(BIO 386) Lec 18 (17) - Carbon cycle

The carbon cycle

Changes in CO2 due to imbalances in these processes are slow

• Inputs to atmosphere

• Outputs from atmosphere

• Upshot: Imbalance an take millions of year to affect atmospheric CO2

Slow changes in CO2: Time scale reflects differences in rate or periodicity of disturbances

1) Very long-term processes

• Geological factors

  • Volcanism

  • Weathering

• Evolution of plants

2) Medium-term processes

• Changes in solar radiation

• Ocean/atmosphere interactions

Weathering as a global thermostat

When Earth is warmer:

• Weathering increases

• CO2 declines

• Earth cools

When Earth is cooler:

• Weathering decreases

• CO2 increases

• Earth warms

NEGATIVE FEEDBACK LOOP: holds temp in a certain range

Change in solar luminosity _______ with temperature

increases

Counteracting solar intensity, CO2 concentrations have ______ over earth’s history

fallen

Living processes occur _______ than geologic processes

100x

Changes in CO2 could ________ response by increasing surface temperature

amplify

• Changes would be global because CO2 stays in atmosphere for much longer

CO2 feedback

Possible oceanic mechanisms:

1) Temperature effect on bicarbonate system

2) Higher export of production to depth

Possible terrestrial mechanisms:

1) Temperature effects on plant biomass

2) Sequestration or release of soil C

Solubility of CO2 (and other gasses) _________ with increasing temp

decreases

• If phytoplankton do not use CO2, warming of water causes CO2 to leave water and enter atmosphere

  • This can enhance warming, result to more CO2 release from oceans

  • Cooling does the opposite

  • Occurs near ITCZ

What happens to CO2 in ocean during warming?

• Warmer conditions result in less absorption of CO2 by sinking water

• The water that upwells to the surface also looses more CO2

What happens to CO2 in the ocean during cooling?

Cooler conditions result in more absorption of CO2 by sinking water at poles

• The water that upwells to the surface in tropics looses less CO2 because it warms less

• This results in a net movement of CO2 from the atmosphere to the ocean

Reestablishing equilibrium after warming/cooling slows down takes about…

200-1000 years

The effects on the carbon cycle in the future are complex and uncertain because

• In the past, non-human processes initiated the change

• Now humans are altering the carbon cycle directly in many ways

Not all of CO2 emitted stays in the

atmosphere

Burning of biomass causes a release of

CO2

Main causes of loss of vegetation and global C cycle

• human-set fires

• shifting fire regimes

Net effect of fire intensity can only be…

as large as the pool

What is the expansion of woody vegetation through succession

Caused by:

• Succession and abandonment of farmland in temperate zones

• Northward expansion

Effect is limited by the amount of vegetation that can be supported in a particular environment

Has the opposite effect of fire

Net effects on vegetation

Traditional slash and burn

• At low human population densities - forests have time to regrow

• Eventually frequency of burning is too fast to allow any of forest to attain maximum biomass

Intensive farming and animal agriculture

• Suppresses regrowth of forest in certain locations

• Animal agriculture requires much more land in rangeland or grassland state

• Spatial changes in vegetation carbon results from changing centers of agriculture

What will happen to soil C as temperature and CO2 increase

Could go up

• Temperature allows range expansion

• CO2 fertilizes plants

• Precipitation allows for better growth

Could go down

• Increase in temperature results in decomp of soil carbon

• Likewise for increasing precipitation

• Fires could burn the accumulated peat with loss of permafrost

The solubility pump8