NATS 101-06

Lecture 20 Anthropogenic Climate Change

What is Climate Change?• Climate change - A significant shift in the mean state

and event frequency of the atmosphere.• Climate change has been a normal component of the

Earth’s natural variability. • Climate change occurs over a wide range of time and

space scales.• A plethora of evidence exists that indicates the climate

of the Earth has changed over time.• Humans are radically altering the concentrations of

greenhouse gases in the atmosphere which is likely causing a new type of climate change

Our changing climate: Key Questions

• Climate modelers have predicted the Earth’s surface will warm because of manmade greenhouse gas (GHG) emissions

• So how much of the warming is manmade?• How serious are the problems this is

creating?• What, if anything, can and should we do?

Climate Change: Changing Earth’s Energy Balance

• Changing the – Incoming solar energy– Outgoing energy (IR emitted to space)– Internal transport of energy within the Earth;s

atmosphere and oceans

will cause a shift in climate

Global Energy Balance

Ahrens, Fig. 2.14

IR OutIR Out

In a stable climate, Solar Energy IN = IR Energy OUT

Solar inSolar in

Global Energy Imbalance

Ahrens, Fig. 2.14

AtmosphereAtmosphere

IR OutIR Outis reducedis reduced

Increasing GHG concentrations decrease the IR Energy outSo Energy IN > Energy OUT and the Earth warms

Solar inSolar in

Change in IR Emission to Space

• Notice that because of the greenhouse gases in Earth’s atmosphere, 91% (=64/70) of the IR emitted to space comes from the atmosphere and only 9% (=6/70) comes from Earth’s surface

• When additional GHG’s are added to the atmosphere, the altitude of IR emission to space rises and less IR from the surface makes it to space

• Since air temperature in the troposphere decreases with altitude, the temperature of the emission to space decreases

• Therefore Earth’s energy emission to space decreases because the IR energy flux emitted by an object decreases with decreasing temperature

• Therefore total Energy flux IN > Energy flux OUT

• Earth warms until Energy flux IN = Energy flux OUT

Change in IR Emission to SpaceBEFORE GHG increase: IN=OUT AFTER GHG increase

NH SHAhrens, Fig. 2.21

Alti

tud

e

Temperature Temperature

1. Altitude of IR emission to space rises

Altitude of IR emission to space

Temperature of IR emission to space

2. Temperature of IR emission to space decreases

3. IR emission to space decreasesbecause of colder emission temperature

IR emission to space

Change in IR Emission to Space (cont’d)

AFTER GHG increase IN>OUT Eventual solution IN=OUT

SHAhrens, Fig. 2.21

Temperature

4. Atmosphere warms until…

5. Temperature of IR emission to space increases to original temperature

6. IR emission to space increases until it matches the original IR emission before GHG increases

SHAhrens, Fig. 2.21

Temperature

1. Altitude of IR emission to space rises

2. Temperature of IR emission to space decreases

3. IR emission to space decreasesbecause of colder emission temperature

Change: Atmo IR Emission to SurfaceBEFORE GHG increase: AFTER GHG increase

Energy DOWN=UP at surface more IR emission into surface

NH SHAhrens, Fig. 2.21

Alti

tud

e

Temperature Temperature

1. Altitude of IR emission to surface decreases

Altitude of IR emission to surface

Temperature of IR emission to space

2. Temperature of IR emission to surface increases3. Atmo IR emission

to surface increasesbecause of warmer emission temperature

Atmospheric IR emission to surface

Out-of-Equilibrium Issues

• Land temperature increases faster than Ocean temperature

• Atmospheric lifetime of CO2 is ~50 years

• System response is NOT instantaneous• System takes a while to heat up in response to

increase in CO2

• Initial temperature increase is smaller than final

Anthropogenic Climate Change

The data indicate that global-mean land and sea-surface temperatures have warmed about ~0.6oC during the last 35 years.

Is this the early stages of a man-made global warming?

Two main anthropogenic forcing mechanisms:

Greenhouse gas concentrations => rising.

Aerosol concentrations => also increasing.

We will focus attention on CO2 increases.

Greenhouse Gas Concentrations

• The two dominant greenhouse gases are H2O and CO2.

• There are several other GHGs with smaller concentrations: O3, CH4, N2O

• H2O vapor is categorized separately because it is controlled indirectly by evaporation and condensation and changes in response to other changes in climate (temperature, circulation)

Absorption 20% of incident Visible (0.4-

0.7 m) is absorbed

O2 an O3 absorb UV (shorter than 0.3 m)

Infrared (5-20 m) is selectively absorbed

H2O & CO2 are strong absorbers and emitters of IR

Little absorption of IR around 10 m – atmospheric window

Visible

IR

Ahrens, Fig. 2.9

Global Warming Potential (GWP)

Different gases has different warming potentials which are defined relative to the warming effect of CO2

Ahrens, Fig 2.10

Gas GWPCarbon dioxide (CO2) 1Methane (CH4) 21Nitrous oxide (N2O) 310Hydrofluorocarbons 560-12,100Perfluorocarbons 6,000-9,200Sulfur hexafluoride 23,900

CO2 makes the biggest contribution to the climate forcing

Ice Core from Vostok, Antarctica

During last ice age (>18,000 years ago)Temps 6oC colder

CO2 levels 30% lower

CH4 levels 50% lower

H2O levels were lowerthan current interglacial.

135,000 years ago it was a bit warmer than today

50% increase in CO2 was 50% increase in CO2 was associated with 6-8associated with 6-8ooC C increase in temperatureincrease in temperature

6-8oC decrease in temperature produced incredibly different climate: Ice Age

Changing CO2 concentrations• CO2 concentrations have varied naturally by ~30-50%

over the past few hundred thousand years (ice ages)• Fossil fuel burning since the industrial revolution has

created a recent sharp increase in CO2 concentrations• CO2 concentrations are now higher than at any time in

past few hundred thousand years • And concentrations are increasing faster with time

Last 4 Ice Age cycles:400,000 years

See http://epa.gov/climatechange/science/recentac.html

Man made

You are here

Changing CO2 concentrations

• CO2 concentrations measured very precisely since 1958

• Over past 45 years they’ve increased by ~21%

• Presently increasing at 0.6%/yr

See http://www.cmdl.noaa.gov/ccgg/trends/co2_data_mlo.php

You are here

US (5% of world population) now causes 24% of total pollution.See http://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions

• Currently, 7 gigatons per year of CO2 are injected into the air by burning fossil fuels (80%) and forests (20%).

• Half accumulates in atmosphere, where it resides for 50+ years.

• If the burning fossil fuels and forests totally ceased, it would still take 50 years for CO2 levels to return to 50% above pre-

industrial levels.

Missing Carbon

Sink

• CO2 is accumulating in the atmosphere more slowly than expected (believe it or not)

• Based on our understanding of CO2 emissions and ocean and atmosphere uptake, there is a missing sink/uptake of about 25% NASA OCO mission

Natural Variations in Climate

Milankovitch Theory of Ice Ages

• Attempts to explain ice ages by variations in orbital parameters

• Three cycles:

Eccentricity (100,000 yrs)

Tilt (41,000 yrs)

Precession (23,000 yrs)

• Changes the latitudinal and seasonal distributions of solar radiation.

Milankovitch Theory of Ice Ages

• Most recent Ice Ages have occurred for past 2 million years

• Ice Ages occur when there is less radiation in summer to melt snow.

• Partially agrees with observations, but many questions unanswered.

What caused the onset of the first Ice Age?

MilankovitchTheory

Change in daily solar radiation at top of atmosphere at June solstice

Changes as large as ~15% occur

Global Temperatures and CO2

There is a very strong relationship between CO2 levels and past global temperatures.

CO2 levels are now higher than during any period of the past 450,000 years.

Will global temperatures responding accordingly?

Long-Term Climate Change

250 million years ago, the world’s landmasses were joined together and formed a super continent termed Pangea.

As today’s continents drifted apart, they moved into different latitude bands.

This altered prevailing winds and ocean currents.

NAE-A

AfSAIndia

NAIndiaAf

SA

E-A

AntAus

Ant

Aus

180 M BP Today Ahrens, Fig 13.6

Long-Term Climate Change• Circumpolar ocean current

formed around Antarctica 40-55 MY ago once Antarctica and Australia separated.

• This prevented warm air from warmer latitudes to penetrate into Antarctica.

• Absence of warm air accelerated growth of the Antarctic ice sheet.

http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

Long-Term Climate Change• Circumpolar seaway leads to

large latitudinal temperature gradient.

• Circum-equatorial seaway leads to small latitudinal temperature gradient

http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html

Climate System Feedbacks

CO2 and the Greenhouse Effect

If the atmosphere were dry, we could predict with high confidence that a doubling of CO2 (likely before 2100) would increase the global mean surface temperature by ~2C.

The presence of oceans, ice, water vapor and clouds complicates the analysis significantly.

Ahrens, Fig 2.10

Complexity of Climate System

The climate system involves numerous, interrelated components.

Closer Look at Climate System

Climate Feedback Mechanisms

Climate System Feedbacks• Feedbacks are what makes the climate problem so

difficult• Positive: Increase the impact of the original change• Negative: Reduce the impact of the original change

• EXAMPLE: Ice albedo (Positive Feedback): – Temperatures increase because of increased CO2

1. Ice melts2. Reduces Earth’s reflectivity (albedo)3. Earth absorbs more sunlight4. Earth warms5. More ice melts….

Positive and Negative Feedbacks

• Assume that the Earth is warming.

- Warming leads to more evaporation from oceans, which increases water vapor in atmosphere.

-More water vapor increases absorption of IR, which strengthens the greenhouse effect.

-This raises temperatures further, which leads to more evaporation, more water vapor, warming…

“Runaway Greenhouse Effect”

Positive Feedback Mechanism

NET EFFECT: Water vapor feedback increases the warming of the Earth, roughly doubling the effect of CO2 increase alone

Positive and Negative Feedbacks

• Again assume that the Earth is warming.

- Suppose as the atmosphere warms and moistens, more low clouds form.

- More low clouds reflect more solar radiation, which decreases solar heating at the surface.

- This slows the warming, which would counteract a runaway greenhouse effect on Earth.

Negative Feedback Mechanism

Cloud Feedback• Clouds affect both solar & IR radiation

– Clouds increase solar reflection (albedo) cooling the Earth

– Clouds absorb and emit IR radiation like a GHG

• Currently their net effect is to cool the Earth– Solar effect > IR effect

• Not clear what they will do in the future– If the amount of low clouds increases, the albedo effect wins

and they cool the Earth (Negative feedback)

– If the amount of high clouds increases, the IR effect wins and the Earth warms (Positive feedback)

• There are other subtler cloud feedbacks as well involving aerosols in particular

Positive and Negative Feedbacks

• Atmosphere has a numerous checks and balances that counteract climate changes.

• All feedback mechanisms operate simultaneously.

• All feedback mechanisms work in both directions.

• The dominant effect is difficult to predict.

• Cause and effect is very difficult to prove at the “beyond a shadow of a doubt” level.

Climate Model Predictions

Projections of Global Warming

• Atmosphere and coupled Atmo-Ocean models are run for hundreds of years to simulate future climates

• They assume continued increases in the levels of greenhouse gasses in the atmosphere.

• The models have sophisticated physics…

But they have coarse spatial grid separations!

Atmosphere: 250 km horizontal, 30 levels vertical

Ocean: 125-250 km horizontal, 200-400 m vertical

How Well Do Models Capture Current Observed Climate?

• Performance Varies by Weather Element

In general…

Excellent for Surface Temperature

Skillful for Sea-Level Pressure (SFC Winds)

Marginal Skill for Precipitation

Fig. 8.4 IPCC Report

Global Temperature Outlook

Assume CO2 levels rise at rate of 1% per year until 2070.

• Good agreement for past climate and CO2 levels leads to high confidence.

• Rather close agreement among models.

Consensus of several model runs indicates an average warming of 2oC

Fig. 9.3 IPCC Report

Global Precipitation Outlook

Fig. 9.3 IPCC Report

• Marginal performance for past climate and CO2 levels means low confidence in outlook.

• Large differences exist among models.

Consensus of several model runs indicates an average increase of 2% in global precipitation

Regional Consistency: Warming

Fig. 10.1.1 IPCC Report

A + or - symbol denotes 7 out of 9 models agree.A2: no sulphate aerosols; B2: has sulphate aerosols

Regional Consistency: Rainfall

Fig. 10.1.2 IPCC Report

A + or - symbol denotes 7 out of 9 models agree.A2: no sulphate aerosols; B2: has sulphate aerosols

GDFL Model

Lets look at some details from a 500-year simulation by a specific climate model.

Examine Two Scenarios:

1% CO2 increase per year for 70 years 2X Total Increase in CO2

1% CO2 increase per year for 140 years 4X Total Increase in CO2

Good agreement for CO2 levels of the past 150 yrs

www.gfdl.noaa.gov

Mandatory before use as global warming model

Average Global Surface Temperatures… Warm 2oC for 2X CO2 Warm 4o C for 4X CO2

Sea Level Rises… Equilibrium Not Reached until after 500 years

North Atlantic Ocean… Circulation Weakens

CO2 Increases 1% per Year

www.gfdl.noaa.gov

CO2 Increases 1% per Year

• Surface Temperatures for GFDL Model

2X CO2 Temperature Animation Remote

2X CO2 Temperature Animation Local Disk

BIG File (8.2 MB)!!!

Climate response is out of equilibrium for a while

• Continental temperatures rise faster than ocean temperatures – Water has much larger heat capacity than land – Evaporation cools the ocean surface

• Ocean temperatures take long time to reach a new equilibrium1. Warming we have seen thus far is NOT the total warming even

if GHG concentrations stop increasing• Water saturation vapor pressure over land increases faster than

that over the oceans• Most of the atmospheric water vapor ultimately comes from the

ocean • Therefore expect the relative humidity over land to decrease in

general, at least until ocean temperatures catch up with the land temperatures.

2. Continental interiors will get drier in general

Annual Energy Balance

Latitudinal imbalance between visible in and IR out is compensated by meridional energy fluxes

Heat transfer from low latitudes to high latitudes is accomplished by winds and ocean currents

Differential heating and spinning Earth drive winds and currentsThis horizontal energy transfer will likely change as climate

changes

NH SH

Radiative WarmingRadiative

CoolingRadiative Cooling

Ahrens, Fig. 2.21

Mid-Latitude Cyclones

mP

cP

mT

Ahrens, Meteorology Today, 5th Ed.

•Winter storms move tropical air poleward and polar air toward the tropics.

•Net result is to transport energy poleward

Ocean Currents of World

Ahrens Fig. 7.24

www.gfdl.noaa.gov

4X CO2 Sea Ice Animation4X CO2 Sea Ice Local Link

July Temperature over Southeastern U.S.

• Warms 5-9oC

July Heat Index over Southeastern U.S.

• Rises 7-14oC due to increase H2O vapor

Consistent Signal

Warmer Southern U.S.

CO2 Increases 1% per Year

www.gfdl.noaa.gov

Soil Moisture• Much Drier over U.S.

60% Soil Moisture DecreaseHigher Evapo-TranspirationAltered Balance between Evaporation-Precipitation

• Agricultural ImplicationsHow do we hydrate and feed ourselves?

CO2 Increases 1% per Year

www.gfdl.noaa.gov

Increasing CO2 concentrations• How high will they go? How warm will it get???• If CO2 concentrations stay within factor of 2 of pre-industrial,

then warming of 3+1oC is expected• If concentrations go still higher => larger uncertainty because

the climate is moving into unprecedented territory

Last 4 Ice Age cycles:400,000 years Man made

You are here

Ice age CO2 range

You are going to be somewhere in here

See http://epa.gov/climatechange/science/futureac.html