What's in a Number? New Carbon Dioxide Level Unseen in Human History

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."

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