MET 10 Global Climate Change-Chapter 14 Global Climate Change Dr. Craig Clements San José State University

Review: Why is CO 2 So Important?  Carbon Dioxide is a greenhouse gas.  Greenhouse gases are those gases that cause the greenhouse effect.  The greenhouse effect makes a planet’s surface temperature warmer than it would otherwise be.  The stronger the greenhouse effect, the warmer the surface (other factors being equal).  Consider the blanket analogy

Earth’s Energy Balance  Energy entering top of atmosphere  Energy entering the Earth’s surface = Energy leaving top of atmosphere = Energy leaving Earth’s surface Conservation of Energy

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiation  Each ‘beam’ of incoming sunlight can be either: –Reflected back to space:  Clouds  Atmosphere  Surface –Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface. Albedo

Longwave radiation is emitted from surface. Some surface radiation escapes to space Most outgoing longwave is absorbed in atmosphere (by greenhouse gases) Greenhouse gases emit longwave upward and downward Some atmospheric radiation escapes to space Some atmospheric radiation is absorbed at the surface

Greenhouse Effect Sequence of steps: 1.Solar radiation absorbed by earth’s surface. 2.Earth gives off infrared radiation. 3.Greenhouse gases absorb some of the Earth’s infrared radiation. 4.Greenhouse gases (water and CO 2 ) give off infrared radiation in all directions. 5.Earth absorbs downward directed infrared radiation Result: warmer surface temperature

Energy Balance  Assume that the Earth’s surface is in thermodynamic equilibrium:  Thermodynamic Equilibrium: –The flow of energy away the surface equals the flow of energy toward the surface Surface Average surface temperature = 15°C

Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface. Sudden Removal of all Greenhouse Gases

Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface. Thus, average surface temperature starts to decrease. Sudden Removal of all Greenhouse Gases

As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops Sudden Removal of all Greenhouse Gases

As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops and equilibrium is restored. Average surface temperature = -18°C Result: A Very Cold Planet!

Earth’s Greenhouse Effect  Without the greenhouse effect, the surface temperature of Earth would be –Way Cold (-18°C)  Greenhouse gases play an important role in shaping climate. –More GHGs – warmer climate –Less GHGs – cooler climate

Recent Climate Change

Modeled temperature changes

IPCC (2007)

(b) Additionally, the year by year (blue curve) and 50 year average (black curve) variations of the average surface temperature of the Northern Hemisphere for the past 1000 years have been reconstructed from “proxy” data calibrated against thermometer data (see list of the main proxy data in the diagram). The 95% confidence range in the annual data is represented by the grey region. These uncertainties increase in more distant times and are always much larger than in the instrumental record due to the use of relatively sparse proxy data. Nevertheless the rate and duration of warming of the 20th century has been much greater than in any of the previous nine centuries. Similarly, it is likely 7 that the 1990s have been the warmest decade and 1998 the warmest year of the millennium. 7

Latest global temperatures

…“Over the last 140 years, the best estimate is that the global average surface temperature has increased by 0.6 ± 0.2°C” (IPCC 2001)  So the temperature trend is: 0.6°C ± 0.2°C  What does this mean?  Temperature trend is between 0.8°C and 0.4°C  The Uncertainty (± 0.2°C ) is critical component to the observed trend

CO 2 Concentration in Atmosphere

Short Term Carbon Cycle  One example of the short term carbon cycle involves plants  Photosynthesis: is the conversion of carbon dioxide and water into a sugar called glucose (carbohydrate) using sunlight energy. Oxygen is produced as a waste product.  Plants require  Sunlight, water and carbon, (from CO 2 in atmosphere or ocean) to produce carbohydrates (food) to grow.  When plants decay, carbon is mostly returned to the atmosphere (respiration)  During spring: (more photosynthesis)  atmospheric CO 2 levels go down (slightly)  During fall: (more respiration)  atmospheric CO 2 levels go up (slightly)

Current CO 2 : ~387 ppm

What Changed Around 1800?  Industrial Revolution –Increased burning of fossil fuels  Also, extensive changes in land use began –the clearing and removal of forests

Burning of Fossil Fuels  Fossil Fuels: Fuels obtained from the earth are part of the buried organic carbon “reservoir” –Examples: Coal, petroleum products, natural gas  The burning of fossil fuels is essentially –A large acceleration of the oxidation of buried organic carbon

Land-Use Changes  Deforestation: –The intentional clearing of forests for farmland and habitation  This process is essentially an acceleration of one part of the short-term carbon cycle: –the decay of dead vegetation  Also causes change in surface albedo (generally cooling)

Climate Feedbacks

 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. Earth’s Climate Stable 1 2 3

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

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

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

Climate Stability  Internal Forcing mechanisms - processes that are internal to the climate system that involve the various elements: ice, water vapor, CO2  External Forcing mechanisms - some forcing that can alter the system without itself being altered. - solar variability, axis wobble, etc.

Climate Feedbacks  Positive feedback: –initial change reinforced by another process. –Trends towards instability  Negative feedback: –initial change counteracted by another process. –Trends towards stability

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

Fill in the blanks Earth Warms Ice Coverage ___________ Albedo _____________ Absorption of Sunlight _______ Initiating Mechanism 1.increases, decreases, decreases 2.Decreases, decreases, increases 3.Increases, increases, increases 4.Decreases, decreases, decreases

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

Fill in the blanks 1.Increases, increases, increases 2.Increases, decreases, decreases 3.Decreases, increases, increases 4.Decreases, decreases, decreases

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

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

Negative Feedbacks  Processes that reduces an imposed change - Trends towards stability  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

Which feedback is positive? 1.Left 2.Right