Atmospheric Carbon Dioxide
Dangerous Level of Atmospheric Carbon Dioxide
As people have burned fossil fuels, they have overloaded the atmosphere with carbon dioxide (CO2).
Let's examine how much carbon dioxide (CO2) we have already put into the atmosphere.
Atmospheric Carbon Dioxide Over Time
For the last 2 million years the concentration of CO2 has varied from 180 parts per million (ppm) during the ice ages to about 280 ppm in the warm periods. For the last 10,000 years, since the beginning of agriculture, the CO2 concentration in the atmosphere has been very stable at about 280 ppm. In the last 150 years people have burned fossil fuels which has increased the concentration of CO2 to an alarming peak of 391 ppm, a level not seen in millions of years.
Right now lead atmospheric scientists have stated that 300 ppm of atmospheric CO2 is a limit that we should never have crossed. Numbers as high as 325 to 350 ppm are given as an "acceptable" limit for atmospheric CO2 -- if we can scrub the atmosphere of other critical greenhouse agents such as nitrous oxides, CFC's, black carbon, ozone and methane. Limiting these agents is fraught with difficulties.
As we are at 389 ppm atmospheric CO2 and rising 2+ ppm every year -- we have overshot the mark and are at dangerous levels right now. Yikes!
More warming in the pipeline
So why are things relatively OK? We haven't experienced the full extent of the warming that will come with the CO2 we have already put into the atmosphere. The reason for this delay is that it takes time to heat up the oceans.
Anyone who has crept under a blanket has experienced the time delay before the blanket effectively traps your body heat and keeps you warm. By analogy we have applied a CO2 blanket to the earth's atmosphere and we are now accumulating the heat from the sun.
In other words, we have signed up for more heat than we have yet to experience but the blanket is in place.
Worse yet, we are under a CO2 blanket that we cannot easily remove.
Once excess CO2 goes into the atmosphere, about 40% of it remains there for a long, long, time. How long? For many thousands of years. Higher peak concentrations of atmospheric CO2 levels raise the set point to which we can expect to return CO2 in terms of human life spans. The decay curve for elevated atmospheric CO2 has a long, slow tail. This is because the ocean can absorb only so much CO2 before equilibrium with the deep ocean sets in, at which point further lowering of atmospheric CO2 becomes dependent on much slower deep ocean cycles.
It would be extremely foolish of us to push atmospheric levels of CO2 beyond another 10 to 12 parts per million above where it is now. In recent communications (Oct, 2010), Dr. James Hansen has indicated that 400 ppm should be the ceiling for atmospheric levels of CO2. As we are at 389 ppm and increasing at a rate of more than 20+ ppm per decade, we have precious little time left to avoid dangerous climate change. With known and unknown positive feedbacks possible, there is the very real possibility that the process can become runaway. We are playing Russian roulette with the climate with four chambers loaded. We have no margin for error.
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