Weather, Climate & SocietyATMO 325

Climate Projections

What is Climate Change?

• Climate change - A significant shift in the mean state and event frequency of the atmosphere.

• Climate change is a normal component of the Earth’s natural variability.

• Climate change occurs on all time and space scales.

• A plethora of evidence exists that indicates the climate of the Earth has changed. (Next Lecture)

What is Climate Change?

• If we can’t predict the weather more than 7-10 days in advance, why should I believe that we can provide useful 100 year climate outlooks?Weather forecasts attempt to predict specific weather at specific times for point locations. A climate “forecast” attempts to replicate changes in the statistics of weather.

- Average temperature, rainfall, etc. - Distribution of weather events

Causes of Climate Change

• Atmospheric Composition - Anything that changes the radiative properties of the atmosphere (carbon dioxide, clouds, aerosols).

• Astronomical - Anything that alters the amount or distribution of solar energy intercepted by the Earth (solar variations, orbital variations).

• Earth’s Surface - Anything that alters the flow of energy at the Earth's surface or changes its distribution (snow cover, continental drift).

Global Climate Model: Take 2

A = albedo - % solar reflected to space

(1-) emitted to space

= emissivity - % absorbed by air

FOUTSFC

FINSFC

(S0/4) AS0/4(1-) FOUT

SFC

Surface TSFC

Atmosphere TATM

FOUTATM

FOUTATM

Let’s model the Earth system as a planetary surface with an absorbing atmosphere above the surface.

(1-) emitted to space

absorbed by air

FOUTSFC=T4

SFC

(1-A) S0/4

(S0/4)AS0/4(1-) FOUT

SFC

Surface TSFC

Atmosphere TATM

T4ATM

FOUTATM= T4

ATM

A planetary surface with an IR absorbing atmosphere above the surface.

Global Climate Model: Take 2

1. An OLR absorbing atmosphere slows the net energy flow out from surface (relative to no atm).

2. An increase in atmosphere’s absorptivity causes surface T to increase.

3. Radiation reaching space from the Earth is a combination of emission from a warm surface and colder atmosphere.

It must be equal to (1-A)S0/4 at equilibrium.

1-Layer Model Summary

Courtesy J. Thornton UW

Global Climate Model: Take 2

1.The atmosphere absorbs only Outgoing Long wave Radiation (no absorption of solar radiation)

2. The atmosphere absorbs the same fraction of OLR at each wavelength.

3. The atmosphere has a uniform temperature.

4. Fin = Fout for each component and whole system.

Simplifying Assumptions

Courtesy J. Thornton UW F =T 4

Global Climate Model: Take 2

By now, this is tattooed on your brain

Courtesy J. Thornton UW

T

SFC=

1−A( )S0

4 1− 2( )

⎡

⎣⎢

⎤

⎦⎥

14

Global Climate Model: Take 2

If ~ 0.75, then TSFC ~ 288 K

Courtesy J. Thornton UW

T

SFC=

239.44 1− 2( )

⎡

⎣⎢

⎤

⎦⎥

14

How can you warm the TSFC?

1. Increase the solar output S0

2. Decrease the reflectivity A

3. Increase the absorptivity

T

SFC=

1−A( )S0

4 1− 2( )

⎡

⎣⎢

⎤

⎦⎥

14

To increase TSFC by 1˚C…

1. Increase S0 by ~20 W/m2

2. Decrease A by ~1%

3. Increase absorptivity by ~0.02

ΔTSFC

TSFC

≈ΔS0

4S0

−ΔA

4 1−A( )+

Δ4 2−( )

ΔTSFC ≈TSFC

ΔS0

5472−

ΔA2.8

+Δ5

⎛

⎝⎜⎞

⎠⎟

Can GHG increases explain warming?

IPCC Fig. SPM.1

IPCC WG1 Fig. 6.10

IPCC SYR 1-1

Other changes are consistent with warming

• CO2 has increased from 290 ppm in 1900 to 380 ppm today

Evidence of Warming

Temperatures have increased 0.6 K the past 100 years

0.6oC warming past century

Ahrens, Fig 13.5

Simple approach to GHG warming

If a 33% increase in CO2 could alone produced a 0.6 K warming, would a projected doubling of CO2 the next 100 years produce a 1.8 K warming?

Can it possibly be that simple?

Not Really

Climate Feedbacks

Feedbacks more than double the response of the temperature to increasing concentrations of greenhouse gases

Closer look at climate system reveals couplings between

physical processes

Ice/Albedo Feedback

Surface Temperature

(+)

Snow and Ice Cover

Planetary Albedo

Temperature/Water Vapor Feedback

Surface Temperature

(+)

Atmospheric H2O vapor

Greenhouse Effect

Temperature/Low Clouds Feedback

Surface Temperature

(-)

Atmospheric H2O vapor

Low CloudsPlanetary

Albedo

Temperature/Infrared Flux Feedback

Surface Temperature

Outgoing IR Flux

(—)

Multiple feedbacks complicate the response of the climate

system

Positive and Negative Feedbacks

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

• All feedback mechanisms…

Operate simultaneously.

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.

Synthesis Fig i-1

Observed distribution of temperature changes show

warming over land

IPCC SYR 1-2

Many sources are responsible for the increase in potent

GHG’s

IPCC SYR 2-1

Industrialized sectors are the biggest per capita

contributors to GHG’s

IPCC SYR 2-2

But underdeveloped sectors are among the biggest overall

contributors per GDP

IPCC SYR Fig 2-2

WG1 SPM.2

Attribution of RF changes (1750-2005)

WG1 SPM.4

IPCC SYR 3-1

GHG Scenario

s

Integrated world, fossil fuel emphasis, 2050 pop max

Integrated world, balanced fuels, 2050 pop max

Integrated world, non-fossil fuel emphasis, 2050 pop max

Divided world, rapid unchecked pop growth

Integrated world, more ecol. friendly, 2050 pop max

Divided world, more ecol. friendly, slower pop rise

WG1 SPM.5

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Divided world, rapid unchecked pop growthIntegrated world, balanced fuels, 2050 pop maxIntegrated world, more ecol. friendly, 2050 pop max

WG1 SPM.5

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WG1 SPM.6

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Integrated world, balanced fuels, 2050 pop max

Divided world, rapid unchecked pop growth

Integrated world, more ecol. friendly, 2050 pop max

WG1 SPM.6

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B1

A1B

A2

WG1 SPM.7

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Precipitation Changes (%)

IPCC SYR Fig. 3.5

Projections and model consistency of relative changes in runoff by the end of

the 21st century

“Some systems, sectors and regions are likely to be

especially affected by climate change.”

• “particular ecosystems:- terrestrial: tundra, boreal forest and mountain regions because of sensitivity to warming; Mediterranean-type ecosystems because of reduction in rainfall; and tropical rainforests where precipitation declines

- coastal: mangroves and salt marshes, due to multiple stresses

- marine: coral reefs due to multiple stresses; the sea-ice biome because of sensitivity to warming”

“Some systems, sectors and regions are likely to be

especially affected by climate change.”

• “water resources in some dry regions at mid-latitudes and in the dry tropics, due to changes in rainfall and evapotranspiration, and in areas dependent on snow and ice melt”

• “agriculture in low latitudes, due to reduced water availability”

• “low-lying coastal systems, due to threat of sea level rise and increased risk from extreme weather events”

• “human health in populations with low adaptive capacity.”

“Uptake of anthropogenic carbon since 1750 has led to

the ocean becoming more acidic…”

“Increasing atmospheric CO2 concentrations lead to further acidification.”

“Projections based on SRES scenarios give a reduction in average global surface ocean pH of between 0.14 and 0.35 units over the 21st century.”

“Progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species.”

IPCC SYR Fig. 3.6

Extreme Weather Events

“Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems.”

IPCC Synthesis Report

Hurricane Intensities Hurricane Intensities Increase by 10%Increase by 10%

Reason: Warmer Sea Reason: Warmer Sea Surface TempsSurface Temps

www.gfdl.noaa.gov

Increases in the frequency and/or the intensity of extreme weather events is a likely consequence of global warming. GFDL Super Typhoon

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99%

90%

90%

66%

66%

66%

“Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate

change. “Partial loss of ice sheets.”“Rapid sea level rise on century time scales cannot be excluded.”

“There is medium confidence that approximately 20 to 30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5 to 2.5 ｡ C (relative to 1980-1999). As global average temperature increase exceeds about 3.5 ｡ C, model projections suggest significant extinctions (40 to 70% of species assessed) around the globe.”

5 in 10 chance

“Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate

change. “It is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st century.

“Impacts of large-scale and persistent changes in the MOC are likely to include changes in marine ecosystem productivity, fisheries, ocean CO2 uptake, oceanic oxygen concentrations and terrestrial vegetation. Changes in terrestrial and ocean CO2 uptake may feed back on the climate system.”

90% chance

Key Points: CO2 Warming• CO2 levels are rising and will likely double by 2070.• The greenhouse relationship between higher CO2

levels and warmer temperatures is indisputable.• Even with perfect knowledge of future CO2 levels,

there is significant uncertainty about how much warming would occur and how fast it would occur.

• Model results suggest ~2oC global warming, with strongest warming in polar regions, and an overall increase in global precipitation.

• Shifts in precipitation are much more uncertain, as are the consequences on water resources.

Intergovernmental Panel onClimate Change

A consensus document of of a wide sampling of the scientific community

IPCC Reports 2007

“Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations” --2007

Available for download at http://www.ipcc.ch/