May 10, 2013  -- Something happened on Earth Thursday that scientists believe the planet hasn't experienced in as many as three to five million years.

Atop the Mauna Loa Observatory on the Big Island of Hawaii, sensors tracking all the substances swirling around in Earth's atmospheric soup are -- for the first time -- recording carbon dioxide at levels higher than at any time in human history.

Carbon dioxide, shorthanded as "CO2," is the primary greenhouse gas emitted into the air when people burn fossil fuels. It traps heat in the atmosphere that would normally radiate back into space. Simply put, the more carbon dioxide, the hotter we get.

Scientists express the level of carbon dioxide according to how many molecules of CO2 are floating around in a million molecules of air -- the "parts per million" measurement, or "ppm."

On Thursday, CO2 in the atmosphere surpassed a new milestone of 400.03 ppm, according to the National Oceanic and Atmospheric Administration. The Scripps Institution of Oceanography, which also tracks the number, recorded a slightly lower number of 399.73.

The fact that CO2 levels have hit 400 ppm comes as no surprise to scientists who have watched the number steadily climb since around 1780, the start of the Industrial Revolution. Back then, CO2 levels were about 280 ppm. By 1958, when scientist Charles Keeling began measuring CO2 on Mauna Loa (known today as the "Keeling Curve"), the number had risen to 317 ppm.

Scientists have long predicted that an unprecedented period of human prosperity built on the use of oil and coal would come with dark side effects for the climate.

"This generation has altered the composition of the atmosphere on a global scale," President Lyndon Johnson told Congress in 1965, through "a steady increase in carbon dioxide from the burning of fossil fuels."

What worries scientists in 2013 is not only the amount of CO2 in the atmosphere, but how fast it continues to build up without showing any sign of slowing or even stabilizing.

Today's rate of carbon dioxide increase is more than 100 times faster than the increase that occurred when the last ice age ended, NOAA said today.

At this rate, even 400 ppm will soon vanish in the rearview mirror. Unless emissions are slowed, scientists tell us that babies being born today will enter their thirties as the CO2 level reaches 450 ppm.

Without a concerted worldwide effort to curb greenhouse gas emissions, scientists predict global warming will produce a cockeyed climate concoction laced with increasing numbers of heat waves, melting glaciers, higher sea levels and more extreme weather.

ABC News asked climate scientists, experts and historians to weigh in, in their own words, with their thoughts on the 400 ppm milestone and what it means for the rest of us:

Waleed Abdalati, Director, Earth Science and Observation Center, Cooperative Institute for Research in Environmental Sciences

"Four-hundred parts per million. On the face of it, it is just a number, not much different from 375 of a decade ago or the 425 we can expect 10 years from now. The true significance is not in the number itself, but in the fact that this point lies on a disturbing trajectory. One has to ask: On a trajectory to what? This level of CO2 -- much of which has been introduced by human activity -- is solidly outside of the naturally occurring bounds of nearly the last million years, and it is only getting worse. So this nice round number is significant, not for what it is, but for what it says about what will be. There is an opportunity here to give this number another significance, by turning into a milestone in which awareness of the trajectory is recognized and meaningfully addressed. Whether or not this is the case remains to be seen."

Richard C. J. Somerville, Distinguished Professor Emeritus and Research Professor, Scripps Institution of Oceanography, University of California San Diego

"Today we are watching human-caused climate change occur. It is not a problem for the future. It is happening here and now. Melting Arctic sea ice and rising sea level are examples. Extreme weather events today occur in a changed environment. For example, Hurricane Sandy, which killed hundreds of people and caused some 75 billion dollars in property damage, occurred in a climate with higher ocean temperatures and more water vapor in the air than only a few decades ago. The heat-trapping gases and particles that humanity emits into the atmosphere increase the odds of severe weather events, just as steroids taken by a baseball player can increase the odds of home runs. Today we are seeing climate change on steroids. To limit global warming to moderate or tolerable amounts, the entire world must act quickly to reduce these emissions. That we have failed to do this is a tragedy."

Dr. George Luber, Associate Director for Climate and Health, Centers for Disease Control and Prevention

"Climate change is already impacting health in the United States. Wildfires and extreme weather events are becoming more frequent, air pollutants and allergens are becoming more concentrated, and the habitats for disease-carrying rodents and insects are shifting. We can manage the unavoidable, by adapting to the effects that are already occurring, but we also need to avoid the unmanageable, by preventing the most severe impacts. As climate-related threats increase, our opportunities to adapt may be limited. Acting now to prevent the worst impacts of climate change will have the most health benefits. Responding to climate change now will have real impacts for the health of people in the United States. There are strategies that can improve well-being in other ways too. For instance, generating electricity from renewable sources of energy can reduce air pollution, and encouraging alternative means of transportation, such as bicycling, can increase social interaction and physical activity."

Kevin Trenberth, Distinguished Senior Scientist, Climate Analysis Section, National Center for Atmospheric Research

"Carbon dioxide concentrations are up some 43 percent from 280 ppm just over 100 years ago. Moreover, more than 60 percent of the increase has occurred since 1970 and attempts to slow the rate of rise, like the Kyoto Protocol, have failed. It is a sign of a flourishing industrialized economy and human well-being that is unprecedented. But it has unintended consequences in air quality and climate change. It provides a strong sign of our inability to control excesses that provide threats not so evident now but in the future. Yet in 2012 the U.S. experienced a taste with the enhanced drought, heat waves and wildfires. Superstorm Sandy was another harbinger of expected stronger storms and deluges. The often-unrecognized, tremendous costs already present rain down on the innocent, and we all suffer the consequences. There are too many people with too big an environmental footprint effectively stomping on the future generations!"

Naomi Oreskes, Professor of History and Science Studies, University of California San Diego

"In 1965, atmospheric carbon dioxide stood at 320 ppm -- only 10 percent higher than the pre-industrial value. But President Johnson knew that it was cause for concern because a group of scientists, led by Charles Keeling, had explained that rising CO2 would mean a warmer world in which droughts and heat waves could threaten food supplies, and higher sea level could inundate cities and infrastructure around the globe. Keeling and his colleagues thought these effects would become be evident by the end of the 20th century. Here we are, well into the 21st century, CO2 is about to pass the 400 ppm mark, and drought, heat waves, and rising seas are all underway. Isn't it past time we did something about it?"

Veerabhadran Ramanathan, Distinguished Professor of Climate and Atmospheric Sciences, Scripps Institution of Oceanography, University of California San Diego

"It is still not too late. If we can stabilize CO2 concentration below 450 ppm and reduce short-lived warming pollutants such as methane, black carbon and ozone in the lower atmosphere and HFCs making maximum use of available technologies, we can cut the rate of warming during this century by two-thirds and sea-level rise from now until 2100 by one-third. By so doing, we can also save about 4.5 million lives that are now lost to indoor smoke and outdoor air pollution and as much as 100 million tons of crops lost to ozone pollution. This would be the best legacy we can give to future generations and we are fortunate we still have time to accomplish this."

Richard Norris, Professor of Paleobiology and Curator, SIO Geological Collections, Scripps Institution of Oceanography

"The last time Earth saw 400 ppm CO2 in the atmosphere about 3-4 million years back, there were camels and forests in the Arctic, the tropics were locked in a near constant el Nino -- the kind that typically floods the western U.S. -- and large expanses of the U.S. East Coast, Florida and the Gulf States were underwater. Still, we are not going to 'go back to the future' overnight since it takes millennia to melt the ice that would raise global sea levels, but 400 ppm is milestone on the road to a constantly changing world for the foreseeable future. Settled human society has grown up in a remarkably stable period in the last 8,000-9,000 years. Archaeologists note that people started to grow crops as soon as the climate was constant enough to have some assurance we could make a harvest the next year. But, beyond 400 ppm is a shift into heat waves, deep droughts and torrential rains, and a generally less-predictable world of "Snowpocalypse," "Hurricane Sandy" and Texas-sized fires. The worst of it is that we are already committed to several thousand years of unsettled climate thanks to our approximately about 150 years of carbon pollution. But, if we do nothing, CO2 emissions will hit approximately 1,000 ppm by the end of the century -- a scenario that results in more than 10,000 years of radically changing climate. Tell your kids not to go into farming, disaster insurance or beachfront real estate since these, and many other jobs, are going to be increasingly a way to lose your shirt."

Andrew Gettelman, Climate Scientist, National Center for Atmospheric Research

"The carbon dioxide concentration in air tells two stories. It varies like a clock, rhythmically marking the annual cycle of the biosphere (plants) breathing in CO2 in the spring, and exhaling in the fall. There is also a general upward tick from year to year. The trend is from society exhaling CO2 as the waste product of energy and agriculture: it feeds us, heats us and moves us. We borrow CO2 from the past (fossil fuels), and give it to the future (in the atmosphere). The laws of physics say that has consequences. Warmer temperatures on average. Shifts in rain patterns. Extreme wet and dry conditions we have not seen in recorded climate. We can choose how much CO2 we emit. As we slowly push it up, we could slowly push it down. Someday, our choices will be recorded in the CO2 record itself. Where will the trend go? How hard will the planet be breathing? That is the story we continue to write into the record."

Jeffrey Kiehl, Senior Scientist, National Center for Atmospheric Research

"In 1978, when I began my graduate career in climate science, atmospheric carbon dioxide levels were around 335 ppm. I was working on a simple climate model to study the effects of a doubling of carbon dioxide on temperature. Scientists did this thought experiment to gauge the sensitivity of Earth's climate, as is still done today. Back then, most of us viewed this as a theoretical problem. Clearly, we would be off of fossil fuels in 30 or 40 years given all the talk of nuclear and solar power. After all, fossil fuels were going to run out. In the very near future we will cross the 400 ppm level hurtling our way towards 560 ppm (a doubling of the pre-industrial levels). Continuing on our current energy path will lead to projected carbon dioxide levels of around 1,000 ppm by 2100, a mere 90 years in the future. At that point, we will have returned Earth's carbon dioxide levels to that of the extremely warm Eocene a time more than 30 million years ago, in which there were no large ice sheets. We now realize that the increase in carbon dioxide is not just a theoretical problem, but one of immense importance for the well-being of all life on Earth. Do we now have the wisdom to avoid a return to Earth's deep warm past?"

Prashant Sardeshmukh, Senior Research Scientist, Cooperative Institute for Research in Environmental Sciences, University of Colorado

"Reaching the 400 parts per million milestone for carbon dioxide should not by itself affect our general concern about rising carbon dioxide levels in the atmosphere. It would be cause for special alarm if 400 ppm were a climate tipping point or close to a tipping point, but no evidence exists for such a claim. Still, something's gotta give as carbon dioxide levels continue to rise. And if that something makes the occurrence of an ice-free Arctic or a mega drought or a monster Nino or a jumbo storm virtually certain in the next several decades, then a tipping point with serious consequences would indeed have been reached. It's rather like a Chinese water torture, not knowing precisely when or even if this will happen. Both natural climate variability and climate modeling errors, not to mention uncertainty in future climate forcing, prevent us from answering the question at present."

Michael Mann, Director, Earth System Science Center, Pennsylvania State University

"The soon-to-be-breached atmospheric CO2 concentration of 400 ppm has important symbolic significance. It is a reminder of just how uncontrolled this dangerous experiment we're playing w/ the planet really is. The last time we're confident that CO2 was sustained at these levels is more than 10 million years ago, during the middle of the Miocene period. This was a time when global temperatures were substantially warmer than today, and there was very little ice around anywhere on the planet. And so sea level was considerably higher -- around 100 feet higher -- than it is today. It is for this reason that some climate scientists, like James Hansen, have argued that even current-day CO2 levels are too high. There is the possibility that we've already breached the threshold of truly dangerous human influence on our climate and planet."

James Butler, Director, Global Monitoring Division, NOAA Earth System Research Laboratory

"Atmospheric carbon dioxide (CO2) reaching 400 ppm at Mauna Loa is not in itself a significant event. It is, however, a noteworthy marker of what is significant -- the accelerating growth and persistence of CO2 in the atmosphere. CO2, far and away the dominant greenhouse gas emitted by humans, is responsible in good part for recent climate change, and, once emitted, will remain in the ocean-atmosphere system for thousands of years, warming the planet, changing climate, and driving acidification of the oceans. Atmospheric CO2 has been about 280 ppm through almost all of human civilization, yet, primarily in the past century, humans have driven it up to around 400 ppm mainly by burning fossil fuels. These emissions continue to accelerate unabatedly. Last year, NOAA reported that CO2 at all arctic sites reached 400 ppm for the first time. This month, the iconic Mauna Loa site reports 400 ppm, with the global average not far behind. South Pole, the last to reach this marker, will do so in 4-5 years."

Pieter Tans, Climate Scientist, NOAA Earth System Research Laboratory

The modern measurements of atmospheric CO2 started in 1958 near the summit of Mauna Loa, Hawaii. The station is still the best known global "benchmark station" because the site was very carefully chosen to be representative of most of the northern hemisphere. The round number of 400 ppm can be seen as a milestone. It reminds us that CO2 is now higher than in the last several million years. In addition, the rate of increase, over the last decade more than 2 ppm per year, is at least 100 times faster than what has been observed in ice cores over the last 800,000 years. We know for a fact that today's CO2 increase is entirely due to human activities, especially the burning of coal, oil and natural gas.

Mark Serreze, Director, National Snow and Ice Data Center

"It seems that we've made our decision regarding what to do about climate change. We are going to do nothing and hope that the effects will be manageable. The history of humanity is filled with shortsighted, illogical choices. We can now add another one to our quiver."

Robert Corell, Principal, Global Environment and Technology Foundation Center for Energy and Climate Solutions

"For the past 800,000 years, the maximum average concentration of C02 in the atmosphere has been about 270 ppm, but with fossil fuels use accelerating, CO2 concentration will now exceed 400 ppm. The CO2 levels in the atmosphere will fly through 400 ppm towards 500 ppm by 2060, leading to serious food shortages for the poorest of the poor, water scarcity for half of the world and extreme floods and droughts. At current rates of fossil fuel emissions, the CO2 levels in the atmosphere will double over the maximum level of CO2 in the atmosphere experienced during the past 800,000 years with devastating consequences. At current rates of fossil fuel emissions, sea level globally will very likely rise 1.5 feet by 2050 and global average surface temperatures will pass the international target of 2 degrees centigrade by 2050."