The planet hit an ominous milestone on Friday: The first day global warming crossed a threshold, albeit briefly, that climatologists warned could have catastrophic consequences. Preliminary data show that global temperatures have averaged more than 2 degrees Celsius (3.6 degrees Fahrenheit) above the historical norm since before humans began consuming fossil fuels and emitting planet-warming greenhouse gases. That doesn't mean efforts to limit global warming have failed—yet. Temperatures would have to exceed the 2-degree benchmark for months and years before scientists would consider it exceeded. But it's a striking reminder that the climate is moving into uncharted territory. Friday marked the first time that daily swings in global temperature norms, which have been steadily rising for decades, pushed the planet beyond a dangerous threshold. It comes after months of record-breaking heat that stunned many scientists and defied some expectations of how quickly temperatures would accelerate this year. (Scott Dance)