| Summary: | 6 p. The concentration of carbon dioxide (CO2) in the atmosphere has achieved its highest levels in the last 800,000 years, and probably even in the last 2.1 million years, recently topping briefly the atmospheric concentration target of 400 ppm. Whereas this mark does not set Earth’s climate in an apocalyptic mode, it does represent a grave global sociopolitical risk, because it highlights the inaction and indifference of government and society to our self-triggered climate changes and their consequences, especially for the poor and the weak. *Since pre-industrial times (i.e. since 1750), atmospheric CO2 concentrations have increased by over 40%, primarily from fossil fuel emissions and decondarily from net land use change emissions, at a rate unprecedented in the last 22,000 years, reaching an average of 2 ppm/ year in the last decade. About 30% of the emitted anthropogenic CO2 has been absorbed by the ocean, causing ocean acidification that poses serious risks to marine ecosystems, resources, and services. *Ice core paleoclimate records teach us that, under typical conditions, global surface temperature never changes much in the long term (of centuries) without a corresponding change in atmospheric CO2 concentration, and vice-versa. In order to explain the amount of warming observed in the temperature records, one must take into account the greenhouse effect caused by the corresponding Atmospheric CO2 concentrations in that period. This does not preclude, however, the occurrence of short-term (decadal) climate variability, which can enhance or counteract the prevailing temperature trend (e.g. the current 15-year hiatus in global temperature rise). *In a business as usual scenario, atmospheric CO2 concentrations by the middle of the 21st century would reach just over 500 ppm, a change of 25% above the present value, which would probably lead to an increase of more than 2ºC in the global mean surface temperature On the other hand, reducing emissions by 2% per year starting no later than 2020 would limit the ...
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