What is climate change denial?

Climate change denial is a term used to describe attempts to downplay or discredit the extent of global warming, its significance, or its connection to human behavior. The sources of climate change denial are generally associated with the energy lobby, industry advocates and free market think tanks.1
The August 13, 2007 Newsweek cover story "The Truth About Denial" reported that

 "since the late 1980s, this well-coordinated, well-funded campaign by contrarian scientists, free-market think tanks and industry has created a paralyzing fog of doubt around climate change. Through advertisements, op-eds, lobbying and media attention, greenhouse doubters (they hate being called deniers) argued first that the world is not warming; measurements indicating otherwise are flawed, they said. Then they claimed that any warming is natural, not caused by human activities. Now they contend that the looming warming will be minuscule and harmless."

 The article went on to say that individual companies and industry associations—representing petroleum, steel, autos and utilities, among others—formed lobbying groups to enlist greenhouse doubters to "reposition global warming as theory rather than fact," and to sow doubt about climate research just as cigarette makers had about smoking research.
 
One example given in the article points to ExxonMobil as a corporate sponsor of the climate misinformation.

"Warming of the climate system is unequivocal," concluded a report by 600 scientists from governments, academia, green groups and businesses in 40 countries. Worse, there was now at least a 90 percent likelihood that the release of greenhouse gases from the burning of fossil fuels is causing longer droughts, more flood-causing downpours and worse heat waves, way up from earlier studies.
 
A conservative think tank long funded by ExxonMobil had offered scientists $10,000 to write articles undercutting the new report and the computer-based climate models it was based on.

Paul Krugman wrote in one of his NY Times columns last year:

To fully appreciate the irresponsibility and immorality of climate-change denial, you need to know about the grim turn taken by the latest climate research.

Temperature increases on the scale predicted by the M.I.T. researchers and others would create huge disruptions in our lives and our economy. As a recent authoritative U.S. government report points out, by the end of this century New Hampshire may well have the climate of North Carolina today, Illinois may have the climate of East Texas, and across the country extreme, deadly heat waves — the kind that traditionally occur only once in a generation — may become annual or biannual events.
In other words, we’re facing a clear and present danger to our way of life, perhaps even to civilization itself. How can anyone justify failing to act?