World at riskClimate change and

its causes

Climate change

• Climate change can be assessed across short, medium and long timescales. • Short-term (recent) climate change is on a timescale of decades, e.g. global

warming. • The medium-term (historical) timescale covers changes over the last few

thousand years.• Long-term climate change has occurred on geological timescales, over

hundreds of thousands to millions of years.

Geological timescales (1)

Atmospheric carbon dioxide concentration measured from the Vostok ice core, East Antarctica

Geological timescales (2)

• Ice cores, pollen analysis and past sea-level changes all indicate that climate

has changed in the past.• Ice ages and interglacials (warmer periods) seem to occur on a cycle of about

100,000 years.• The last ice age (the Devensian) ended approximately 10,000 years ago and the

current interglacial period (the Holocene) began.

Historical climate change

• Written records, pictures, tree rings and the extent of glaciers suggest climate

has changed on historical timescales.• During the Little Ice Age (1400–1850) the climate was around 1C colder than

it was in the twentieth century.• During the Medieval Warm Period (800–1400) the climate was around 1C

warmer than in the twentieth century.

Recent climate change (1)

Global temperatures, 1850–2008

Recent climate change (2)

• Global temperatures fluctuated considerably between 1860 and 1970 (see

graph on previous slide).• Since the late 1970s there has been a marked warming of around 0.5C.• This corresponds to the ‘era of global warming’. • Accurate instrumental measurements of air and ocean temperatures as well as

ice cover testify to this record of global warming.• Increasingly ecosystems are changing in response to rising temperatures.

Possible causes of climate change

Ice ages in geological time

(glacial/interglacial cycles)

(1,000s–10,000s years)

Milankovitch cycles — changes in the amount and distribution of

solar energy received at the Earth’s surface caused by natural

variations in its orbit around the sun and the tilt of the Earth’s

axis.

Historical changes

(Little Ice Age/Medieval Warm Period)

(several hundreds of years)

Variations in solar output and volcanic activity. The output of

the sun naturally varies as sunspots grow and shrink, thereby

changing the amount of solar energy received by the Earth.

Volcanic dust, ash and sulphur dioxide have a short-term

cooling effect.

Global warming

(several decades since 1970)

Anthropogenic (human) greenhouse gas emissions (carbon

dioxide, methane) trap outgoing radiation in the atmosphere,

thereby creating a warming effect.

El Niño Southern Oscillation (years) Variations in Pacific Ocean currents cause short-term changes in

climate around the world.

The greenhouse effect

The greenhouse effect

• The greenhouse effect is the natural process whereby gases in the

atmosphere, principally carbon dioxide, trap some outgoing solar radiation.• This process warms the planet.

The enhanced greenhouse effect

• Gases released by burning

fossil fuels have enhanced the

greenhouse effect and made it

more powerful.• This has a net warming effect.• The level of carbon dioxide in

the atmosphere has risen

steadily since accurate

recording began in the 1950s.

Carbon dioxide concentrations, Hawaii, 1959–2005

Anthropogenic greenhouse gas sources (1)

• Transport, industry, electricity and

heat account for over 50% of all

greenhouse gas emissions.• Carbon dioxide is the most important

greenhouse gas by volume.• Methane is a more powerful

greenhouse gas per molecule.

Global greenhouse gas emissions by economic sector, 2000

Anthropogenic greenhouse gas sources (2)

• The average carbon footprint in

the developed world is five to ten

times greater than in the

developing world. • This confirms that global warming

is a problem created largely by

the developed world.

North America 24.1

Europe 10.5

Asia 3.4

Sub-Saharan Africa 2.3

South America 5.3

Oceania 19.1

Greenhouse gas emissions per person (tonnes of carbon

dioxide equivalent, 2005)

Unprecedented global warming?

• Average global temperatures have risen by 0.8 C since 1880.• The decades from 1980–2000 were the hottest for at least 400 years.• Measured warming in the Arctic is twice that for the rest of the world.• Arctic sea ice in 2007 was at its lowest recorded extent.• Carbon dioxide levels, at 380 ppm in 2007, are over 100 ppm higher than pre-

industrial ‘natural’ levels.

Concern about global warming centres on key data, which are increasingly taken to

be ‘fact’ by the majority of scientists: