MET 10 Global Climate Change-Chapter 14

Global Climate ChangeDr. Craig Clements

San José State University

Review: Why is CO2 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 BalanceEarth’s Energy Balance

Energy entering top of atmosphere

Energy entering the Earth’s surface

= Energy leaving top of atmosphere= Energy leaving top of atmosphere

= Energy leaving Earth’s surface= Energy leaving Earth’s surface

Conservation of EnergyConservation of Energy

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiationIncoming 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.

AlbedoAlbedo

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 EffectGreenhouse EffectSequence 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 CO2) 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

SurfaceAverage 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 likely7 that the 1990s have been the warmest decade and 1998 the

warmest year of the millennium.

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

CO2 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 CO2 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 CO2 levels go down (slightly)

During fall: (more respiration) atmospheric CO2 levels go up (slightly)

Current CO2: ~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 Stable

1

2

3

Stability versus instability

Stable equilibrium Unstable 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

Pos

itive

Fee

dbac

k

Fill in the blanks

Earth Warms

Ice Coverage ___________

Albedo _____________

Absorption of Sunlight _______

Initiating Mechanism1. 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

Pos

itive

Fee

dbac

k

Fill in the blanks

1. Increases, increases, increases2. Increases, decreases, decreases3. Decreases, increases, increases4. Decreases, decreases, decreases

Earth Warms

Evaporation

Atmospheric Water Vapor Content

Greenhouse Effect

Initiating Mechanism

Water Vapor Feedback (Warming)

Earth Warms

Evaporation Increases

Atmospheric Water Vapor Content Increases

Greenhouse Effect Strengthens

Initiating Mechanism

Pos

itive

Fee

dbac

k

Water Vapor Feedback (Cooling)

Earth Cools

Evaporation Decreases

Atmospheric Water Vapor Content Decreases

Greenhouse Effect Weakens

Initiating Mechanism

Pos

itive

Fee

dbac

k

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

Figure 12.7 Role of cloud in both warming and cooling the atmosphere.

Increased warming bytrace gases

(including water vapour)

Increased airtemperatures

Increased watervapour

Increased cloudamount

of terrestrial radiationIncreased trappingIncreased reflection

of solar radiation

WarmingCooling

Net warmingor cooling

Which feedback is positive?

Figure 12.7 Role of cloud in both warming and cooling the atmosphere.

Increased warming bytrace gases

(including water vapour)

Increased airtemperatures

Increased watervapour

Increased cloudamount

of terrestrial radiationIncreased trappingIncreased reflection

of solar radiation

WarmingCooling

Net warmingor cooling

1. Left

2. Right