Most of what Howard Brown wrote recently about global warming was correct. The theory of anthropogenic global warming has two parts: burning fossil fuels increases carbon dioxide and increasing carbon dioxide increases temperatures. It's correct that ice-core samples show that temperature and carbon dioxide vary naturally and together. However, if carbon dioxide can vary naturally, how do we know that current carbon dioxide change is man-made?
Austrian physicist Hans Suess discovered the answer. A small portion of carbon atoms are a radioactive isotope called carbon-14 with a half-life of 5,730 years. Because carbon-14 is constantly being created in the atmosphere by cosmic rays, even very old carbon dioxide disolved in the ocean will contain carbon-14 atoms.
If you take two cold cans of soda, open both and put one back in the refrigerator, an hour later, the warm one will be flat, but the cold one will be fizzy. This is because cold water absorbs more carbon dioxide than warm water. This means changing global temperatures will cause the ocean to absorb or emit carbon dioxide.
Therefore, if the increase in carbon dioxide were natural, the ratio of carbon-14 to carbon-12 (normal carbon) wouldn't change. However, if the increase in carbon dioxide is man-made, that carbon dioxide will contain no carbon-14 (it's been underground for millions of years) and the ratio will change at a rate proportional to fossil-fuel use. It does, and therefore fossil fuel use is increasing carbon dioxide.
However, there are two problems with using ice-core samples as evidence that increasing carbon dioxide causes warming. First, scientists use heavy water levels in the ice samples as a proxy for temperature. Unfortunately, this only tells us about Artic and Antarctic temperatures, not global temperatures. The larger problem is that temperatures increase first and then carbon dioxide increases. This is inconsistent with the theory of anthropogenic global warming.