The scientists, in the Netherlands, Switzerland, Denmark, the United States and France, noted a second rise in methane in Medieval times, coinciding with a warm period from 800 to 1200 that also saw Europe's economy emerge from the Dark Ages. That spike might be because population growth in Asia and Europe led to more deforestation for farming. Rates then fell, perhaps partly because factors such as the Black Death cut the population.

A record of the atmosphere trapped in Greenland's ice found the level of heat-trapping methane rose about 2,000 years ago and stayed at that higher level for about two centuries. Methane was probably released during deforestation to clear land for farming and from the use of charcoal as fuel, for instance, to smelt metal to make weapons, lead author Celia Sapart of Utrecht University in the Netherlands told Reuters. "Per capita, they were already emitting quite a lot in the Roman Empire and Han Dynasty," she said of the findings by an international team of scientists in Thursday's edition of the journal Nature. Rates of deforestation "show a decrease around AD 200, which is related to drastic population declines in China and Europe following the fall of the Han Dynasty and the decline of the Roman Empire," the scientists wrote. Mankind's emissions 2,000 years ago, when the world population was an estimated 300 million, were discernible but tiny compared with current levels caused by a population of seven billion. Sapart estimated that methane emissions until 1800 were about 10 per cent of the total for the past 2,000 years, with 90 per cent since the Industrial Revolution. Methane is generated from human sources including burning of forests and fossil fuels, rice paddies, livestock or landfills. Natural sources include wetlands, wildfires or mud volcanoes. The findings by Sapart's team questioned the view by a UN panel of climate scientists that man-made climate change started with the surge in use of fossil fuels during the Industrial Revolution.

One reason philosophy has been less successful than other disciplines in overcoming these biases could be that the subject’s self-image actually makes it more vulnerable. “Philosophers have this special relationship with objectivity,” says Saul, “where we think that we’re better and more rational than everyone else. It’s very well confirmed that people are really bad at judging their own objectivity and systematically over-estimate it. But really interestingly, it’s also been shown that thinking about how objective one is increases bias rather than decreases it. If you form the explicit intention to be unbiased and not affected by gender and race, you will be more affected by these things.”

Philosophy has also been peculiarly indifferent to feminist thinking. “We are the anomaly in the humanities,” says Haslanger. “You can’t go through a graduate programme [in other humanities subjects] and be considered competent in those fields unless you’ve done some work on gender and race issues. Feminist work is mainstream. In philosophy that’s just not true. You could go through a philosophy degree to this day and never have a class by a woman, never have to encounter anything having to do with feminism or gender or race.”

One reason for this is that “many philosophers have the view that in order to be objective you have to be value-free or value-neutral and feminism is by its nature not value-free or value-neutral. So there are a lot of philosophers to this day who think there’s an inherent contradiction in the idea of feminist philosophy or feminist epistemology.