The global CO₁ level, rising rapidly due to the burning of fossil fuels, is going to hit 400 parts per million by 2016 according to the National Oceanic and Atmospheric Administration (NOAA). Before the Industrial Revolution of the 1880s, global average CO₁ was about 280 ppm.
NOAA says CO₁ isn't the only problem we face:
Carbon dioxide is not the only greenhouse gas. NOAA calculates the Annual Greenhouse Gas Index every year, which takes into account the heating effects of other gases that are emitted from human activities (e.g., methane, nitrous oxide, and chemicals called chlorofluorocarbons). When those gases are also considered, the global atmosphere reached a CO2 equivalent concentration of 400 ppm in 1985; and 450 ppm in 2003. Atmospheric CO2 levels are currently higher than they have been at any time during the last 800,000 years.
The following animation shows the last 800,000 years of CO₁ levels:
If there are any deniers left out there after seeing this animation, we'd be very surprised.