Atmospheric concentrations of CO2 equivalent (carbon dioxide and other greenhouse gases) reached 430 parts per million last year, compared with 280ppm before the industrial revolution.

ECONOMIST: Getting warmer

The idea was that countries whose emissions fall under the emissions cap -- the permitted level of CO2 equivalent emissions per year -- could then sell those carbon credits to countries who are not able to meet their own caps.

CNN: All About: Forests and carbon trading

Each allowance represents the right to emit one ton of CO2 or CO2-equivalent GHG.

FORBES: The Climate Change Trap

Emissions increased by 0.5% to 392.2kt of carbon dioxide (CO2) equivalent, compared to 390.3kt in 2010.

BBC: Guernsey greenhouse gas levels rise slightly

This is how much temperature will rise given a doubling of atmospheric CO2 (technically, CO2-equivalent, converting all the methane etc to one handy unit).

FORBES: If the MWP Was Global What Does That Tell Us About Climate Change Now?

Stacking up the numbers, he realised that human activities were adding CO2 to the atmosphere in a quantity and rate equivalent to all natural processes of absorption or emission of the gas.

FORBES: Global Warming Is No Threat?

Here's the problem: According to the Clean Air Act, once the EPA regulates greenhouse gas emissions for autos, it must do so for so-called "stationary sources" that emit the equivalent of more than 250 tons of CO2 per year.

FORBES: Regulations

The Gujarat Fluorocarbons plant northwest of Mumbai, India, for example, is designed to eliminate the equivalent of 3.3 million tons of CO2 a year in a high-temperature furnace.

FORBES: Magazine Article

Drax claims that because shipping is so efficient, it creates less CO2 bringing a boat-load of wood from America - even the West Coast - than it does ferrying the equivalent wood by lorry from forests in Scotland.

BBC: Biomass may hinder climate fight