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The level of carbon dioxide (CO2) in the atmosphere reached a symbolic milestone late last week when it hit historic record levels of 400 parts per million (ppm). Climate scientists warns that the milestone is a wake-up call for people and world leaders as it shows the alarming urgency of reducing our greenhouse gas emissions before it's too late. "Crossing 400 ppm is not a reason for celebration," said Pieter Tans, a scientist with NOAA's Global Monitoring Division, after the latest reading was released from the Mauna Loa Observatory in Hawaii. Because CO2 is the main greenhouse gas contributing to global warming it is imperative that we reduce our CO2 emissions rapidly. But this latest milestone shows the world is moving in the wrong direction. Global CO2 levels have increased since the beginning of the industrial revolution, we passed 300 ppm during early 20th century and since then the rate have increased ever so rapidly. The rate has accelerated since the 1950s from around 0.7 ppm per year to 2.1 ppm per year for the last 10 years. "That increase is not a surprise to scientists," said Tans. "The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration." And once emitted the CO2 stays in the atmosphere for thousands of years, making it more and more difficult to mitigate and adapt to the devastating effects of runaway climate change. According to the National Oceanic and Atmospheric Administration (NOAA), this is the first time in human history that CO2 concentrations in the atmosphere crosses 400 ppm. In fact, the planet haven't experienced these levels of CO2 in millions of years. The last time the level of CO2 was near these levels was during the Miocene Period, about 10 million years ago. Back then the Earth looked completely different from today. Global temperatures then were much hotter. At the poles it was perhaps as much as ten degrees warmer, and the great ice sheets of Greenland and Antarctica had not been formed. Sea level was around 5 to 40 meters higher than today. But "it took nature hundreds of millions of years to change CO2 concentrations through natural processes such as natural carbon burial and volcanic outgassing," Michael Mann, climate change author and director of the Earth System Science Centre at Penn State, told the AFP. "We're unburying it and burning it over a timescale of 100 years, a million times faster." And this rapid speed in which global concentrations of CO2 are increasing is the main concern for climate scientists. "There is no precedent in Earth's history for such an abrupt increase in greenhouse gas concentrations," Mann said. It's easier for living things to adopt to slow changes that take place over tens of millions of years than to adjust to this abrupt change in climate. World leaders want to stop climate from rising further than 2 degrees in global temperatures. A 2C increase in global temperatures will mean that we have to stabilize our greenhouse gas concentrations in the atmosphere at about 445 to 490 ppm. But 350 ppm is the level climate scientists have identified as the safe upper limit for CO2 in our atmosphere. If the current trend is allowed to continue we will not be able to reach neither of these two targets. "We are creating a prehistoric climate in which human societies will face huge and potentially catastrophic risks," Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science, said.