Climate Change: Observations and Projections

Climate Change: Observations and Projections

Dan [email protected]

AP3A90/APMA90 Climate Change and Food Systems

Climate Change: Observations and Projections

In this lecture: What is Climate? Observations of a changing Climate. Climate modelling and Projections of future Climate

Change Uncertainty in Climate modelling and projection.

08/02/2013 2Climate Change: Observations and Projections.

What is Climate?

“Climate is what you expect, Weather is what you get”.

Climate is the statistics of weather, e.g. the average of weather conditions over some period of time.

Expect :

Maldives to be Warm

Antarctica to be Cold

Atacama Desert Dry

Bergen Wet


The Climate System

Land

Ocean

Atmosphere

Ice

Vegetation08/02/2013 4Climate Change: Observations and Projections.

The Atmosphere


Atmospheric Composition

Dry air contains:– Nitrogen 78% by volume– Oxygen 21% by volume– Argon 0.9%– The remaining 0.1%

• Carbon Dioxide (CO2)• Methane (CH4)• Nitrous Oxide (N2O)• + other trace gases

PLUS Water vapour (variable amounts ~1%)


Nitrogen Oxygen Argon Other

Greenhouse Gases

The Greenhouse Effect


Electromagnetic Radiation

All objects emit Electromagnetic Radiation (Light). Very hot objects emit visible light (Shortwave). Cooler objects emit infrared light (Longwave). This radiation carries Energy away from an object

which can then be absorbed by another object.

Fir0002/Flagstaffotoshttp://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License


08/02/2013

Main Greenhouse Gases

Although small fraction of Atmosphere, large impact. Greenhouses gases are almost transparent to

Shortwave radiation from the Sun, but almost opaque to Longwave radiation from the Earth.

Hence Greenhouse gases trap some outgoing Longwave radiation -> Disequilibrium -> Warming.

With no Greenhouse gases, average temperature of the Earth’s surface would be -19ºC rather than 14ºC.

Greenhouse GasesThe remaining 0.1%

Carbon Dioxide (CO2)Methane (CH4)Nitrous Oxide (N2O)

Water vapour

9Climate Change: Observations and Projections.

The Greenhouse Effect


Observations of Climate Change


Climate Change

Climate can be defined as the average of weather. Climate is what we Expect. A Change in Climate means e.g. a change in the

average weather conditions & change in what we expect e.g.:– Warmer summers– Wetter winters

But also can talk about the Climate in terms of other system components e.g.– The height of the sea.– The number of forest fires each summer.

08/02/2013 Climate Change: Observations and Projections. 12

Observing the Weather

People have always watched and noted the weather, but Objective measurements using scientific instruments began only ‘recently’:– Thermometers around since 1600’s– Early 1700’s Daniel Gabriel Fahrenheit manufactured first

reliable mercury thermometers. The Central England Temperature record is the

Oldest instrumental record of temperature in the world. – Monthly measurements back to 1649.

08/02/2013 Climate Change: Observations and Projections. 13/46

CET


Observing the Weather

Observations continued in this uncoordinated way for many years.– Individual Weather diaries– Marine weather logbooks

Once weather forecasting began, it was realised that coordinated, reliable, regular measurements of the weather were required.

World Meteorological Organisation (1950) set up to coordinate this observation across the World.


Global Observing Network


Locations of land, ship and buoy observations across the world at 6am 14 January 2008

Land observations concentrated in inhabited areas and mainly in the Northern Hemisphere

1970

Has the Earth Surface Warmed?


IPCC AR4

Different estimates

(smoothed)

~ 0.8ºC

Is the rate of warming increasing?


IPCC AR4

Global Trends

More warming over land than oceans Some regions have cooled.


IPCC AR4

Temperature trends at each point on the Earth

The ten warmest years on record

1. 19982. 20053. 20034. 20025. 20096. 20047. 20068. 2007 9. 200110.1997

Eight of these are from the last decade

All are from the last 13 years

(Data taken from the Hadley Centre)08/02/2013 20Climate Change: Observations and Projections.

1. 19982. 20103. 20054. 20035. 20026. 20097. 20048. 20069. 2007 10.2001

Nine of these are from the last decade

All are from the last 15 years

2012

Is it unusual?

Global average temperature rose in the 20th Century

Is this unusual? Have temperatures changed like this in

the past? Problem: Very few temperature

measurements before 1900. How can we measure temperatures

before the invention of the thermometer?

Natural Thermometers!08/02/2013 Climate Change: Observations and Projections. 21

Tree Rings

Trees grow outwards and lay down a new ring of wood every year.

More vigorous growth = thicker ring.

Growth dependent on temperature, rainfall etc.

Can estimate temperature from the width of rings.

~1000 years.


Estimates of past Northern Hemisphere Temperature


IPCC AR4

Recent warming unprecedented

Have other things changed?

Arctic Sea Ice Area Glacier Lengths Sea level height


Have other things changed?

Oceans are Warming:


Three Estimates of the Amount of heat in the upper ocean.

Greenhouse Gases

Carbon Dioxide– Burning of Fossil Fuel– Manufacture of Cement (~5% global)– Deforestation

Methane– Agriculture– Natural Gas– Landfill decomposition

Nitrous Oxide– Artificial fertilizers– Burning of Fossil Fuel

Are these changes unusual?


Ice Cores

Ancient gases trapped in bubbles in Antarctic Ice.

Can recover ice & gases that have been stored for 10 000s of years.

Can measure levels of Greenhouse gases in Ancient atmosphere.


Unprecedented?

Rate of increase of greenhouse gases unprecedented in last 20 000 years.


IPCC AR4 End of Last Ice Age

Observations of Climate Change: Summary Many 1000s of measurements form estimate of

changing climate. Global average (mean) surface temperature rose by

~0.8ºC during the 20th Century. 9/10 of the last 10 years were the warmest on record

globally. These warm temperature are likely highest in the

past ~1000 years. Also

– Sea level & Upper ocean heat content rose– Arctic sea ice and Glaciers melted

Concentrations of Major Greenhouse gases risen over 20th Century.– Largest seen in last 20 000 years.


Modelling the Climate


Climate System Components

Land

Ocean

Atmosphere

Ice

Vegetation08/02/2013 31Climate Change: Observations and Projections.

Why?

Why do we need a model of the Climate System? Most of Experimental Science

– Take some part of the World.– Make some change.– Measure any Effect.

The Climate System is the World. Can’t do real experiments on the

Whole Climate System. Need Climate Models.


Modelling the Climate System Do experiments to find out how Climate components

(e.g. water) behave. Write down a mathematical description of this

behaviour. Convert this into a numerical form for use in a

computer.

Can then use model Earth climate to perform experiments not possible with the real Earth Climate.

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Physics

Is the model Correct?

If we use our Climate Model to simulate 20th Century climate we can compare to our past measurements of the real 20th Century climate.– Model Validation


Real World ModelAnnual Rainfall

IPCC AR4

Is the model Correct?

Models simulate some variables better than others.– Temperature is predicted accurately– Rainfall less so

Climate models reproduce many of the features of the real climate– Global Patterns temperature & rainfall– Seasonal retreat of ice caps– North Atlantic Gulf Stream

these are not explicitly built into the model, they emerge from the physics.– More confidence that models are reliable.


Two Experiments

EXPT1: Use our climate models to simulate 20th Century climate as it was.– Known Increases in Greenhouse gases– Known changes in Natural Forcings

• Volcanoes• The slight variations of light from the Sun.

EXPT2: Use our climate models to simulate 20th Century climate as it might have been– No changes in Greenhouse gases– Known changes in Natural Forcings


Results EXPT1: Greenhouse

gases + Natural Forcings.– Black line: measured 20th

Century Global average Temperature.

– Red Line: Average of same experiment with many different climate models.

EXPT2: Natural Forcings Only.

Implication: Increases in Global average Temperature due to increases in Greenhouse gas emissions.

08/02/2013 Climate Change: Observations and Projections. 37IPCC AR4

Projections of future Climate

Change


Projected global temperatures

A2/A1B/B1 – different estimates of future GHG emissions.– A2 = High Emissions– B1 = Low Emissions

Large range by end of century.

Some warming even if we stopped emitting all GHG today.


Projected regional temperature changes 2090-2099


IPCC AR4

B1 (low)

A2 (high)

Northern latitudes warm more.– Melting sea ice –

feedbacks Land warms more than

oceans.

Projected regional Rainfall Changes 2090-2099


Northern Winter

Northern Summer

Different regions show different rainfall changes.

Northern high latitudes: increased rainfall in N. Hemisphere winter.– Wetter, more flooding.

Southern Africa & Mediterranean reduced rainfall in N. Hem. Summer– Drier, more droughts.

Grey shading – regions where we are confident.

Extremes

These are projections of the increase in the average temperatures and rainfall.

If the average temperature increases, then the chance of heat waves occurring increases.


2003 European heatwave– Probably hottest EU summer in last

500 years.– Stott et al (2004) likely to be

considered a cool summer in 2100.

Uncertainty in Climate modelling

and projection.


Climate Projections: Sources of Uncertainty

In Science it’s important to understand what you don’t know.

Three sources of uncertainty in climate projections: Uncertainty in future Greenhouse gas levels

(Scenario Uncertainty) Uncertainty in the formulation of the Climate models

(Model Uncertainty) Intrinsic uncertainty in the climate system.

(Internal Variability)


Scenario Uncertainty We don’t know what future Greenhouse gas emissions

will be. Produce ‘Scenarios’ – estimates of future emissions

based on estimates of future– Population Projections– Economic Development– Structural and Technological Change

Vary from High growth (A1) to Lowgrowth (B2).


Emissions Scenarios IPCC,2000

Model Uncertainty

Climate models are only approximate models of the real world. They have low resolution

– A region of 200km x 200km represented by a single point. They cannot represent small scale processes directly

– Formation of rain represented by statistical rather than physical model. They cannot include all processes in the Climate System

– Ocean biology, Dynamic vegetation, Agriculture, Economics are only just beginning to be included.

There are many Climate models (17 used in last IPCC report)


Intrinsic Uncertainty (Internal Variability)

If we start our Climate model and watch how the temperature evolves.

If we start the model again, but with e.g. slightly cooler initial temperatures.

The temperature evolves differently.


Butterfly effect. Sensitivity to the initial conditions. Because we do not know the exact state of the Climate

system now, we cannot predict the exact future state of the climate.

Working with uncertainty

Although these sources of uncertainty exist we can assess and quantify the scale of uncertainty.

This allows us to assign a level of confidence to climate projections.

This allows the IPCC AR4 to say:– “Most of the observed increase in global average

temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”.


Summary

In this lecture we have discussed:

What Climate is. Observations of a Changing Climate. Climate Modelling and Projections of future Climate

Change Uncertainty in Climate Modelling and Projection.


Next Lecture

The next lecture:

Climate Change and LivestockLes Crompton

Jan 15th

Dan [email protected]


Climate Change: Observations and Projections

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