Dyson suggested that philosophers can be broadly, if simplistically, divided into lumpers and splitters. These roughly correspond to Platonists, who regard the world as made up of ideas, and materialists, who imagine it divided into atoms. Dyson agreed that technically humans and additional CO emissions contribute to warming, however, he felt that any such contribution was minor relative to natural effects and that the benefits of additional CO outweighed any associated negative effects. He said that in many ways increased atmospheric carbon dioxide is beneficial, and that it is increasing biological growth, agricultural yields and forests. He believed that existing simulation models of climate change fail to account for some important factors, and that the results thus contain too great a margin of error to reliably predict future trends. Dyson's views on global warming were criticized. Climate scientist James Hansen said that Dyson "doesn't know what he's talking about ... If he's going to wander into something with major consequences for humanity and other life on the planet, then he should first do his homework – which he obviously has not done on global warming." Dyson replied that "[m]y objections to the global warming propaganda are not so much over the technical facts, about which I do not know much, but it's rather against the way those people behave and the kind of intolerance to criticism that a lot of them have." Dyson argued that political efforts to reduce the causes of climate change distract from other global problems that should take priority. Since originally taking interest in climate studies in the 1970s, Dyson suggested that carbon dioxide levels in the atmosphere could be controlled by planting fast-growing trees. He calculated that it would take a trillion trees to remove all carbon from the atmosphere. In a 2014 interview he said, "What I'm convinced of is that we don't understand climate ... It will take a lot of very hard work before that question is settled." Dyson was a member of the academic advisory council of the Global Warming Policy Foundation, a climate sceptic think tank chaired by Nigel Lawson. He compared belief in climate change to religion. At the British Bomber Command, Dyson and colleagues proposed removing two gun turrets from the RAF Lancaster bombers, to cut the catastrophic losses due to German fighters in the Battle of Berlin. A Lancaster without turrets could fly faster and be much more maneuverable. On hearing the news of the bombing of Hiroshima:  In 1967, in his capacity as a military adviser, Dyson wrote an influential paper on the issue of possible US use of tactical nuclear weapons in the Vietnam War.