MET 12 Global Climate Change - Lecture 5 Climate Feedbacks Shaun Tanner San Jose State University Outline Stability/instability Feedbacks Examples Activity

That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear. The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

Earth’s Climate The Earth’s climate is fairly stable in terms of temperature This can be visualized using in the following system diagram. The idea is that even though the system may change away from it’s initial point, it will have the tendency to go back to ‘normal’ eventually. 2 3 1 Stable Stable

Stability versus instability Stable: Instability: Stable equilibrium Unstable equilibrium

Stability versus instability Stable: Given a perturbation, the system tends to return to original state Instability: Given a perturbation, the system moves to another state. Stable equilibrium Unstable equilibrium

States of equilibrium The system may have multiple states of equilibrium 2 3 1 Stable to small perturbations, until a big force perturbs the system into a new equilibrium

Climate Stability The Earth’s climate changes as a result of internal/external forcing: Changes in solar radiation Changes in the earth’s orbit Plate tectonics Volcanoes Human pollution etc. These forcings can be thought of as a perturbation (or push) to climate stability. These changes can be enhanced or diminished by positive or negative feedbacks

Feedbacks Positive feedback: Negative feedback:

Climate Feedbacks Positive feedback: initial change reinforced by another process. Negative feedback: initial change counteracted by another process.

Positive Feedbacks Processes that accelerate a change Note: Feedbacks cannot initiate change; they can only alter the pace of change Important climate examples: Ice-albedo feedback Water-vapor feedback Cloud feedback

Ice-Albedo Feedback (Cooling) Initiating Mechanism Earth Cools Ice Coverage Increases Albedo Increases Absorption of Sunlight Decreases

Ice-Albedo Feedback (Cooling) Initiating Mechanism Earth Cools Somehow this happens Ice Coverage Increases Positive Feedback Albedo Increases Absorption of Sunlight Decreases

Fill in the blanks Initiating Mechanism Earth Warms 31 of 31 Initiating Mechanism increases, decreases, decreases Decreases, decreases, increases Increases, increases, increases Decreases, decreases, decreases Earth Warms Ice Coverage ___________ Albedo _____________ Absorption of Sunlight _______

Ice-Albedo Feedback (Warming) Initiating Mechanism Earth Warms Ice Coverage Albedo Absorption of Sunlight

Ice-Albedo Feedback (Warming) Initiating Mechanism Earth Warms Ice Coverage Decreases Positive Feedback Albedo Decreases Absorption of Sunlight Increases

Water Vapor Feedback (Warming) Initiating Mechanism Earth Warms Evaporation Atmospheric Water Vapor Content Greenhouse Effect

Fill in the blanks Increases, increases, increases 33 of 36 Fill in the blanks Increases, increases, increases Increases, decreases, decreases Decreases, increases, increases Decreases, decreases, decreases

Water Vapor Feedback (Warming) Initiating Mechanism Earth Warms Evaporation Increases Positive Feedback Atmospheric Water Vapor Content Increases Greenhouse Effect Strengthens

Water Vapor Feedback (Cooling) Initiating Mechanism Earth Cools Evaporation Atmospheric Water Vapor Content Greenhouse Effect

Water Vapor Feedback (Cooling) Initiating Mechanism Earth Cools Evaporation Decreases Positive Feedback Atmospheric Water Vapor Content Decreases Greenhouse Effect Weakens

Negative Feedbacks Processes that reduces an imposed change Important examples: Cloud feedback Chemical weathering Note: Positive/negative feedbacks have no relation to ‘good versus bad’, but are about how a system responds to a change.

Possible Role of Cloud in Warming or Cooling the Atmosphere Finally in relation to CO2 or greenhouse warming there is the vexatious question of the role of cloud. GO THROUGH To some extent the role of cloud depends on its height in the atmosphere. Low level cloud tends toward warming through long wave trapping, while high level cloud leads to cooling through solar radiation reflection In our final slide today, lets briefly look at surface characteristics.

Which feedback is positive? Left Right 32 of 36

Cloud Feedbacks (positive and negative) Positive Feedback Negative Feedback Finally in relation to CO2 or greenhouse warming there is the vexatious question of the role of cloud. GO THROUGH To some extent the role of cloud depends on its height in the atmosphere. Low level cloud tends toward warming through long wave trapping, while high level cloud leads to cooling through solar radiation reflection In our final slide today, lets briefly look at surface characteristics.

This is a positive feedback. This is a negative feedback. As a result of the earth’s warming over the last 100 years, plants have been growing at a faster rate. Because plants absorb CO2, the increase in plant growth means that more CO2 is being absorbed from the atmosphere by plants. This is a positive feedback. This is a negative feedback. This has nothing to do with feedbacks.

Activity Imagine the Earth was to warm for some reason (initiating mechanism or perturbation) Identify two positive feedbacks that would influence the earth’s climate and explain how each one works. Identify two negative feedbacks that would influence the earth’s climate and explain how each one works. Which feedback do you think is more uncertain. Explain your reasoning.