state

but Gain → 1 when ice-line reaches ~30° latitude

5 4022.5Local temperature (C)

Gro

wth

rate

Effect of environment on life (temperature on daisy growth)

5

40

22.5

Tem

pera

ture

(C)

Areal coverage

Effect of life on environment (black daisies on temperature)

5 4022.5

Temperature (C)

Life

(are

al c

over

age)

Life

Temp.

+

+

Life

Temp.

-+

Positive feedbackregime

Negative feedbackregime


Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Sequentialselection

Individualselection

Groupselection

Oceancirculation

Rockweathering



Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction



Communityassembly

Sequential selection

• “I imagine that “learning” through repetitions over time alone in a sufficiently complex system has to be shown able to replace the currently understood (and I am sure much more powerful) “learning” through repetitions over both time and space that is natural selection as we know it”

W. D. Hamilton (letter to J. Lovelock 19/1/1997)

Appeared in Gaia Circular (2007)

and as Hadley Centre Technical Note 77 (2008)http://www.metoffice.gov.uk/media/pdf/9/l/HCTN_77.pdf

Sequential selection for the Earth

Effect on environment

Stability?

Evolutionary innovation

Startlife

Persistence

Yes

Environment

Life

E→L L→E

Sequential selection for the Earth

Eliminate destabilising

effects

ResetEvolutionary innovation

Effect on environment

Stability?

Startlife

No

Approach bounds of habitability

e.g. Snowball Earth

Persistence

Yes

Environment

Life

E→L L→E

W. Ross Ashby’s Ultrastability

W. R. Ashby (1952) Design for a Brain

The Homeostat(1948)

Evolutionary regime shifts

Williams & Lenton (2010) Oikos

Population

Nutrients

Recycling

Environment

Earth history

Hadean Archean Proterozoic Phan.

4 3 2 1 0Gyr ago

Earth history• There have been a series of habitable states


4 3 2 1 0Gyr ago

Life

Environment

Originof life

Oxygenicphotosynthesis

AnimalsEukaryotes Us

No O2 Low O2 Mid O2 High O2

Earth history• There have been a series of habitable states• Separated by extreme environmental changes


4 3 2 1 0Gyr ago

Life

Environment

Originof life

Greatoxidation


SnowballEarth


???No O2 Low O2 Mid O2 High O2

Origin ofrecycling

Earth history• There have been a series of habitable states• Separated by extreme environmental changes• Driven by co-evolution of life and the planet


4 3 2 1 0Gyr ago

Life

Environment

Originof life

Greatoxidation


SnowballEarth


???No O2 Low O2 Mid O2 High O2

Origin ofrecycling

The remaining puzzle? Progress!

• Biosphere free energy capture increases in steps– New forms of photosynthesis, land colonisation– Explicable in terms of natural selection?

• Biological complexity progressively increases– Just a random walk bounded by zero on one side???

• Earth system ‘master variables’ have a trend over time and appear to get more tightly regulated (?)– e.g. atmospheric oxygen (O2) and CO2…

Oxygen over Earth history

PAL = Present Atmospheric Level

Lenton (2016) Earth System Science: A Very Short Introduction (OUP)


Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Observerself-selection

Sequential selection

Individualselection

Groupselection

Oceancirculation

Rockweathering



Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction



Communityassembly

Observer self-selection• The explanation of last resort • The history of Earth that we

see has to be consistent with our existence as conscious observers– Rise in oxygen to ~1 PAL– Increasing biological

complexity without elimination (e.g. stem group animals in Snowball Earth)

– Tight regulation of O2, CO2, T...

• Invoke observer self-selection to explain apparent progress

0

0.05

0.1

0.15

0.2

0.25

0.3

1 2 3 4 5 6 7 8 9 10

Biospherestableenoughforcomplexlife

Numberofstabilisingfeedbacks(n)Probability(n)

Observer self-selection for the tail of a distribution?

Watson (1999) GSL Special PublicationWatson (2008) Astrobiology 8: 175-185

Welcome to Teleological Gaia!

Lenton ‘Earth system science: A very short introduction’ (OUP, 2016)

Thank you

Stuart Daines

Peter Cox

Rich Boyle Colin Goldblatt Andy Watson

Jim Lovelock Noam Bergman

Ben Mills

Documents

University of Exeter › aosta › LecturesSeminars › Lenton_1.pdf · 2016-07-05 · Glaciation Plate tectonics Soil erosion Abiotic processes. 1s 1h 1yr 103yr 10 6yr 10 9yr Timescale