In addition to its orbit, the one other thing that has contributed to our salubrious climate is our atmosphere. We have just the right concentration of carbon dioxide in the air to ensure that enough radiated heat remains trapped near the surface to support life. If this balance is tampered with, the atmosphere could either bleed too much heat out to space, thereby freezing the oceans, or trap so much heat near the surface that the oceans boil over. The acceptable upper limit of carbon dioxide in the Earth’s atmosphere is 350 parts per million. Up to that level, ................................................................. If the percentage of carbon dioxide in the atmosphere rises beyond those levels, the atmosphere will trap more and more heat, raising the surface temperature uncomfortably, till the polar ice caps melt and the ocean levels rise significantly.