Even by the standards of the super-rich, Charles and David Koch are extraordinarily wealthy. Together they own most of Koch Industries, one of the largest private conglomerates in America with annual revenues of around $100bn (£62.5bn), and interests as diverse as energy, petrochemicals, pulp and paper.
The two brothers share a similar political outlook. They are right-wing libertarians who believe in minimal regulation of industry, smaller government, lower corporate taxes and less generous social services. They are also closet "sceptics" when it comes to climate science.
Charles, 77 and the elder of the two, has effectively run the family business since the death of their father, Fred, in 1967. He is described as a hands-on executive, whereas David has played a more junior role. Nevertheless, the spectacular growth of the family firm has put them both among the richest men in America – and the world.
Each of the Koch brothers has his own charitable foundation and they have given generously to organisations that share their free-market outlook. They have both funded opposition campaigns to many of the policies of the Obama administration – so many, in fact, that their opponents have dubbed their ideological network "The Kochtopus".
Together, the two brothers have given millions of dollars to non-profit organisations that criticise environmental legislation and support lower taxes for industry.
The Kochs have also contributed vast sums to promote scepticism towards climate change, more even than the oil industry according to some estimates. Greenpeace, for instance, has calculated that ExxonMobil spent $8.9m on climate-sceptic groups between 2005 and 2008; over the same period the Koch brothers backed such groups to the tune of nearly $25m.
Americans for Prosperity, a group established by David Koch, did much to foment public outrage over the "climategate" emails stolen from the computers of the University of East Anglia (UEA) in 2009. The organisation claimed the emails proved that global warming was the "biggest hoax the world has ever seen".
Meanwhile, Charles and his wife Liz have established a little-known organisation called the Knowledge and Progress Fund, which appears to have been set up to channel Koch money into a third-party organisation called the Donors Trust.
The Donors Trust is a "donor advised" fund, which means it has special status under the US tax system. One of the benefits of donor-advised funds for billionaires such as Charles is that their names are not linked to funds given out by the Donors Trust to other organisations.
Among the beneficiaries of Donor Trust money is the Competitive Enterprise Institute, which is being sued for defamation by Professor Michael Mann of Pennsylvania University, an eminent climatologist, whose affidavit claims that he was accused of scientific fraud and compared to a convicted child molester.
Professor Mann was one of the scientists named in the stolen emails from the UEA and has been a target of climate sceptics keen to rubbish his work on temperature records, which produced the now famous "hockey stick" graph showing a rapid temperature rise in the 20th century.
According to Robert Brulle, a sociologist at Drexel University in Philadelphia, the Donors Trust and its sister organisation Donors Capital Fund have become key players in the climate "countermovement", which he says is dedicated to maintaining the status quo on energy policy.
For instance, in 2003 Donors received just 3 per cent of the funding that eventually went to climate sceptic groups such as the ultra-conservative Heartland Foundation, which has led the attack on climate scientists. Professor Brulle said this had risen by 2009 to about a quarter of the total funding of the climate countermovement dedicated to denying the link between greenhouse gas emissions from fossil fuels and global warming.
The link between the Koch family and Donors was discovered by Professor Brulle and John Mashey, a retired software engineer living in California, who has trawled through hundreds of official tax records, including the US Inland Revenue Service's "990" forms of the Knowledge and Progress Fund, which mention that it had given Donors millions of dollars for "general support".
"The Kochs decided it was better to go underground and foster all these think-tanks. Charles Koch has often said publicly that anyone should be able to give money to anyone without having to make it public," Dr Mashey said. "The point of all this is that Koch wants to anonymise his giving as much as possible."
Whitney Ball, the chief executive of Donors Trust, said the fund has given more than $400m to about 1,600 beneficiaries since it began in 1999 but that only a few of them are involved in promoting climate scepticism.
"We don't disclose our list of donors, any more than other donor-advised funds… We are not legally required to do so. We have been referred to as a black box but this is a misleading and unfair characterisation. We are no different from any other donor-advised fund," she said.
"There are all kinds of reasons why people want to be anonymous. Imagine you lived a fairly modest lifestyle and you wanted to give charitably. You could do it through a donors-advised fund and no one would treat you differently," she said.
Asked whether the Donors Trust or Donors Capital Fund have any connection with the Koch family, Ms Ball said: "I wouldn't say we have a relationship. We are knowledgeable about some of the groups that they fund and we are ideologically sympathetic."
Asked if she could explain how the money from the Knowledge and Progress Fund was spent, she said: "There's been a lot of hyperventilating about this… Like other donor-advised funds, we don't disclose that information."
The Charles Koch Foundation and Koch Industries declined to comment.
Lawson's sceptics double secret donations in a year
Anonymous donations to Britain's leading climate-change sceptic organisation have more than doubled over the past year – but the registered charity has consistently refused to comment on the identity of its backers.
The Global Warming Policy Foundation, which was set up in 2009 by Lord Lawson, pictured, the former Conservative Chancellor, received £408,641 from anonymous donations in 2012, compared with £158,008 in 2011.
The foundation has repeatedly refused to name its financial supporters, although a leaked email last year suggested that Michael Hintze, a wealthy Tory donor and philanthropist, was one of its main backers.
Benny Peiser, a founder member of the foundation, said the charity does not receive any corporate money from the fossil-fuel industry, but denied it is being secretive about its backers.
"Our concern is about transparency of the science, not the funding. We don't take funding from vested interests and we make sure there is no conflict of interest before accepting donations," Dr Peiser said. "The main reason why they want to remain anonymous is so that people like you [the media] don't harass them," Dr Peiser explained.
From freeze to flood weekend forecast
One last blast of snow is expected over much of Britain today before the present week-long freeze gives way to a thaw and wet weather, which may combine to produce flooding.
Up to a foot of snow may fall over the Scottish Highlands, with up to six inches possible in northern England, four inches in the Midlands and up to two inches in the South.
"A combination of rain and snowmelt over the weekend will increase the risk of flooding, especially in South West England, Wales, the West Midlands and northern England," said the Environment Agency's Phil Rothwell. "We are closely monitoring the situation and have teams ready to respond to any potential flooding."
The recent severe weather has been blamed for at least nine deaths and has caused widespread disruption to travel and schooling.