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 1 C colder than it was in the twentieth century.
During the Medieval Warm Period (800–1400) the climate was around 1 C 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 0.5 C.
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 Variations in Pacific Ocean currents cause short-term changes in climate around the world. El Ni ñ o Southern Oscillation (years) 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.
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.
Greenhouse gas emissions per person (tonnes of carbon dioxide equivalent, 2005) North America 24.1 Europe 10.5 Asia 3.4 Sub-Saharan Africa 2.3 South America 5.3 Oceania 19.1
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: