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Climate Sensitivity and Feedback

Climate Changes Tectonic-Scale Climate Changes Orbital-Scale Climate Changes Deglacial and Millennial Climate Changes Historical Climate Change Anthropogenic Climate Changes

Tectonic Scale Tectonic Scale: the longest time scale of climate change on Earth, which encompasses most of Earth’s 4.55-billion years of history. Tectonic processes driven by Earth’s internal heat alter Earth’s geography and affect climate over intervals of millions of years. On this time scale, Earth’s climate has oscillated between times when ice sheets were presented somewhere on Earth (such as today) and times when no ice sheets were presented.

Orbital Scale Orbital-scale climate changes are caused by subtle shifts in Earth’s orbit. Three features of Earth’s orbit around the Sun have changed over time:      (1) the tilt of Earth’s axis,      (2) the shape of its yearly path of revolution around the Sun      (3) the changing positions of the seasons along the path. Orbital-scale climate changes have typical cycles from 20,000 to 400,000 years.

Major Climate Feedback Processes Water Vapor Feedback - Positive Snow/Ice Albedo Feedback - Positive Longwave Radiation Feedback - Negative Vegetation-Climate Feedback - Positive Cloud Feedback - Uncertain

Water Vapor Feedback Mixing Ratio = the dimensionless ratio of the mass of water vapor to the mass of dry air. Saturated Mixing Ratio tells you the maximum amount of water vapor an air parcel can carry. The saturated mixing ratio is a function of air temperature: the warmer the temperature the larger the saturated mixing ration.      è a warmer atmosphere can carry more water vapor      è stronger greenhouse effect      è amplify the initial warming      è one of the most powerful positive feedback

Snow/Ice Albedo Feedback The snow/ice albedo feedback is associated with the higher albedo of ice and snow than all other surface covering. This positive feedback has often been offered as one possible explanation for how the very different conditions of the ice ages could have been maintained.

Longwave Radiation Feedback The outgoing longwave radiation emitted by the Earth depends on surface temperature, due to the Stefan-Boltzmann Law: F = s(Ts)4.       è warmer the global temperature       è larger outgoing longwave radiation been emitted by the Earth       è reduces net energy heating to the Earth system       è cools down the global temperature       è a negative feedback

Vegetation-Climate Feedbacks

Cloud Feedback Clouds affect both solar radiation and terrestrial (longwave) radiation. Typically, clouds increase albedo è a cooling effect (negative feedback)      clouds reduce outgoing longwave radiation è a heating effect (positive feedback) The net effect of clouds on climate depends cloud types and their optical properties, the insolation, and the characteristics of the underlying surface. In general, high clouds tend to produce a heating (positive) feedback. Low clouds tend to produce a cooling (negative) feedback.

El Nino-Southern Oscillation (ENSO) ENSO is a interannual (year-to-year) climate variability in the eastern tropical Pacific Ocean. ENSO is found to have profound impacts on global climate.

1982-83 El Nino

1997-98 El Nino

El Nino Comparisons

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Decadal Changes of ENSO

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ENSO and PDO

Pacific Decadal Oscillation “Pacific Decadal Oscillation" (PDO) is a decadal-scale climate variability that describe an oscillation in northern Pacific sea surface temperatures (SSTs). PDO is found to affect  Alaska salmon production cycles. PDO is found to link to the decadal variations of ENSO intensity.

Decadal Variations (PDO Index)

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North Atlantic Oscillation The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia. The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low. The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

Positive and Negative Phases of NAO

Dynamics Behind NAO The North Atlantic Oscillation is considered as a natural variability of the atmosphere. However, processes in the ocean and stratosphere and even the anthropogenic activity can affect its amplitude and phase. Surface winds of the NAO can force sea surface temperature variability in the Atlantic Ocean. Feedbacks from the ocean further affect NAO variability.

North Atlantic Oscillation = Arctic Oscillation = Annular Mode

Decadal Timescale of Arctic Oscillation The Arctic Oscillation switches phase irregularly, roughly on a time scale of decades. There has been an unusually warm phase in the last 20 years or so, exceeding anything observed in the last century.