Raymond T. Pierrehumbert The University of Chicagogeosci.uchicago.edu/~rtp1/outbox/MBI2006.pdf ·...

Mathematical Biology Institute, Columbus 2006

How does biology manage the Climate Commons?

Raymond T. Pierrehumbert

The University of Chicago

1


The basic statement of the problem

• The Earth’s climate is a commons that affects all species

• It is in turn strongly affected by these species

• The managers of this commons have very little foresight, and no realidea of what kind of climate change they will cause, and whether ornot it will be good for them

• What happens if an innovator species changes the climate in a waythat makes the Earth much less habitable for itself?

2


A few possibilities

• Fouling the nest: population grows, carrying capacity goes down, ev-erybody survives (barely), in misery.

• Catastrophe strikes, all die, and Gaia says ”Game over, Try again?”

• Make life worse for yourself, hang on in misery, but make life better forsomebody else (who can’t maintain the ”improved” climate on its own).

• Make life worse for yourself, but brethren emerge who are betteradapted to the altered climate and can maintain it without relying onyour miserable skin-of-the-teeth existence

3


Two examples

• Transition from anaerobic methane greenhouse world to O2/CO2

world

• Effect of land plants on planetary climate

4


Faint young sun and climate regulation

• Sun was 30% dimmer 4 billion years ago

• Gets gradually brighter over time

• With present atmospheric composition, Earth would have been frozenover during most of its history (in fact, would still be frozen!)

• Greenhouse gases must have been higher in the past, and adjustedover time to maintain equable temperatures

• Main players: CO2, CH4,H2O

• Water vapor is a feedback amplifying climate change due to changesin longer-lived GHG’s

5


Silicate weathering and CO2

6

Inorganic C process #5: carbonate metamorphism

(emits CO2, balances loss by silicate weathering)

CaCO3 + SiO2 --> CaSiO3 + CO2 (released by volcanoes)


How the feedback loop works: Walker, Hayes and Kasting, muchelaborated by B.La.G.

• Weathering increases with T or continental runoff

• Continental runoff also tends to increase with T

• CO2 builds up until corresponding T and runoff yield a weathering ratethat balances outgassing rate

• Phanerozoic outgassing rate is now believed to be rather constant.Rate may have varied more in the deeper past.

7


Enter methane

• Per molecule, a stronger GHG than CO2

• Most important source is methanogens (now and in past)

• Methanogens can be primary producers, given H2 as a feedstock

• Could also live off organic CH2O produced by non-oxygenic photo-synthesis on the Early Earth

• Climate still driven by CO2 outgassing. Methanogens intercept CO2

and convert it into CH4

8


More methane, less CO2

• Temperature adjusts until silicate weathering equals CO2 outgassing

• The more methane there is in the atmosphere, the less CO2 neededto maintain this temperature

• This process can even drive CO2 to zero.

9


Oxygen vs. Methane

• Without O2, CH4 is slowly destroyed by photochemical processes andtholin formation.

• In presence of O2, CH4 rapidly oxidizes to CO2 (12 year time con-stant)

• With any plausible methanogen ecosystem, high amounts of CH4 canonly be maintained if the atmosphere is nearly anoxic

• If oxygenation is gradual enough (1 million years or so), CO2 has timeto accumulate and make up for methane loss

• Rapid oxygenation is a climate peril

10


Snowball Earth

0

100

200

300

400

500

600

700

800

200 220 240 260 280 300 320 340

OLRL=1517L=1685L=1854L=2865

Flux

(W

/m2 )

Surface Temperature

H

Sn1

Ice covered Ice Free

Sn2

Sn3

AB

A'

B'

11


220

240

260

280

300

320

1000 1200 1400 1600 1800 2000

Surf

ace

Tem

pera

ture

Solar Constant (W/m2)

prad

= 670mb

220

240

260

280

300

320

350400450500550600

Surf

ace

Tem

pera

ture

Radiating Pressure (mb)

L = 960 W/m2

12


The snowball is important because it introduces a catastrophe into thesystem, making it harder for climate and biology to evolve gradually

towards a mutually agreeable solution.

13


In other words: Finding an optimum by hill-climbing doesn’t work too wellwhen the hill has cliffs!

14


How to recover from Snowball Earth?

• Silicate weathering shuts off because all precip is snow

• CO2 builds up until it gets warm enough to deglaciate.

15


But how much CO2 does it take to get out?

Pierrehumbert, Nature 2004, J. Geophys Res 2005.

16

160

180

200

220

240

260

-90 -60 -30 0 30 60 90

January Ice-masked air Temperature

100ppm400ppm1600ppm12800ppm.1bar.2bar

Tem

pera

ture

(D

egre

es K

)

latitude


And then...

295

300

305

310

315

320

325

-90 -60 -30 0 30 60 90

Jan T (200mb)Jan T (100mb)

Oce

an T

empe

ratu

re (

K)

Latitude

17

Ascona Neoproterozoic Conference: July,2006.

Land temperature becomes very high

18


A freeze-fry cycle

Biology has to cope with:

• 10 million years of deep-freeze with ice-covered ocean

• Followed by 10 million years of 320K surface waters at the tropics, with300K at poles

• Deep ocean critters are sheltered from freeze-fry but what about pho-tosynthesis?

19


• Photosynthesizers poison anaerobic methanogen ecosystem (re-duces competition)

• However, they can cause a methane collapse if they do it too fast

• Freeze-thaw cycle could wipe out cyanobacteria

• Later on, by Neoproterozoic, we also have to worry about how fragilephotosynthetic eukaryotes could survive a snowball

20


Oxygen and snowballs: A rough start

Mak

gany

ene

SturtianMarinoan

21


Banded Iron Formations also indicate a rough start

22


Part II: Land plants and the CO2 world

• High temperature and precipitation increase silicate weathering, andstabilize CO2

• Vascular land plants greatly increase silicate weathering rate, all otherthings being equal, leading to a colder climate.

• No snowball when land plants evolved!

– Because plants start to die off when it gets too cold?

– Because the fully vegetated state is cooler than unvegetated, butstill too warm to permit a snowball?

23


Results without vegetation feedback Donnadieu, Fluteau andPierrehumbert, G3, in review

• Coupled geochemical climate model (weathering,CO2,climate)

• Time slices of paleo-geography over Phanerozoic

• CO2 outgassing rate held constant

• Vegetation cover fixed (conifer forest)

• Breakup of Pangea greatly increases weathering

24

Late Permian Early Triassic

Middle Late Triassic Early Middle Jurassic

Late Cretaceous

Middle CretaceousEarly Cretaceous

A) B)

C) D)

E) F)

G)

DONNADIEU ET AL., 2005

FIGURE 5

Late Permian Early Triassic

Middle Late Triassic Early Middle Jurassic

Late Cretaceous

Middle CretaceousEarly Cretaceous

A) B)

C) D)

E) F)

G)

DONNADIEU ET AL., 2005

FIGURE 6


25


What would vegetation feedback do?

• Dry hot climates would reduce vegetation cover, reduce weathering,get warmer

• But what is happening in the Cretaceous, when predicted CO2 is toolow?

• Interactive vegetation model might reduce Cretaceous vegetation, in-crease CO2

• However, we need to learn a lot more about how vegetation affects soilchemistry and silicate weathering

• ... and how this depends on the ecosystem structure

26


Conclusions and prospects

• Microbial ecology of methanogens and early cyanobacteria is cruciato understanding climate evolution

• What determines the rate of oxygenation? (And when did cyanobac-teria really evolve?)

• How does evolution respond to catastrophes that wipe out innovators?How is an equilibrium achieved between ecosystem and climate?

• Essential to learn how to model vegetation effects on weathering; Cre-taceous CO2 is too low with fixed vegetation effects

• For both methane and land plant problems, what are the prospects forcoupled climate-ecosystems-evolution modeling?

27

Raymond T. Pierrehumbert The University of Chicagogeosci.uchicago.edu/~rtp1/outbox/MBI2006.pdf ·...

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