If you want a challenge, predicting the fate of the world’s climate is a good place to start. The science involves massive systems that are well understood on small scales, but much more difficult to predict at global scales.

Take the carbon dioxide (CO2) that rings our planet. We know two things for certain. First, the recent increase in CO2 is from humans burning fossil fuels (we can recognize its isotopic fingerprint in the atmosphere). Second, CO2 absorbs the sun’s radiation and releases it as heat, some of it straight back to the surface of our planet.

Everything that comes next is more complicated. First, there’s the effect of the oceans, which absorb heat and release it later, causing the cycle of global warming to lag. This calculation is educated guesswork, but scientists believe the ocean effect has already locked in a decade of increasing temperatures and melting ice, even if all carbon emissions were to end today.

But that’s only the beginning. As the heat climbs, feedback loops kick in that can take changes from mild to massive. Will melting permafrost release huge amounts of methane, itself a potent greenhouse gas? Will temperature increases sicken forests, de-greening the planet? Will ocean currents shift, bringing less cool water to the surface or reducing the ocean’s ability to absorb carbon from the air? What about the heat-trapping effect of increased water vapor entering the air as oceans warm? The loss of reflective ice surfaces that deflect the sun’s radiance at our poles? The possibility of air currents changing and clouds dispersing, reducing their sun-filtering effect? Any of these changes can amplify what we’ve already done ourselves — and they’ve occurred before, on geological timescales throughout the past of our planet.

The fixed-year limit to save the planet was never a scientific concept.

You can see why climate researchers were alarmed. They stared into the face of a challenge with civilization-altering possibilities, but one that moved slowly and with just enough of a delay that ordinary people could ignore it. They knew that expressions of concern weren’t enough to motivate anyone. Their solution was to try to inspire action with speculative deadlines.