NASA scientist Dr. James Hansen came up with the number 350 PPM after decades of climate research. He first testified that global warming is happening in 1988, and since then he has observed the world, done computer simulations, and studied data regarding ancient climates in order to come up with that number. The second url cited below is a link to part of his statement about global warming and 350 PPM. Overall, 350 PPM is the number we should aim for if we wish to keep earth in the general climate zone that our life and species developed in.
I’m not sure what you mean by “not entirely sure of what we’re doing”. We’re never entirely sure of anything, especially in science.
350 ppm comes from the fact that during ice ages, atmospheric CO2 is around 180 ppm, and during interglacials (warm periods) it’s at around 280 ppm. Atmospheric CO2 hasn’t been above 350 ppm in millions of years.
The 350 ppm comes mainly from looking at past climates and CO2 levels. Scientists like James Hansen who have studied climate changes over tens of millions of years have concluded that anything above 350 ppm will lead to dangerous warming in the long-term. However, this figure is based on slow-acting feedbacks like melting ice. In the short-term, 450 ppm is the target which we don’t want to exceed, and then eventually we want to get the level down to 350 ppm.
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