Since 1998, globally averaged surface temperatures have remained relatively flat, despite continued warming of the climate system and carbon dioxide concentrations reaching a new high of 400 parts per million in 2013. Scientists are debating how and why the global atmosphere seems to be bucking the influence of steadily increasing greenhouse gases. As Kevin Trenberth and John Fasullo, scientists at the National Center for Atmospheric Research (NCAR) point out in a 2013 paper, the climate system’s innate variability and dynamics make this a less-than-surprising reality.
While seasonal fluctuations don’t seem out of the norm, with warmer-, wetter-, or sunnier-than-normal summers occurring one year, and cooler-, drier-, or cloudier-than-normal occurring the next, many people, write Trenberth and Fasullo, expect that human-made climate change will result in seasonal temperatures growing increasingly warm each year. However, natural variability within a dynamic system and people’s experience with seasonal fluctuations should indicate that such an assumption is neither typical nor likely. Over the last several years, scientists including Trenberth, Fasullo, and NCAR colleague Jerry Meehl have used observations and models to show that “pauses” in global atmospheric warming lasting a decade or more can be expected, thanks in large part to the huge role of oceans in modulating Earth’s climate.
Most climate scientists agree that the current warming “hiatus” does not indicate a stalling of the effects of a warming world. Instead, this heating hiatus, a result of natural variability, may be caused by fluctuating patterns linked to both the atmosphere and ocean, like the El Niño/Southern Oscillation that results in El Niño and La Niña events. For instance, the 1998 El Niño event caused notable changes in global weather patterns because heat came out of the oceans – thereby cooling the ocean – and invigorating weather systems, while recent La Niña events have reduced sea surface temperatures, resulting in cooler global average surface temperatures even as the ocean as a whole warms. In other words, the warming of the surface ocean often goes in the opposite direction to the global mean surface temperatures. Additionally, natural events such as volcanic eruptions and reduced solar activity caused by the Sun’s recent quieter-than-average state can cause a reduction in the amount of incoming radiation.
A number of studies, including Trenberth and Fasullo’s, indicate that the excess heat generated by anthropogenic emissions seems to be melting the Arctic sea ice and warming the world’s oceans, with the deep ocean – below 700 meters – currently taking up a third of the excess heat. This appears to be related to the cool (negative) phase of the Pacific Decadal Oscillation (PDO) that has prevailed since the late 1990s. When the PDO is in its cool phase, there tends to be a net storage of heat in the global oceans—similar to El Niño, but on a longer time scale.
The PDO switches from warm to cool (positive to negative) about every 20 to 30 years. Trenberth and Fasullo speculate that the record-strong El Niño of 1997–98 released so much heat from the ocean that the PDO’s switch to a heat-storing negative mode may be having some type of compensating response. They caution that the dynamics that drive shifts in the PDO have not been conclusively determined, and climate models don’t yet seem fully capable of predicting such shifts.
Trenberth believes an El Niño might be the trigger to push the current PDO in the other direction. If this happens, some of the “missing” atmospheric warming may once more be felt, potentially causing global temperature to rise at rates on a par with that experienced during the 1970s to 1990s and pushing global average readings to new record highs. This will likely cause global decision-makers some concern, given that even with the current hiatus in increasing land-surface temperatures, the first decade of the 21st century is the warmest since at least the 1850s, when instruments began regularly and reliably measuring weather phenomenon.
Even in the current global pause, the United States experienced by far the warmest year on record in 2012, accompanied by widespread and costly drought. The evidence suggests that global warming of the planet is continuing, explains Trenberth, it just gets manifested in different ways at times.