In my debates with climate sceptics, they like to throw out numbers like 1,000 – 1,200 ppm as being good for the planet in that plants thrive on increased CO2 levels in greenhouses so why shouldn’t they do so in the natural environment.
As such, I decided to do some of my own research and found this very good article in Scientific American that gives a nice succinct overview from well-respected scientists who state the figures that they believe need to be achieved for our future well-being.
To quote directly from the article:
“… how much heating and added CO2 are safe for human civilization remains a judgment call. European politicians have agreed that global average temperatures should not rise more than two degrees C above preindustrial levels by 2100, which equals a greenhouse gas concentration of roughly 450 ppm. “We’re at 387 now, and we’re going up at 2 ppm per year,” says geochemist Wallace Broecker of Columbia University. “That means 450 is only 30 years away. We’d be lucky if we could stop at 550.”
Goddard’s James Hansen argues that atmospheric concentrations must be brought back to 350 ppm or lower—quickly. “Two degrees Celsius [of warming] is a guaranteed disaster,” he says, noting the accelerating impacts that have manifested in recent years. “If you want some of these things to stop changing—for example, the melting of Arctic sea ice—what you would need to do is restore the planet’s energy balance.”
Other scientists, such as physicist Myles Allen of the University of Oxford, examine the problem from the opposite side: How much more CO2 can the atmosphere safely hold? To keep warming below two degrees C, humanity can afford to put one trillion metric tons of CO2 in the atmosphere by 2050, according to Allen and his team—and humans have already emitted half that. Put another way, only one quarter of remaining known coal, oil and natural gas deposits can be burned. “To solve the problem, we need to eliminate net emissions of CO2 entirely,” Allen says. “Emissions need to fall by 2 to 2.5 percent per year from now on.”
Climate scientist Jon Foley of the University of Minnesota, who is part of a team that defined safe limits for 10 planetary systems, including climate, argues for erring on the side of caution. He observes that “conservation of mass tells us if we only want the bathtub so high either we turn down the faucet a lot or make sure the drain is bigger. An 80 percent reduction [in CO2 by 2050] is about the only path we go down to achieve that kind of stabilization.”
The full article can be viewed here