[UPDATE: read Bill McKibben's recent blog post, "The Science of 350, the Most Important Number on the Planet"] 

350 parts per million is what many scientists, climate experts, and progressive national governments are now saying is the safe upper limit for CO2 in our atmosphere. Accelerating arctic warming and other early climate impacts have led scientists to conclude that we are already above the safe zone at our current 390ppm, and that unless we are able to rapidly return to 350 ppm this century, we risk reaching tipping points and irreversible impacts such as the melting of the Greenland ice sheet and major methane releases from increased permafrost melt. 

There are three numbers you need to really understand global warming, 275, 390, and 350. For all of human history until about 200 years ago, our atmosphere contained 275 parts per million of carbon dioxide. Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere. 275 ppm CO2 is a useful amount—without some CO2 and other greenhouse gases that trap heat in our atmosphere, our planet would be too cold for humans to inhabit. So we need some carbon in the atmosphere, but the question is how much? Beginning in the 18th century, humans began to burn coal and gas and oil to produce energy and goods. 

The amount of carbon in the atmosphere began to rise, at first slowly and now more quickly. Many of the activities we do every day like turning the lights on, cooking food, or heating or cooling our homes rely on energy sources like coal and oil that emit carbon dioxide and other heat-trapping gases into the atmosphere. We're taking millions of years worth of carbon, stored beneath the earth as fossil fuels, and releasing it into the atmosphere. By now—and this is the second number—the planet has 390 parts per million CO2 – and this number is rising by about 2 parts per million every year. Scientists are now saying that's too much – that number is higher than any time seen in the recorded history of our planet – and we're already beginning to see disastrous impacts on people and places all over the world. Glaciers everywhere are melting and disappearing fast—and they are a source of drinking water for hundreds of millions of people. 

Mosquitoes, who like a warmer world, are spreading into lots of new places, and bringing malaria and dengue fever with them. Drought is becoming much more common, making food harder to grow in many places. Sea levels have begun to rise, and scientists warn that they could go up as much as several meters this century. If that happens, many of the world's cities, island nations, and farmland will be underwater. The oceans are growing more acidic because of the CO2 they are absorbing, which makes it harder for animals like corals and clams to build and maintain their shells and skeletons. Coral reefs could start dissolving at an atmospheric CO2 concentration of 450-500 ppm. These impacts are combining to exacerbate conflicts and security issues in already resource-strapped regions. 

The Arctic is sending us perhaps the clearest message that climate change is occurring much more rapidly than scientists previously thought. In the summer of 2007, sea ice was roughly 39% below the summer average for 1979-2000, a loss of area equal to nearly five United Kingdoms. Many scientists now believe the Arctic will be completely ice free in the summertime between 2011 and 2015, some 80 years ahead of what scientists had predicted just a few years ago. Propelled by the news of these accelerating impacts, some of the world's leading climate scientists have now revised the highest safe level of CO2 to 350 parts per million. 

That's the last number you need to know, and the most important. It's the safety zone for planet earth. As James Hansen of America's National Aeronautics and Space Administration, the first scientist to warn about global warming more than two decades ago, wrote recently, "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm." 

That will be a hard task, but not impossible. We need to stop taking carbon out of the ground and putting it into the air. Above all, that means we need to stop burning so much coal—and start using solar and wind energy and other such sources of renewable energy –while ensuring the Global South a fair chance to develop. If we do, then the earth’s soils and forests will slowly cycle some of that extra carbon out of the atmosphere, and eventually CO2 concentrations will return to a safe level. By decreasing use of other fossil fuels, and improving agricultural and forestry practices around the world, scientists believe we could get back to 350 by mid-century. 

But the longer we remain in the danger zone—above 350—the more likely that we will see disastrous and irreversible climate impacts. [solutions images] Every year since 1992, the United Nations hosts a two-week long conference for world leaders to meet and discuss what to do to about the global threat of climate change. In December of 2009, this meeting will be in Copenhagen, Denmark. There, delegates, non-governmental organizations, and businesses from every nation will meet to finalize a new global climate change agreement. It is crucial that decision-makers at this meeting understand and are held accountable to crafting policy that is informed by the most recent science. Just over a year old, 350 is a relatively new target being discussed in the scientific community, compared to 450ppm or 2 degrees Celsius that the Intergovernmental Panel on Climate Change supports. 

Currently many policy-makers, institutions, and NGOs are still supporting targets that are out of date and greatly increase the risk of catastrophic climatic changes. Yet at the last UN climate negotiations in Poland at the end of 2008, the 350 target began to attract more endorsers as new scientific reports and evidence of early impacts made it clear that we are already above the safe level for CO2. In his annual speech, Nobel laureate Al Gore told delegates to the most recent climate negotiating session that we must now ‘toughen our goal’ to 350ppm. 

At the same meetings, 40 of the most vulnerable nations who will feel the impacts of climate change first and worst, the Alliance of Small Island States (AOSIS) and the Least Developed Countries (LDC’s), included in their policy statements the need to adopt a much stronger target than those currently being debated, and to support a 350ppm target. Said Leon Charles, chair AOSIS, “Two degrees C is really not a safe level for small island states. For many of them it would be like a death sentence in the long run.” It's no small task, but for people and nations everywhere, we need to make sure all of the world’s decision makers pay attention to the most recent science that is telling us 350 is the right target to aim for that can ensure an equitable future safe from climate catastrophe. 

With your help, we can spread this important piece of information to our fellow citizens, communities, countries, and the world. 

For more in-depth information on climate science, policy, and solutions, please see our list of recommended resources below. 

Sources: 

A Safe Operating Space for Humanity. Nature 461, 472-475 (24 September 2009) | doi:10.1038/461472a; Published online 23 September 2009 Hansen, James, et al. 

Target Atmospheric CO2: Where Should Humanity Aim? Submitted April 7, 2008. NASA climate scientist James Hansen's paper about the 350ppm target. Hansen, James, et al. 

Target Atmospheric CO2: Supporting Material. Submitted April 7, 2008. The IPCC 4th Assessment Report – link to the latest report by the Nobel-prize winning United Nations Intergovernmental Panel on Climate Change, supported by the world's leading climatologists. Baer, Paul, Tom Athanasiou and Sivan Kartha. 

"The Right to Develop in a Climate Constrained World: The Greenhouse Development Rights Framework" - an important policy framework for how to mitigate climate change while ensuring an equitable path to development for the Global South. The United Nations Framework Convention on Climate Change - link to the official UNFCCC website with information about the UN climate policy process. 

NASA - scientific reports, interactive maps, resources for kids, and more 

RealClimate.org - a blog of climate science, written by climate scientists Climate Safety - a very useful new report about current climate science, policy, and solutions Pew Center on Climate Change - helpful information about climate science and international policy