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."