Notes

Outline

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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 is the largest interannual (year-to-year) climate variation signal in the coupled atmosphere-ocean system that has profound impacts on global climate.

ENSO-Related Research

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El Nino and Southern Oscillation

Pioneers in Modern Meteorology & Climatology

Coupled Atmosphere-Ocean System

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1997-98 El Nino

1982-83 El Nino

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Delayed Oscillator: Wind Forcing The delayed oscillator suggested that oceanic Rossby and Kevin waves forced by atmospheric wind stress in the central Pacific provide the phase-transition mechanism (I.e. memory) for the ENSO cycle. The propagation and reflection of waves, together with local air-sea coupling,  determine the period of the cycle.

Wave Propagation and Reflection

Why Only Pacific Has ENSO? Based on the delayed oscillator theory of ENSO, the ocean basin has to be big enough to produce the “delayed” from ocean wave propagation and reflection. It can be shown that only the Pacific Ocean is “big” (wide) enough to produce such delayed for the ENSO cycle. It is generally believed that the Atlantic Ocean may produce ENSO-like oscillation if external forcing are applied to the Atlantic Ocean. The Indian Ocean is considered too small to produce ENSO.

ENSO Simulation by ESS CGCM

Decadal Changes of ENSO

Decadal Changes in ENSO Period

Changing Role of Indian Ocean in the Tropical Climate System

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 link to the decadal variations of ENSO intensity.

ENSO and PDO

PDO Index

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Subduction

How El Nino Changes When Climate Warms?

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.

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.

Tectonic-Scale Climate Change The faint young Sun paradox and its possible explanation. Why was Earth ice-free even at the poles 100 Myr ago (the Mesozoic Era)? What are the causes and climate effects of changes in sea level through time? What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?

Faint Young Sun Paradox Solar luminosity was much weaker (~30%) in the early part of  Earth’s history (a faint young Sun). If Earth’s albedo and greenhouse effect remained unchanged at that time, Earth’s mean surface temperature would be well below the freezing point of water during a large portion of its 4.5 Byr history. That would result in a “snowball” Earth, which was not evident in geologic record.

Three Possible Solutions Solution 1: Additional heat sources must have been presented       Unlikely: The geothermal heat from the early Earth is sometimes suggested one such additional heat source to warm Earth. However, the geothermal heat flux is not big enough to supply the required energy. Solution 2: The planetary albedo must have been lower in the past       Unlikely: It would require a zero albedo to keep the present-day surface temperature with the 30% weaker solar luminosity in the early Earth. Solution 3: Greenhouse effect must have been larger       Most Likely: The most likely solution to the faint young  Sun paradox is that Earth’s greenhouse effect was larger in the past.       But (1) why and (2) why that stronger greenhouse effect reduced to the present-day strength?

Chemical Weathering The precipitation process in the atmosphere dissolve and remove CO2 from the atmosphere. Rocks exposed at Earth’s surface undergo chemical attack from this rain of dilute acid. This whole process is known as chemical weathering. The rate of chemical weathering tend to increase as temperature increases. Weathering requires water as a medium both for the dissolution of minerals and for the transport of the dissolved materials to the ocean       è The rate of chemical weathering increases as precipitation increases.

Negative Feedback From Chemical Weathering The chemical weathering works as a negative feedback that moderates long-term climate change. This negative feedback mechanism links CO2 level in the atmosphere to the temperature and precipitation of the atmosphere. A warm and moist climate produces stronger chemical weathering to remove CO2 out of the atmosphere è smaller greenhouse effect and colder climate.

Earth’s Thermostat – Chemical Weathering Chemical weathering acts as Earth’s thermostat  and regulate its long-term climate. This thermostat mechanism lies in two facts:       (1) the average global rate of chemical weathering depends on the state of Earth’s climate,       (2) weathering also has the capacity to alter that state by regulating the rate which CO2 is removed from the atmosphere.

Plate Tectonics and Climate How can one account for the alternating periods of climatic warmth and coolness observed in the geologic record?              è Part of the answer must lie in the tectonic activity and the positions of  the continents.

Circulation of the Solid Earth

Twenty Rigid Plates What can happen to the cold boundary? The lithosphere has broken into a number of rocky pieces, called plates. There are a few large plates plus a number of smaller one comprise the Earth’s surface (a total of 20 plates). The plates range from several hundred to several thousand kilometers in width.

Three Ways for Solid Earth to Affect Climate  Polar position hypothesis  Chemical Weathering Hypothesis  Seafloor Spreading Hypothesis

The Polar Position Hypothesis The polar position hypothesis focused on latitudinal position as a cause of glaciation of continents. This hypothesis suggested that ice sheets should appear on continents when they are located at polar or near-polar latitudes. To explain the occurrence of icehouse intervals, this hypothesis calls not on worldwide climate changes but simply on the movements of continents on tectonic plates. This hypothesis can not explain the climate of the Late Proterozoic Era, when both  continents and glaciers appear to have been situated at relatively low latitudes. It can not explain the warm Mesozoic Era when high-latitude continents were present but were almost completely ice-free.

Climate Changes in the Last 500 Myr Climate in the past 500 million years have alternated between long periods of warm climate and short periods of cold climate. During the last 500 million years, major continent-size ice sheets existed on Earth during three icehouse ear: (1) a brief interval near 430 Myr ago, (2) a much longer interval from 325 to 240 Myr ago, and (3) the current icehouse era of the last 35 million year.

Tectonic Control of CO2 Input – The Seafloor Spreading Rate Hypothesis During active plate tectonic processes, carbon cycles constantly between Earth’s interior and its surface. The carbon moves from deep rock reservoirs to the surface mainly as CO2 gas associated with volcanic activity along the margins of Earth’s tectonic plates. The centerpiece of the seafloor spreading hypothesis is the concept that changes in the rate of seafloor spreading over millions of years control the rate of delivery of CO2 to the atmosphere from the large rock reservoir of carbon, with the resulting changes in atmospheric CO2 concentrations controlling Earth’s climate.

Negative Feedback in Seafloor Spreading Hypothesis The seafloor spreading hypothesis invokes chemical weathering as a negative feedback that partially counters the changes in atmospheric CO2 and global climate driven by changes in rates of seafloor spreading.

Tectonic Control of CO2 Removal – The Uplift Weathering Hypothesis The uplifting weathering hypothesis asserts that the global mean rate of chemical weathering is heavily affected by the availability of fresh rock and mineral surfaces that the weathering process can attack. This hypothesis suggests that tectonic uplifting enhances the exposure of freshly fragmented rock which is an important factor in the intensity of chemical weathering. This hypothesis looks at chemical weathering as the active driver of climate change, rather than as a negative feedback that moderates climate changes.

Can These Two Hypotheses Explain Tectonic-Scale Climate Changes?

Orbital-Scale Climate Change Changes in solar heating driven by changes in Earth’s orbit are the major cause of cyclic climate changes over time scales of tens to hundreds of thousands of years (23k years, 41k years, and 100k years) . Earth’s orbit and its cyclic variations: tilt variations, eccentricity variations, and precession of the orbit. How do orbital variations drive the strength of tropical monsoons? How do orbital variations control the size of northern hemisphere ice sheets? What controls orbital-scale fluctuations of atmospheric greenhouse gases? What is the origin of the 100,000-year climate cycle of the last 0.9 Myr (ice sheets melt rapidly every 100,000 years)?

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.

Seasonal Insolation Changes The 23,000-year cycle of precissional change dominants the insolation changes at low and middle latitudes. The 41,000-year cycle of tilt change dominants the insolation changes at higher latitudes. Eccentricity changes (the 1000,000 or 413,000-year cycles) is not a significant influence on seasonal insolation chanes.

Insolation Control of Monsoons Monsoon circulations exit on Earth because the land responds to seasonal changes in solar radiation more quickly than does the ocean. Changes in insolation over orbital time scales have driven major changes in the strength of the summer monsoons. Changes of 12% in the amount of insolation received at low latitudes have caused large changes in heating of tropical landmass and in the strength of summer monsoons at a cycle near 23,000  years in length.

Orbital-Scale Changes in Methane The Vostok ice record shows a series of cyclic variations in methane concentration, ranging between 350 to 700 ppb (part per billion). Each Ch4 cycle takes about 23,000 years. This cycle length points to a likely connection with changes in orbital procession. The orbital procession dominates insolation changes at lower latitudes.

Insolation Control of Ice Sheets Ice sheets reacted strongly to insolation changes. Summer insolation control the size of ice sheet by fixing the rate of ice melting.

Evidence of Ice Sheet Evolution This figures shows a North Atlantic Ocean sediment core holds a 3 Myr d18O record of ice volume and deep-water temperature changes. There were no major ice sheets before 2.75 Myr ago. After that, small ice sheets grew and melted at cycles of 41,000 and 23,000 years until 0.9 Myr ago. After 0.9 Myr ago, large ice sheet grew and melted at a cycle of 100,000 years.

Conceptual Phases of Ice Sheet Evolution