Within the next few days, the atmospheric concentration of carbon dioxide is likely to hit 400 parts per million – a record high since the evolution of modern humans. As far as we know, CO2 concentrations have not been at such levels since the Pliocene some 3-5 million years ago – a period that was considerably warmer than today.

We have been in the 390s for a while, and although 400ppm is certainly worse, the world won’t plunge into chaos in May because we crossed that threshold. But that doesn’t make 400ppm just a symbolic level, argues SEI Executive Director Johan L. Kuylenstierna.

…it is a point along a continuum that we know leads to ever-worse consequences, with potentially irreversible impacts on water resources, biodiversity, ecosystems, and other key aspects of our planet’s functioning. And, as always, impacts on humans will be worst for those with the least capacity to deal with changes.

Hitting 400ppm also reminds us of the failure of the international community to reach an agreement that would permanently bend the emissions curve in the right direction. … Of course, we need to respect the complexity of the negotiations; after all, it involves the core of our development, including tomorrow’s energy systems, our consumption patterns, global trade and other fundamental aspects of our societies and lives. Still, this new CO2 record should add to the sense of urgency – making it clear that climate systems cannot be put on hold while we make a new agreement.

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