There are some scientists among the critics of the theory that global warming is caused by human activity. For example, Dr. Fred Singer observed that "CO<sub>2</sub> changes have lagged about 800 years behind the temperature changes. Global warming has produced more CO<sub>2</sub>, rather than more CO<sub>2</sub> producing global warming."<ref>S. Fred Singer and Dennis T. Avery, "Unstoppable Global Warming: Every 1,500 Years" (Rowman & Littlefield, 2007)</ref> | There are some scientists among the critics of the theory that global warming is caused by human activity. For example, Dr. Fred Singer observed that "CO<sub>2</sub> changes have lagged about 800 years behind the temperature changes. Global warming has produced more CO<sub>2</sub>, rather than more CO<sub>2</sub> producing global warming."<ref>S. Fred Singer and Dennis T. Avery, "Unstoppable Global Warming: Every 1,500 Years" (Rowman & Littlefield, 2007)</ref> |