Science of climate change[fr][de] 

Early climate models in the 1970s only took a limited number of factors into account: carbon dioxide (CO2) concentrations, heat from the sun (radiation) and rain but not clouds (for an introduction to climate science, see BBC ). 

Climate science has evolved greatly since then. Models now take into account many more factors such as land surface, ice sheet cover (which reflects the sun’s radiation), deserts (which also reflect radiation), forests (which absorb CO2), ocean currents, and more (for a description of main climate models, see Wikipedia ). 

Drawing on peer-reviewed work from hundreds of scientists worldwide, the reports by the Intergovernmental Panel on Climate Change (IPCC) are seen as a reference in climate change science. Their purpose is to inform policy-makers about the causes of climate change, its potential impacts and draw up possible response options. The reports have laid the scientific foundations for some countries to embark on ambitious policies to try to reduce global warming. The UN Framework Convention on Climate Change (UNFCCC) and its related Kyoto Protocol directly stem from conclusions drawn up by the IPCC. 

Global warming has already had multiple effects, such as rising sea-levels, melting glaciers and shrinking ice caps, increasing temperatures and acidity of the oceans, as well as possible increases in the intensity of tropical cyclones. 

The first IPCC report on climate change appeared in 1990 and was still rather cautious. Subsequent IPCC reports tried to narrow down uncertainties and refine predictions. 

A summary of conclusions from the IPCC’s 4th Assessment Report - Working Group I (AR4) was agreed on 2 February 2007. The document concludes that: “most of the observed increase in globally averaged temperature since the mid-20th century is very likely due to the observed increase in anthropogenic (human) greenhouse gas concentrations.” The assertion “very likely” amounts to a probability of than 90% in UN terminology. It is stronger that the panel’s previous conclusions which, in 2001, referred to human responsibility as being “likely”, or equivalent to a 66% probability. 

Other main conclusions of the IPCC’s fourth report - WG I (20007): 

• Eleven of the last 12 years rank among the 12 warmest years on record since 1850
• Global average temperatures have increased by 0.74°C over the last 100 years, with the trend over the last 50 years standing at twice the average (0.13 °C per decade). 
• Global average temperature will likely increase by a further 1.8-4.0°C this century.
• Sea levels likely to rise by 28 to 43 cm.

Greenfacts has a more detailed and structured summary  of the first part of the IPCC 2007 report.

IPCC Working Group II presented its part of the 4th assessment on 6 April. Their report "Climate change impacts, adaptation and vulnerability " highlighted that global warming is already having a dramatic negative effect on animals, plants and water in different regions of the world. It also underlined that it will be "the poorest of the poor" who will be the main victims of climate change.

Based on the IPCC’s second report (1995), the EU has adopted binding targets to reduce GHG emissions with the ambition to keep global warming under control. EU environment ministers agreed in 1996 that global average temperature “should not exceed 2°C above pre-industrial levels”. 

They based their decision upon IPCC calculations which showed that, in order to keep temperatures below this limit, concentration of GHG in the atmosphere should stay below 400 parts per million CO2 equivalent (ppm), Conversely, if concentrations were to rise above 550 ppm, then the target would likely be missed. IPCC reports have repeatedly come under attack for being politically biased as conclusions need to be adopted by consensus among participating nations. Criticisms have focused both on an “alarmist” bias and a “conservative” bias in future warming prediction. 

A UK House of Lords report in June 2005 said that “there are some positive aspects to global warming” and that “these appear to have been played down in the IPCC reports”. 

But the most well-known debate is the so-called “hockey stick” controversy which focused on the reconstruction of temperature fluctuations in the Northern hemisphere over the past 1,000 years, as presented in the IPCC’s third assessment report in 2001. The reconstruction showed that the recent warming observed since the 1950s is unprecedented in history, rising up sharply in the later part of the graph, making the curve bend at the end like a hockey stick. The hockey stick theory was fiercely challenged by scientists and US republicans who claimed that the scientists compiling the date were deliberately playing down previous variations in temperatures to make the case for a more dramatic rise today. The dispute was eventually settled upon request from the US Congress by a panel of scientists which broadly supported the hockey stick depiction. 

The UK Dept. for Environment Food and Rural Affairs (DEFRA) organised an international symposium on “Avoiding dangerous climate change” in February 2005 with the aim to advance scientific understanding on the issue. It concluded that “a stabilisation (of GHG concentrations) at 450 ppmv CO2 equivalent would imply a medium likelihood (~50%) of staying below 2°C warming”. 

The most striking theories developed in the symposium concerned so-called ‘tipping points’ where warming leads to consequences which, once started, cannot be reversed. The most widely commented of these ‘tipping points’ related to the possible closure of the Gulf Stream (North-Atlantic Thermohaline Circulation – THC) which keeps Northern Europe relatively warm even though it is at the same latitude as Canada. 

According to different protection methods, the odds that the Gulf Stream would shut down ranged to anything between none to a 2 in 3 chance in the next 200 years to a 30% possibility by 2100. Further modelling experiments and observation data were essential for more robust answers, one scientist noted. 

A report by the European Environment Agency issued in August 2004 concluded that Europe is warming faster that the global average with temperatures projected to climb by a further 2.0-6.3°C this century as emissions of GHG continue building up. 

Once a stern challenger of mainstream climate science, US oil major ExxonMobil now recognises that something needs to be done about manmade greenhouse gas emissions. “We know enough now to say that we need to be on a path to start addressing anthropogenic emissions,” said Ken Cohen, vice-president for public affairs at ExxonMobil in a recent interview with EurActiv. 

But Cohen insists that policies to tackle global warming should be flexible enough to “be adjusted as we learn more on the science side”. “Some have said for instance that we need to stabilise CO2 emissions at 550 parts per million. But that is more of a political conclusion than a scientific conclusion. It may be that we’ll learn that 550 ppm is not an aggressive enough target. It may be that science will tell us that the target needs to be something lower than 550 ppm,” said Cohen. “We need to keep the science effort going and we need to keep in mind the economic impacts of the policies,” Cohen concluded. 

For environmental NGOs, the scientific evidence is now beyond any doubt. “The science is clear, Humans are causing climate change,” said the WWF after the IPCC issued the summary conclusions of its fourth assessment report in February 2007. “The report embodies an extraordinary scientific consensus that climate change is already upon us, and that human activities are the cause,” said James P. Leape, Director General of WWF International. 

For Friends of the Earth, “scientists say that while all climate change is dangerous, a global average temperature increase of more than 2°C would have catastrophic consequences, putting the lives and livelihoods of millions of people at risk.” “The 2003 European heat-wave has killed 33,000 people in Europe and has caused €13 billion in economic damages while tremendous forest fires in the south of Europe have destroyed large ecosystems with serious effects on the tourism sector,” said Friends of the Earth Europe.