Greenhouse gases are the gases present in the earth's atmosphere which reduce the loss of heat into space and therefore contribute to global temperatures through the greenhouse effect. Greenhouse gases are essential to maintaining the temperature of the Earth; without them the planet would be so cold as to be uninhabitable. However, an excess of greenhouse gases can raise the temperature of a planet to lethal levels, as on Venus where the 90 bar partial pressure of carbon dioxide (CO2) contributes to a surface temperature of about 467 °C (872 °F). Greenhouse gases are produced by many natural and industrial processes, which currently result in CO2 levels of 380 ppmv in the atmosphere. Based on ice-core samples and records (see graphs) current levels of CO2 are approximately 100 ppmv higher than during immediately pre-industrial times, when direct human influence was negligible.