The 2000s were Earth’s warmest decade in record keeping, but it wasn’t until 2010 that a single year broke past the mark for warmest year on record, previously set in 1998. In other words, the warming trend had flattened out for a little bit. Why was this?
According to researchers at the National Center for Atmospheric Research (NCAR) and the Bureau of Meteorology in Australia, Earth’s deep oceans at times absorb enough heat to flatten the rate of global warming. In fact, they can do so and affect the global warming for up to a decade at a time.
“We will see global warming go through hiatus periods in the future,” says NCAR’s Gerald Meehl, lead author of the study. “However, these periods would likely last only about a decade or so, and warming would then resume. This study illustrates one reason why global temperatures do not simply rise in a straight line.”
The researchers used the software tool known as the Community Climate System Model, which was developed by scientists at NCAR and the Department of Energy with colleagues at other organizations, to run five simulations of global temperatures based on projections of future greenhouse gas emissions from human activities.
The simulations all indicated that temperatures would continue to rise by several degrees this century, and they all showed that there would be decades where the temperatures would stabilize before climbing again.
One simulation showed the global temperature average rising by 1.4 degrees Celsius between 2000 and 2100, but included two decade-long hiatus periods as well.
During these hiatus periods, the simulations showed that the extra energy entered the deeper layers of the oceans due to changes in the oceanic circulation. According to the simulations, the ocean below 1,000 feet (300 metres) warmed by 18% to 19% more during these hiatus periods than at other times. In comparison, the global oceans above 1,000 feet warmed by 60% less than during the non-hiatus periods in the simulation.
“This study suggests the missing energy has indeed been buried in the ocean,” said NCAR researcher Kevin Trenberth. “The heat has not disappeared, and so it cannot be ignored. It must have consequences.”