Monckton has been described in some quarters as a "former science adviser to British prime minister Margaret Thatcher and a world-renowned scholar." Monckton is critical of the theory of anthropogenic causes for climate change and the stated scope of it, which he regards as a controversy catalyzed by "the need of the international left for a new flag to rally round" following the fall of the Berlin Wall in 1989. He has expressed doubt about the reality of global warming in a number of newspaper articles and papers.
In two Sunday Telegraph articles published in November 2006, Monckton disputed whether global warming is man-made, suggested that it is unlikely to prove catastrophic, and criticized the science presented by the Intergovernmental Panel on Climate Change (IPCC). In particular, he has criticized the IPCC's interpretation of the Medieval Warm Period, cited the "hockey stick" controversy as evidence of faulty science, argued that the science in the IPCC reports has misapplied the Stefan–Boltzmann law, and supported the solar variation theory as a possible explanation of global warming. In an apparent reference to claims made by Gavin Menzies, he further stated "There was little ice at the North Pole: a Chinese naval squadron sailed right round the Arctic in 1421 and found none."
The British writer and environmentalist George Monbiot has criticized Monckton's arguments, labelling them "cherry-picking, downright misrepresentation and pseudo-scientific gibberish." In response, Monckton argued that he "got the science right", claiming that Monbiot got "too many facts wrong" and had shown "ignorance of the elementary physics".
In response to the U.K. government's Stern Review on the Economics of Climate Change, he has argued that the review's recommendation to invest 1% of global GDP in climate change mitigation would be ineffective, as would the introduction of carbon taxes and emissions trading as a means of curbing carbon emissions. He has proposed instead that the best solution should be to "go nuclear and reverse 20th-century deforestation."
In February 2007, he published a critique of the IPCC Fourth Assessment Report on climate change. His calculations of climate sensitivity to increased atmospheric concentrations of carbon dioxide have been published in the Quarterly Economic Bulletin.
Monckton played a key role in a legal challenge heard in the High Court of Justice in October 2007 in a bid to prevent An Inconvenient Truth from being shown in English schools. In an interview with the conservative American talk radio host Glenn Beck, Monckton stated that he had prompted an unnamed friend to fund the case "to fight back against this tide of unscientific freedom-destroying nonsense" and had played a direct role in the litigation against the British government.
In March 2007, Monckton ran a series of advertisements in The New York Times and Washington Post challenging Al Gore to an internationally televised debate on climate change. The former U.S. Vice President did not respond. The Science and Public Policy Institute provided funding for Monckton to produce a response to An Inconvenient Truth, titled Apocalypse?, No!, described as "showing Monckton presenting a slide show in a vitriolic attack on climate change science." The film includes footage of Monckton giving a presentation given on 8 October 2007 at the Cambridge Union in which he asserted that Gore and the IPCC had systematically falsified and exaggerated the evidence for global warming.
During the autumn of 2009, Monckton embarked on a tour of North America to campaign against the December 2009 United Nations Climate Change Conference. His warning that US President Barack Obama intended to sign a treaty at the conference which would "impose a communist world government on the world" was picked up by numerous commentators.