Outline of problem and basic jargon

• Greenhouse gas (GHGs)
  • gas present in the atmosphere which traps heat thereby warming the Earth, e.g. carbon dioxide (CO₂)
• Greenhouse effect
  • warming of the Earth caused by greenhouse gases (has some similarities to the way a greenhouse works)
  • natural greenhouse effect – due to natural levels of GHGs; Earth is 33°C warmer due to this
  • enhanced greenhouse effect (global warming) – extra warming due to human emissions of GHGs
• Climate change
  • extra heat trapped in atmosphere leads to changes in air/ ocean circulation patterns, ie a change in climate

Climate change: the details

• Human activities releasing large amounts of greenhouse gases [1]

GHG	Main artificial sources	Increase in atmospheric level (1750 to 2000)	% contribution to global warming
Carbon dioxide (CO2)	burning fossil fuels (coal, oil, gas); deforestation	31%	60%
Methane (CH4)	gas pipeline leaks; cattle; paddy fields	151%	20%
Nitrous oxide (N2O)	nylon production; artificial fertilisers	17%	6%
‘F’ gases (HFCs, PFCs, SF6)	refrigerators; air-conditioning; electronics industry	(did not exist in 1750)	14%

• CO₂ levels are now higher than for at least 420,000 years, and probably higher than at any time in the last 20 million years [1]

• These GHG emissions are causing the average global temperature to rise [1, 2]
  • Current temp is 0.7°C higher than in ~1900 – higher than at any time in the last 1000 years
  • Predicted temperature in 2100 will be between 1.4°C and 5.8°C higher than in 1990, representing a faster increase than at any time since the transition from the last Ice Age 10,000 years ago. (For comparison, the last Ice Age was 3-5°C cooler than now.)
  • Figure 1 (below) – recent & projected warming (‘hockey stick’ graph)

• Broad climatic effects [1]
  • Sea level rise due to water expansion + ice melting
  • ‘More energetic hydrological cycle’ – ie more severe weather, such as storms, floods in some areas, with droughts in others
  • Large regional changes in climate
  • Risk of dramatic/ irreversible climate shifts (see below)

• Main impacts on humans and wildlife [3]
  • more water shortages, heat-stress deaths
  • more storm/ flood/ landslide deaths & damage
  • expansion in ranges of some infectious diseases, eg malaria
  • disruption of agriculture
  • increase in environmental refugees
  • poorer countries and peoples are most vulnerable
  • loss of wildlife, especially endangered specie

• What effects are visible already? [1, 4, 6]
  • sea level – 10-20cm rise over last century
  • ice cover – thinning of Arctic ice sheet, retreat of most glaciers, some melting of permafrost
  • extreme weather – some increase in storm activity, some increase in occurrence of drought
  • earlier spring/ later autumn – evident in plant, animal changes
  • increase in coral bleaching
  • World Health Organisation study suggested that current global death rate due to climate change could already be around 150,000 a year

• Predictions of effects that are likely to happen in future [1, 3]
  • strongly dependent on what humans do over the next few decades
  • sea level – (mid-level estimate) 0.5m rise by 2100
  • flooding – (mid-level) affect an extra 75 to 200 million by 2080
  • water-stress – (mid-level) affect an extra 3 billion by 2025 (especially poorer countries)
  • hunger and malaria – each could affect hundreds of millions extra
  • most studies have only looked at changes up to 2100 – but effects will continue well beyond that and are likely to be more severe
  • some impacts will be positive, eg reduction in deaths due to cold, but the negative effects will dominate

• Dramatic & irreversible* climate shifts [5]

• 1. Melting of Greenland ice sheet – Climate models suggest that at around 2.7°C warming, the melting of this ice sheet will become irreversible. If the complete sheet melts, sea levels will rise by 7m – although this would take over 1000 years to complete. West Antarctic ice sheet is also at risk.
  2. Shutdown of ocean conveyor – The global ocean current that includes the Gulf stream (which keeps the UK warm) could shutdown as ice dynamics change, eg due to (1) above.
  3. Release of frozen methane – 5 trillion tonnes of methane locked away in frozen hydrates in deep oceans. Warming will start to release it accelerating the warming further.
  4. Destruction of Amazon rainforest – Climate models suggest that later this century rainfall will decline and the Amazon will dry out and die.

• Scale of cuts in GHG emissions needed [6, 7]
  • difficult to be exact because of significant uncertainties in climate models and impacts
  • growing number of scientists argue that we should aim to prevent global temp change of over 2°C above pre-industrial levels to prevent ‘dangerous climate change’
  • this would require keeping CO₂ levels below about 400ppm – we are currently at 380ppm
  • hence global GHG emissions need to begin falling within a few years and reach about 10% of current levels by 2100
  • the is a huge task, but the longer the delay in reduction, the bigger the impacts will be

• How do we know all this? [1]
  • Physics of greenhouse effect well understood, ie observations of planets
  • Wide range of data on past climate – direct temp measurements & ‘proxy’ data which vary with temp, eg tree rings, coral, ice cores
  • Eg ice cores show carbon dioxide and temperature have varied together over past 420,000 years
  • Figure 2 (below) - atmospheric CO₂ and temp variations over 420,000 years
  • Understanding of global carbon cycle (& other mineral cycles) – measurements of how carbon moves through atmosphere, oceans, ecosystems etc
  • Wide range of climate models calibrated on past changes used to predict future changes
  • Evidence gathered together by scientific body Intergovernmental Panel on Climate Change (IPCC). Set up by UN Environment Programme in 1988, it compiles evidence on the issue. The last major assessment in 2001 involved 3000 scientists

References

[1] IPCC (2001a). Climate Change 2001: the scientific basis. (Summary for policy-markers.) Working Group I of the Intergovernmental Panel on Climate Change. Cambridge University Press. http://www.grida.no/climate/ipcc_tar/wg1/index.htm

[2] Houghton J. (2004) Global Warming: the complete briefing. Cambridge University Press. http://www.cambridge.org/catalogue/catalogue.asp?isbn=0521528747

[3] IPCC (2001b). Climate Change 2001: impacts, adaptation and vulnerability. (Summary for policy-markers.) Working Group II of the Intergovernmental Panel on Climate Change. Cambridge University Press. http://www.grida.no/climate/ipcc_tar/wg2/index.htm

[4] World Health Organisation (2003). Climate Change and Human Health – risks and responses. WHO Press. http://www.who.int/bookorders/anglais/detart1.jsp?sesslan=1&codlan=1&codcol=15&codcch=551

[5] Pearce F. (2005). Climate Change: Awaking the sleeping giants. New Scientist, 12 February, p9-11. http://www.newscientist.com/channel/earth/mg18524864.400-climate-change-awaking-the-sleeping-giants.html See also: http://www.stabilisation2005.com/

[6] IPCC (2001c). Climate Change 2001: synthesis report. (Summary for policy-markers.) Intergovernmental Panel on Climate Change. Cambridge University Press. http://www.grida.no/climate/ipcc_tar/vol4/index.htm

[7] Pearce F. (2005). Climate Change: Act now, before it’s too late. New Scientist, 12 February, p8. http://www.newscientist.com/channel/earth/mg18524864.300-climate-change-act-now-before-it-is-too-late.html See also: http://www.stabilisation2005.com/

NB Web-links correct as of 30/08/06