Concentrations of in the atmosphere were as high as 4,000 parts per million (ppm, on a molar basis) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years. Reconstructed temperature records for the last 420 million years indicate that atmospheric concentrations peaked at ~2000 ppm during the Devonian (∼400 Myrs ago) period, and again in the Triassic (220–200 Myrs ago) period. Global annual mean concentration has increased by more than 45% since the start of the Industrial Revolution, from 280 ppm during the 10,000 years up to the mid-18th century to 415 ppm as of May 2019. The present concentration is the highest for 14 million years. The increase has been attributed to human activity, particularly deforestation and the burning of fossil fuels. This increase of and other long-lived greenhouse gases in Earth's atmosphere has produced the current episode of global warming. Between 30% and 40% of the released by humans into the atmosphere dissolves into the oceans, wherein it forms carbonic acid and effects changes in the oceanic pH balance.