evidence: How do we know the build-up of carbon dioxide in the atmosphere is caused by humans? [...] During all of the cycles between ice ages and warm periods over the past million years, atmospheric carbon dioxide never climbed higher than 300 parts per million. At the end of the last ice age around 20,000 years ago, it was 280 ppm. Today it is close to 420 ppm. The increase between the year 1800 and today is 70% larger than the increase that occurred when Earth climbed out of the last ice age between 17,500 and 11,500 years ago, and it occurred 100-200 times faster. Only fossil fuels—which are the remains of millions of years of carbon uptake by plants—contain enough carbon to produce such a massive change in such a short time. [...] As carbon dioxide concentrations in the atmosphere have risen over the past century or more, the ratio of carbon-13 to carbon-12 has fallen, which means that the source of the extra carbon dioxide must be enriched in "light" carbon-12. Meanwhile, the relative amount of carbon-14—radioactive carbon—has declined. The record of carbon-14 in the atmosphere is complicated by nuclear bomb testing after 1950, which doubled the amount of radioactive carbon in the atmosphere. After the nuclear test ban treaty in 1963, the excess atmospheric carbon-14 began to decline as it dispersed into the oceans and the land biosphere.