According to Jim Howell (2012 Quivira Conference speaker), one quarter to one half of the carbon that is currently being added to the atmosphere is due to industrial agriculture’s poor land use alone. Leaving land bare allows carbon from the soil to bond with oxygen in the air, creating carbon dioxide (CO2). This would not happen in nature, as there would always be plants covering the ground, protecting the soil carbon. CO2 is a greenhouse gas, which actively traps heat from the sun on the planet. We need some CO2 in the atmosphere to keep our planet warmer than the vacuum of space, but humans have exponentially increased atmospheric CO2 by burning fossil fuels, which are made of ancient atmospheric carbon stored up under the ground for millions of years. By burning fossil fuels, we have released so much CO2 that it has fundamentally changed the composition of our atmosphere. Scientists say that if we reach 450 ppm (parts per million) of CO2 in our atmosphere we will start an irreversible chain reaction of global warming, and our environment cannot sustain long term CO2 levels above 350 ppm. There are currently 394 ppm of CO2 in our atmosphere, which is the highest amount our planet has seen for at least 800,000 years, and possibly the highest earth has seen in the last 20 million years. At the moment, we are increasing atmospheric CO2 on an average of 2 ppm per year. At this rate, we will reach 450 ppm in 28 years. If we wish to continue living on planet Earth we must begin to not only reduce the CO2 we are releasing into the atmosphere, but we must reverse this process by sequestering atmospheric CO2. Building soil carbon actively sequesters carbon from the atmosphere by absorbing CO2 via plant photosynthesis. According to Fred Provenza, 2012 Quivira Conference Speaker, Every ton of humus (soil organic matter) created removes 3.76 tons of CO2 from the atmosphere.