In 1998, the planet's thermometer set a record that still stands. After that, temperature stalled, even as CO2 emissions continued to climb. Why? Scientists decided to look at the tropics, because there the air is clean and well mixed, like water in an aquarium. If CO2 really warms the atmosphere, it should show up there.
They took satellite data from 1979 to 2007 and removed the influence of El Niño (the warm current in the Pacific Ocean) and volcanoes. What remained was a slight rise: 0.062 degrees per decade. That is almost exactly what CO2 alone would produce, without any amplifiers. In other words, carbon dioxide warms the atmosphere directly, but it does not trigger a chain reaction that would accelerate warming.
So it turns out that CO2 is not as powerful a heater as previously thought. Perhaps nature is more complex, and the mechanisms that should amplify warming are actually weaker. This matters for understanding how to manage climate in the future: if CO2 warms weakly, then rushing to cut emissions may not be as critical as some say. But scientists still need to verify this with longer data sets.
The satellite temperature data for the lower troposphere used in the study come from microwave sounders that measure thermal radiation from oxygen at frequencies around 60 GHz. These measurements are so precise that they can detect changes in the average temperature of an atmospheric layer several kilometers thick to hundredths of a degree.
In Michael Crichton's novel State of Fear (2004), the theme of anthropogenic global warming and the politicization of climate science is treated skeptically. This article, showing that the observed tropical trend corresponds only to a weak CO2 effect without feedbacks, echoes the book's plot, which questions the consensus on the dominant role of humans in climate change.