One of George Bush's top scientific advisor has delivered the strongest statement yet from within the US administration that greenhouse gas emissions caused by human activity are to blame for climate change. At the same time, two leading researchers state global warming is an entirely natural phenomenon and its effects can even be beneficial.

The fact that "finally" President George Bush has seen the light and is endorsing Global Warming as a reality, is seen by most as a change for the better - that the US is no longer in self-denial about "the truth", namely that we are destroying our planet. Some argue that the change in opinion is largely due to one of his top scientific advisor, Professor John Marburger, who has said that it is more than 90 percent likely that Mankind is causing global warming and that the earth may become "unlivable" without reductions in CO2 output.
President Bush has invited world leaders to Washington to discuss climate change, and the United Nations will also debate the subject. Bush has so far admitted that human activity may be a contributory factor to climate change, but famously refused to sign the Kyoto protocol that set targets for limiting emissions. Although he has urged Americans to be less dependent on their cars and supported the development of alternative fuels such as ethanol, sceptics remain within the White House about the extent of human influence over rising global temperatures. Furthermore, these politicians realise that if the US is going to address "the problem", they need to do so with extreme care, as otherwise there may be major repercussions for the US economy.

Prof Marburger's views are in line with the Intergovernmental Panel on Climate Change, a body set up by the UN to review scientific research around the world. Describing controls on CO2 emissions as "arbitrary", he nonetheless said: "The CO2 accumulates in the atmosphere and there's no end point, it just gets hotter and hotter, and so at some point it becomes unlivable."
He added that the state of the science made it difficult to justify a particular target - in efforts to explain why the US did not sign the Kyoto agreements. "It's not clear that we'll be in a position to predict the future accurately enough to make policy confidently for a long time," he said.

This further endorsement of the "reality" of global warming coincides with a scientific study, which argues that the recent climate change is not caused by man-made pollution at all. Instead, it is claimed to be part of a 1,500-year cycle of warming and cooling that has happened for the last million years. Going against the dogma that global warming is manmade, these conclusions were immediately labelled as controversial.

Dennis Avery, an environmental economist, and Professor Fred Singer, a physicist, have looked at the work of more than 500 scientists and concluded that it is very doubtful that man-made global warming exists. They also say that temperature increase is actually a good thing, as in the past sudden cool periods have killed twice as many people as warm spells.
Avery, a senior research fellow at the Hudson Institute, an independent US think-tank, said: "Not all of these researchers who doubt man-made climate change would describe themselves as global warming sceptics but the evidence in their studies is there for all to see. Two thousand years of published human histories say that the warm periods were good for people. It was the harsh, unstable Dark Ages and the Little Ice Age that brought bigger storms, untimely frost, widespread famine, plagues and disease."
In contrast, they say there is evidence that wildlife is flourishing in the current warming cycle with corals, trees, birds, mammals and butterflies adapting well. In addition, sea-levels are not rising dramatically and storms and droughts have actually been less severe and frequent.

The authors claim that the change is not man-made because the most recent period of global warming took place between 1850 and 1940 when there were far less CO2 emissions than today. They state their study shows strong historical evidence of an entirely natural cycle based on data of floods on the Nile going back 5,000 years. Evidence is cited showing records of Roman wine production in Britain in the first century AD.
Prof Singer, a specialist in atmospheric physics at the University of Virginia, said: "We have a greenhouse theory with no evidence to support it, except a moderate warming turned into a scare by computer models whose results have never been verified with real-world events. The models only reflect the warming, not its cause."

They also say that natural temperature change can be caused by fluctuations in the sun. The authors spent months analysing scientific reports and papers for their book, "Unstoppable Global Warming: Every 1,500 Years". Their aim was to undermine claims made by Al Gore, the former US vice-president, in his film "An Inconvenient Truth", which shows the extent of man-made global warming.

The current stand-off is of interest, for until recently, it seemed as if science was trying to convince the White House that global warming was man-made and the White House was seen as largely being in the grasp of the oil industry, which was claimed to be instrumental in denying the phenomenon. Today, it seems the situation has reversed, with the White House endorsing "the cause" and some scientists becoming sceptical about the consensus view. Either way, the momentum in the debate has shifted.