When monitoring of atmospheric CO₂ began in 1957, its concentration stood at 315 parts per million by volume. Before the Industrial Revolution, it is estimated that the atmospheric CO₂ concentration was 280. In May of 2013, it reached 400. There is now a strong consensus among climate scientists that increased concentrations of CO₂ in the atmosphere cause warming both on land and in the ocean. There is also a consensus that this warming will lead to serious environmental problems, such as rising sea levels and extreme weather events.