Some argue that the human effect on global warming has not been proven because some scientists don't think so and that even the scientific community does not give 100% certainty to this assertion.
Consider that Lord Kelvin, a prolific contributor thermodynamics, refused to believe that the Earth could be older than his calculations. These were based on the mass of the Earth and the heat loss into space. Even when radioactivity was shown to be a source of heat, and even when he agreed that radioactivity did produce heat, Lord Kelvin stood by his calculations. See Bill Bryson, "A Short History of Nearly Everything". Just because an eminent scientist like Lord Kelvin said the Earth was only 24 million years old didn't make other calculations uncertain or even false.
Some scientists claim that there have always been cycles of warming and cooling and that the Earth is just in another warming time. However, they and their supporters are ignoring the rate of change, which is unprecedented going back over 500 milliion years. Furthermore, the addition of carbon dioxide to the atmosphere had never exceeded 30 parts per million by volume in a thousand years. It has increased by that rate in the last seventeen years. See "Deep ice tells long climate story", a report by BBC News.
