Exponential growth of human impacts is the greatest threat to Spaceship Earth. The world GDP is currently growing at close to 4% per annum. To find out how many years that it will take the total global GDP to double at this rate of increase, you simply divide the percentage rate into 70. Thus, if the 4% rate continues, the GDP value of all human activity will only take seventeen and half years. This means a doubling of all the stuff that we produce on Spaceship Earth in this time. It took the whole of human history to arrive at our current loading of the planet! This blog from Jeremy Williams elaborates with two excellent links here. The videotaped lecture by Prof. Albert Bartlett is a remarkable feat of undergraduate teaching and one of the best answers available to the question posed by this post. For a 6 minute video that shows how exponential growth is speeding toward limits to growth on Spaceship Earth see this lesson from Chris Martenson’s “The Crash Course”. This free online course for self-study is well worth watching in either its 45-minute or 3-hour version.