Radiocarbon dating, which is used to calculate the age of certain organic materials, has been found to be unreliable, and sometimes wildly so - a discovery that could upset previous studies on climate change, scientists from China and Germany said in a new paper. Their recent analysis of sediment from the largest freshwater lake in northeast China showed that its carbon clock stopped ticking as early as 30,000 years ago, or nearly half as long as was hitherto thought. As scientists who study earth’s (relatively) modern history rely on this measurement tool to place their findings in the correct time period, the discovery that it is unreliable could put some in a quandary. For instance, remnants of organic matter formerly held up as solid evidence of the most recent, large-scale global warming event some 40,000 years ago may actually date back far earlier to a previous ice age. "The radiocarbon dating technique may significantly underestimate the age of sediment for samples older than 30,000 years,” said the authors of the report from the Chinese Academy of Sciences and Germany’s Leibniz Institute for Applied Geophysics. “Thus it is necessary to pay [special] attention when using such old carbon data for palaeoclimatic or archaeological interpretations," they added. Their work was detailed in a paper in the latest issue of the journal Earth and Planetary Science Letters . For over 50 years, scientists and researchers have relied on carbon dating to find the exact age of organic matter. Prior to that, they had to depend on more rudimentary and imprecise methods, such as counting the number of rings on a cross-section of tree trunk. This all changed in the 1940s when US chemist Willard Libby discovered that carbon-14, a radioactive isotope, could be used to date organic compounds. His theory was that all living creatures have a constant proportion of radioactive and non-radioactive carbons in their body because they keep absorbing these elements from the environment. But as soon as the creature dies it stops absorbing these and sheds any trace of carbon-14 at a decay rate of 50 per cent every 5,700 years. By measuring the remaining amount of carbon-14 in a sample, scientists could estimate the time of death up to 60,000 years ago. Before that, all traces of radiocarbon would be too small to detect. But the method had one major flaw: it didn’t account for changes in the proportion of radioactive and non-radioactive carbon in the environment; and if these had changed, the estimate would most likely be wrong. Many events can affect the levels of carbon-14 in the atmosphere, such as the burning of fossil fuel or the detonation of an atom bomb. In the new study using samples taken from Xingkai Lake near the Sino-Russian border in Heilongjiang province, the scientists used both radiocarbon dating and another method known as optically stimulated luminescence. Using light to measure the amount of free electrons trapped in quartz, the team was able to tell how long the samples had been kept away from sunlight, and therefore estimate when it was that they first fell in the lake. By comparing results from the two methods, they found that carbon dating became unreliable beyond a range of 30,000 years. The great lakes are widely believed to have appeared in China due to the massive melting of ice sheets during an exceptionally warm period some 40,000 years ago, and sediment from Xingkai Lake served as key evidence. But the new study suggests that the sediment might be over 80,000 years old, possibly formed during an ice age. "The carbon-14-based mega-lake hypothesis was even incorporated into modelling work to interpret regional climate dynamics,” the paper reported. “[It] traces its link to atmospheric circulation systems such as the Asian monsoon.” The new finding is important because it aligns with rising concern about the reliability of carbon dating, said Professor Liu Jinyi, specimen curator with the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing. "Many alternative methods to date objects are now available, but carbon dating is still the most popular because we have used it for a long time with such ease and comfort," said Liu, who was not involved in the study. "But the method should be limited to young samples, and more efforts should be made to improve its accuracy," he added.