UN summit on climate change in Lima: can deal be reached?

There are hopes that a global deal to halt climate change will finally be thrashed out over the next two weeks at a United Nations conference in South America, which starts today.

Thousands of diplomats from around the world are gathering in Lima, Peru, in a bid to draft an agreement aimed at stopping the global rise of greenhouse gases. It comes after more than two decades of failed attempts to forge a global pact and a year before a landmark summit on climate change in Paris next year.

Diplomats are said to be feeling upbeat after recent pledges from the US and China to reduce emissions.

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A report by the UN Environment Programme warned earlier this month that industrialised countries were falling short of the emissions reductions needed to prevent warming of 2C above pre-industrial levels. This year looks likely to be the warmest on record, according to officials at the National Oceanic and Atmospheric Administration.

But there are still people, including scientists and politicians, who challenge the evidence used to support the concept of climate change and are concerned about the costs of the policies being introduced to tackle it. On top of this, scandals such as "ClimateGate" have shaken the public's faith in global warming science over the last few years. In US politics, global warming has become even more divisive an issue than abortion, gun control and the death penalty. So can scientists and politicians ever agree?

What are the arguments for climate change?

Many scientists believe the earth is heating up because of human activity. Land, sea and air temperatures are increasing, while sea levels are rising and arctic sea ice, glacial ice and northern hemisphere snow cover is shrinking, they say. A 2007 report from the Intergovernmental Panel on Climate Change (IPCC) stated that "warming of the climate system is unequivocal" and it predicted a global temperature increase of 1.8C to 4C by 2100. The IPCC said that most of the perceived changes in global average temperatures since the mid-20th century are "very likely due to the observed increase in anthropogenic [man-made] greenhouse gas concentrations". Floods, droughts and heat waves pose a growing threat to humans, it said, and the impacts are likely to be "severe, pervasive and irreversible".

What do climate change deniers say?

Some sceptics believe there is not enough long-term historical climate data to determine if the earth is truly warming. Others believe the existing data has been interpreted incorrectly or that any increase in global temperature is down to a natural climate shift rather than due to human activity. Many sceptics believe that global warming is happening but not on such a catastrophic scale as has been claimed.

Is there any consensus?

The percentage of scientists who agree on the dangers of climate change is also a bone of contention. President Obama claimed earlier this year that 97 per cent of scientists agree that climate change is "real, man-made and dangerous". Many mainstream organisations such as the Royal Society, IPCC and the US National Academy of Sciences agree with him that humans are causing the climate to change. But some critics, such as Joseph Bast and Roy Spencer in the Wall Street Journal, claim that this consensus came from only a handful of surveys and abstract-counting exercises that have also been contradicted.

Why is climate change so divisive?

The way the issue is handled will have a huge impact on the global economy. The warnings about climate change are severe. The IPCC says it not only poses a threat to life through heat waves, floods and wildfires, it also threatens political stability by reducing basic food and water supplies. "Nobody on this planet is going to be untouched by the impacts of climate change," said Rajendra Pachauri, chair of the IPCC. The global policy changes required to tackle global warming have broad economic consequences, making it a politically divisive subject. Others warn that the economic implications of ignoring it are potentially much more damaging.

What happened in Climategate?

In 2009, the email server at the University of East Anglia's Climatic Research Unit (CRU) was hacked and more than 1,000 emails from the server, including email exchanges among prominent climate scientists, were published. Critics claimed the emails revealed collusion among scientists to manipulate data in favour of global warming and suppress the activities of deniers. A subsequent investigation debunked these accusations, but it did find a serious lack of openness in the way the scientists worked. The IPCC was also forced to admit that it had mistakenly included a statement in its 2007 report that all Himalayan glaciers could melt entirely by 2035. Only a few further minor errors were subsequently found in the report but the scandal temporarily shook the public's faith in global warming science, reports The Guardian.

Is this over and done with now?

No. Five years on, climate change sceptics continue to claim they are being intimidated by other scientists. Earlier this year Lennart Bengtsson, a research fellow at the University of Reading, claimed his paper was rejected by a top academic journal because the publication was intolerant of dissenting views on climate science – an allegation denied by the publisher. Bengtsson compared the "unbearable" pressure from the climate science community to "McCarthyism".

On the other hand, other scientists and commentators fear their sceptical colleagues are preventing politicians from acting on global warming. For example, President Obama failed to get major climate change proposals through Congress in 2010, with Republicans continuing to deny its existence. The next two weeks will be crucial for negotiators hoping to secure a deal that will commit all countries to take action against climate change.