James Hansen, et al., wrote that human activities are altering atmospheric composition. Concern about global warming due to long-lived human-made greenhouse gases - GHGs - led to the United Nations conference with the objective of stabilizing GHGs in the atmosphere at a level preventing dangerous anthropogenic interference with the climate system.
The IPCC and others used several reasons for concern to estimate that global warming of more than 2-3 degrees C may be dangerous. The European Union adopted 2 degrees C above pre-industrial global temperature as a goal to limit human-made warming. Hansen et al. previously argued for a limit of 1 degree C global warming - relative to 2000, 1.7 degree C relative to pre-industrial time, aiming to avoid practically irreversible ice sheet and species loss. This 1 degree C limit, with nominal climate sensitivity of almost 1 degree C per W per m2 and plausible control of other GHGs, implies maximum CO2 of around 450 ppm.
Their current analysis suggests that humanity must aim for an even lower level of GHGs.
Paleoclimate data and ongoing global changes indicate that slow climate feedback processes are not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less. Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is in the pipeline, add urgency to investigation of the dangerous level of GHGs.
A probabilistic analysis previously concluded that the long-term CO2 limit is in the range 300-500 ppm for 25 percent risk tolerance, depending on climate sensitivity and non-CO2 forcings. Stabilizing atmospheric CO2 and climate requires that net CO2 emissions approach zero because of the long lifetime of CO2.
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