Currently, the atmospheric carbon dioxide concentration is well over 400 ppm and climbing, an increase of more than 50%

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Currently, the atmospheric carbon dioxide concentration is wellover 400 ppm and climbing, an increase of more than 50% overtypical values of 260 ppm for an interglacial climate like todaywhen glaciers and ice sheets are relatively small. This representsthe highest atmospheric CO2 concentrations seen inat least 800,000 years and probably longer. These levels areunprecedented for a time when the geologic and biologic sinks(removal processes) for carbon dioxide are relatively strong andfavor an atmosphere with low CO2.
Barring an immediate and dramatic change in the types of energywe use and how we use it, an atmospheric carbon dioxideconcentration of 600 ppm will likely be reached by the end of thiscentury. A higher concentration may even be attained if the oceanand/or biosphere become net sources of carbon dioxide as theyrespond to climate change; currently both are net sinks of carbondioxide and have taken up about 1/3 of the carbon released byfossil fuel use.
From a purely scientific point of view, this is the grandestexperiment ever undertaken by human hands: change the compositionof Earth’s atmosphere and see how the climate, oceans, andbiosphere react to that change. We are only just beginning a largechange in the way the Earth system operates in response toenhanced atmospheric carbon dioxide; these changes will continuefor several generations and some will be irreversible. At whatpoint should we end this experiment? How high will we allowatmospheric carbon dioxide to get (and how much change in climate,ocean, and biosphere will we tolerate) before we choose to addressthis issue in a meaningful way? Or, should the "experiment" beallowed to continue? Why?