The Gaia hypothesis revisited: What does it state?

• Three (at least) propositions of the Gaia hypothesis(Kirchner 2002)

  1. Biologically mediated feedbacks contribute to environmental homeostasis

  2. These make the environment more suitable for life

  3. Feedbacks should arise by Darwinian natural selection

Which of these is most relevant?

• The most popular interpretations of Gaia use 1 and 2.
• The idea that life evolves to influence the feedbacks is more difficult
• The concept that life (the biota) mediates physical processes (e.g. climate) is uncontroversial.
• However: Do biotic mediated feedbacks lead to homeostatasis?

Can we ever understand Gaia through modelling?

• The Earth can be thought of as an ecosystem.

• Ecosystems and complexity.

• A complicated system can be described in full (e.g an internal combustion engine is a complicated system)

• A complex system can never be fully understood and predicted.

  • Complicated systems have a design. Each part has a function.

  • Complex system are not designed. Each part may have numerous roles and functions

  • Some interactions within a complex system can be understood. However the whole system is inherently unpredictable.

  • Complex systems show emergent properties that occur as the result of diverse interactions

Is the earth a homeostatic system?

• Homeostasis returns a systen to its initial state when perturbated.
• A simplistic interpretation implies one single negative feedback loop
• A more refined interpretation implies more negative feedbacks than positive feedbacks
• Attempts to predict measure the strength of each process that leads to feedbacks

Anthropogenic climate change

Positive feedbacks in terrestrial ecosystems

• Warmer temperatures increase soil respiration rates, releasing organic carbon stored in soils
• Warmer temperatures increase fire frequency, leading to net replacement of older, larger trees with younger, smaller ones, resulting in net release of carbon from forest biomass
• Higher atmospheric CO2 concentrations may increase drought tolerance in plants, potentially leading to expansion of shrublands into deserts,thus reducing planetary albedo and atmospheric dust concentrations
• Warming leads to replacement of tundra by boreal forest, decreasing planetary albedo
• Warming of soils accelerates methane production more than methane consumption
• Warming of soils accelerates \(N_20\) production rates

-Increased atmospheric CO2concentrations stimulate increased photosynthe-sis, leading to carbon sequestration in biomass (negative feedback).