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 Department 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.