* Obviously a substance which lingers longer than 100 years in the atmosphere (e.g., carbon dioxide or CFC-13) will exert a GWP worse than the value listed here. But unless we know its rate of decay we cannot exactly quantify the additional harm it does. For instance, CFC-13, which lingers for 400 to 640 years, might show a GWP of nearly [11,700 ×, say, 4] = 46,800 times worse than carbon dioxide’s, or it might fade away much faster. In fact its 500-year GWP is 16,400.