Geology 111G 7 December 2006

Lecture 26. Global Change

Climate System

Greenhouse Effect

Climate Change

Societal Impacts

I. Climate: Average conditions of the surface environment, particularly temperature, and their variation during cycles driven by changes in the amount of solar energy received by the earth. The climate system consists of several components

A. Atmosphere: consists of a variety of gases

1. Nitrogen (N₂) 78%

2. Oxygen (O₂) 21%

3. Argon (Ar) 0.93%

4. Carbon Dioxide (CO₂) 0.035%

5. Other minor gases (Water vapor, methane, etc) 0.035%

6. CO₂, H₂0 and methane (CH₄) are “greenhouse gases,” which absorb and re-radiate infrared (thermal) radiation, thereby insulating the Earth and raising its surface temperature.

B. Hydrosphere: The liquid water on Earth’s surface. Only 1% of water is in lakes, rivers, and groundwater; the rest is in the ocean.

1. Water in ocean currents moves slowly by comparison with atmospheric circulation, but high heat capacity (ability to store heat) of water causes these currents to transport heat efficiently. An important phenomenon is transfer of heat from tropics to polar regions (i.e., Gulf Stream, which brings water from the Caribbean Sea up the western Atlantic to Europe and the north Atlantic Ocean)

C. Cryosphere: ice component of system.

1. Ice reflects almost all solar energy that strikes it.

2. Large volume exchange with hydrosphere results during expansion and contraction of Earth’s ice sheets and glaciers.

a. During last glacial maximum at 18,000 years ago, sea level was ~130 m lower than today.

b. The potential sea level change that can result from melting of global ice is about 200 m

D. Lithosphere: most important part to climate is the land surface, whose composition affects how solar energy is absorbed or reflected.

1. Plate dynamics affect climate through:

a. Topography due to mountain building

b. Arrangement of continents, which affects ocean currrents

c. Volcanism, which affects composition of atmosphere

II. Greenhouse Effect: Greenhouse gases absorb infrared energy (heat) and re-radiate it back toward earth, thereby trapping some heat that would normally be lost through radiation out of atmosphere

A. Albedo: reflectance of land surface. Earth reflects about 31% of incident solar energy.

1. Mean annual surface temperature of the earth: 14^o C (57^o F)

2. Without greenhouse gases it would be: -19^o C (-2^o F)

B. Feedbacks. Changes in components of the climate system cause changes in other components that can either amplify or dampen changes in the first component.

1. Positive Feedback: change in a system component is enhanced by changes it induces in other components. Such feedbacks amplify changes in the system.

a. Temperature increase increases % H₂0 vapor

b. Temperature increase decreases Earth’s albedo by reducing snow and ice cover

2. Negative Feedback: change in a system component is reduced by changes it induces in other components. Such feedbacks stabilize system against further change.

a. A rise in atmospheric temperature increases the amount of infrared energy radiated back into space, which reduces temperature rise. This is termed “radiation damping.”

III. Climate Change: long-term variations in Earth’s surface and atmospheric temperatures are predicted from models that assign values to the components of the climate system; past variations in atmospheric temperature can be understood by reference to geologic “proxies.”

A. Prediction: Predicting long-term climate requires that models of general atmospheric circulation, which are used in short-term weather forecasting, properly address long-term changes in oceanic circulation and changes in greenhouse gases.

B. The surface temperature is driven by amount of solar energy received by the earth, which varies as a result of changes in the earth’s orbit, the tilt of its axis, and wobble of the rotational axis. These orbital parameters change on a regular period of years.

1. For example, orbital eccentricity, or how much Earth’s orbit varies from a circular path, determines which hemisphere receives the most energy at perihelion (nearest approach of Earth to Sun), and thereby alters contrast between summer and winter. Changes in eccentricity take place on a regular period of 100,000 years. Distribution of the continents in the hemispheres influences Earth’s albedo.

C. Understanding Natural Climate Variability: Stratigraphic approaches give us an appreciation for magnitudes and rates of changes that have happened in the past.

1. A geologic proxy is a measurable quantity that serves as a substitute indicator for a value that we cannot measure. Example: the ratio of carbon 18/carbon 16 serves as a proxy for past atmospheric temperature. Lower ratios indicate cooler temperatures.

2. Ice Core data: Long-term change can be assessed from continuous ice cores from Greenland and Antarctica, which record annual cycles of ice formation from snow.

a. Cycles can be counted back like tree rings to create a long-term record of atmospheric temperature.

b. Ice layers trap atmospheric gas, contain isotopic record of temperature change. CO2 and the O18/O16 ratio in the H2O of the ice can be measured directly.

3. Short-term Variation: Evidence from cores of lakes, ice sheets, and sea-floor sediment indicates that temperatures do not vary smoothly through time.

a. During last glacial epoch, short-term (10³ yr) temperature oscillations were superimposed on longer (10⁵ yr) ones

b. Warming trends are jerky and abrupt, taking place in 10-30 years (at 14.5 and 11.5 Ka). Indicates that atmospheric circulation can flip the climate system from glacial to interglacial very quickly.

IV. Societal Impacts. This is why there is such debate about the science

A. Sea-level rise could displace population centers. Melting of continental ice sheets increases volume of ocean water, results in rise in elevation of ocean surface. IPCC (United Nations Intergovernmental Panel on Climate Change) estimates that sea level rose 120 mm (just over 4 inches) during the 20^th Century. Some models predict 1 m of rise during 21^st Century.

B. Rising surface temperatures could shift patterns of agriculture.

C. Increased severity of storms could increase damage/casualties.