It was one of the biggest climate change questions of the early 2000s: Had the planet’s rising fever stalled, even as humans pumped more heat-trapping gases into Earth’s atmosphere?
By the turn of the century, the scientific understanding of climate change was on firm footing. Decades of research showed that carbon dioxide was accumulating in Earth’s atmosphere, thanks to human activities like burning fossil fuels and cutting down carbon-storing forests, and that global temperatures were rising as a result. Yet weather records seemed to show that global warming slowed between around 1998 and 2012. How could that be? After careful study, scientists found the apparent pause to be a hiccup in the data. Earth had, in fact, continued to warm. This hiccup, though, prompted an outsize response from climate skeptics and scientists. It serves as a case study for how public perception shapes what science gets done, for better or worse.
The mystery of what came to be called the “global warming hiatus” arose as scientists built up, year after year, data on the planet’s average surface temperature. Several organizations maintain their own temperature datasets; each relies on observations gathered at weather stations and from ships and buoys around the globe. The actual amount of warming varies from year to year, but overall the trend is going up, and record-hot years are becoming more common. The 1995 Intergovernmental Panel on Climate Change report, for instance, noted that recent years had been among the warmest recorded since 1860.
And then came the powerful El Niño of 1997–1998, a weather pattern that transferred large amounts of heat from the ocean into the atmosphere. The planet’s temperature soared as a result — but then, according to the weather records, it appeared to slacken dramatically. Between 1998 and 2012, the global average surface temperature rose at less than half the rate it did between 1951 and 2012. That didn’t make sense. Global warming should be accelerating over time as people ramp up the rate at which they add heat-trapping gases to the atmosphere.
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