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Climate change experts warned this week that the phenomenon is occurring at a faster rate than the worst-case scenario envisaged by scientists just six years ago.

Tim Flannery, named the 2007 Australian of the Year for his work in alerting the public to the dangers of global warming, said the issue was the greatest challenge facing humanity in the 21st century.

Flannery said predictions in a 2001 UN report, warning the atmosphere was likely to warm by 1.4 to 5.8°C from 1990 to 2100 now appeared conservative.

"In the six years since then, we've collected enough data to (check) whether those projections are valid or not," he said. "It turns out they're not valid, but in the most horrible way – because for the key performance indicators about climate, change is occurring far in advance of the worst-case scenario."

Here, we bring you a timeline on global warming and climate change:

1827

• French scientist Jean-Baptiste Fourier is the first to consider the greenhouse effect: the phenomenon whereby atmospheric gases trap solar energy, increasing Earth's surface temperature, rather than let the heat radiate back into space.

1896

• Swedish chemist Svante Arrhenius blames the burning of fossil fuels (oil, gas and coal) for producing carbon dioxide (CO₂).

1958

• U.S. scientist Charles David Keeling detects a yearly rise in atmospheric CO₂.

1979

• A landmark report by U.S. National Academy of Sciences pins the greenhouse effect to global warming and warns "a wait-and-see policy may mean waiting until it is too late."

1988

• U.N. sets up a scientific authority to vet the evidence on global warming, the Intergovernmental Panel on Climate Change (IPCC).

1990

• First IPCC report says levels of man-made greenhouse gases are increasing in the atmosphere and predicts these will cause global warming.

1992

• Creation of the UN Framework Convention on Climate Change (UNFCCC) at the Rio Summit, which also calls for voluntary cuts in greenhouse gas emissions.

1997

• UNFCCC countries sign the Kyoto Protocol. Under its first commitment period, industrialised countries are required to reduce emissions of six key greenhouse gases to 5.2 per cent below their 1990 levels by the end of 2012. Fleshing out its complex and legally binding rulebook is left to further negotiations.

2000

• 1990s are named as the hottest decade on record.

2001

• IPCC's third report declares the evidence for man-made global warming to be incontrovertible although the effects on the climate are hard to pin down.

• The United States, the biggest single greenhouse-gas polluter, abandons the Kyoto Protocol. President George W. Bush questions scientific consensus on global warming, says the treaty is too expensive for the U.S. economy and unfair as big developing countries escape binding emissions pledges.

• Kyoto signatories minus the U.S. agree on the treaty's rulebook, opening the way to ratification process.

2004

• The International Energy Agency (IEA) says China is now the world's second biggest carbon emitter, due to rising use of fossil fuels.

2005

• Kyoto Protocol takes effect on February 16.

• Global warming takes centre stage at G8 summit in Gleneagles, Scotland, where leaders acknowledge climate change to be "a serious and long-term challenge."

• Awareness, and concern, of global warming surges in U.S. after an exceptional season for tropical storms, punctuated by Hurricane Katrina.

2006

• Former U.S. vice president Al Gore's docu-movie "An Inconvenient Truth" drives global warming up the U.S. political agenda.

• California unveils plans for reducing its greenhouse gas emissions to 1990 levels by 2020 and files lawsuits against six vehicle manufacturers for their contribution to global warming.

• Report by former World Bank economist Sir Nicholas Stern says global warming will cost up to 20 per cent of worldwide gross domestic product if nothing is done.

2007

• The Bulletin of the Atomic Scientists moves the hand of the Doomsday clock forward by two minutes, making it five minutes to midnight, citing global warming and nuclear proliferation.

• IPCC's fourth assessment says glacial shrinkage, ice loss and permafrost retreat are signs that climate change is already underway. Predicts higher risk of drought, floods and more powerful storms this century, increasing the probability of hunger, homelessness and water-borne disease. Forecasts likely warming of 1.8 to 4.0°C and raised sea levels of 18 to 59 cm by 2100.

• September: Meetings are held at the UN in New York and among major emitters in Washington, ahead of talks in December on the issue of deepening cuts after 2012, when the first commitment period under the Kyoto Protocol expires.

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