Group hug.

John McCain popularized this notion during his ill-fated presidential campaign, assuring skeptical conservative audiences that whether or not they believed in climate change, they should support clean energy policy.

The appeal is clear enough: climate change is politically divisive. It’s “environmental” (ew!). It’s associated with Dirty F***ing Hippies (double ew!). If everyone can agree on the same policies for different reasons — national security, energy independence, clean air, whatever — why not just bracket the climate question?

Unfortunately, it doesn’t fly.

The reason is fairly simple: climate change is urgent. Recent research from NOAA demonstrates that irreversible changes are already underway; if humanity continues on its present path those changes will include “irreversible dry-season rainfall reductions in several regions comparable to those of the ‘dust bowl’ era and inexorable sea level rise.” And that’s just if we peak at 600 parts per million of CO2 in the atmosphere. The IPCC says business as usual will lead to 1,000 ppm, which would be an unthinkable apocalypse. Holding atmospheric concentrations to 450 ppm, which many scientists now say offers about a 50/50 chance of avoiding catastrophic and irreversible impacts, means global emissions must peak in the next decade and head down sharply thereafter.

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Climate change sets an unforgiving time limit on the available solutions. It forecloses the option of leisurely conducting basic energy R&D. It forecloses the option of using dirty domestic sources like coal, natural gas, oil shale, and tar sands. It adds problems like reforestation and adaptation to the to-do list. It will require a massive industrial mobilization, on the scale of U.S. preparations for WWII, beginning immediately. No one can understand the challenge facing humanity without understanding climate change. It’s not politically convenient, but it’s the truth.