Why is it 350?

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To see the origins of ‘350’ ppm see the following paper:

Target Atmospheric CO2: Where Should Humanity Aim?- December 2008, James Hansen et al, Goddard Institute for Space Studies

and a quote:

Atmospheric carbon dioxide is already 385 parts per million (ppm) [as of Dec 2008] and it is increasing by about 2 ppm each year as a result of the burning of fossil fuels (coal, oil, and gas), with a smaller contribution from burning of forests. The authors use evidence of how the Earth responded to past changes of CO2 and on-going climate changes to show that atmospheric CO2 has already entered the dangerous zone.
The authors suggest that global policies should have an initial target for atmospheric CO2 of 350 ppm. They note that the optimum CO2 level for maintaining a planet similar to that on which civilization developed is likely to be less than 350 ppm, but a 350 ppm target already reveals that dramatic policy changes are needed urgently. By the time such fundamental changes are achieved, knowledge will exist to help fine-tune the target CO2.

As of August 2019 we were at 415ppm.

Where is it measured?

NOAA Global Monitoring Laboratory reports that CO2 measurements are made at the Mauna Loa Observatory, Hawaii. It's called a background CO2 and is reliable because:

  • The Observatory near the summit of Mauna Loa, at an altitude of 3400 m, is well situated to measure air masses that are representative of very large areas.
  • All of the measurements are rigorously and very frequently calibrated.
  • Ongoing comparisons of independent measurements at the same site allow an estimate of the accuracy, which is generally better than 0.2 ppm.