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Amid climate change debates revolving around limited increases in recent global mean surface temperature (GMST) rates, Kevin Trenberth argues that natural climate fluxes - larger than commonly appreciated - can overwhelm background warming, making plateaued rates, or hiatuses, deceiving in significance. After many years of monitoring, it's clear that the GMST can vary from year to year, even decade to decade; these differences, Trenberth argues, are largely a result of internal natural variability. For example, the Pacific Decadal Oscillation (PDO), a phenomenon where the Pacific Ocean goes through periods of warming and cooling, can have a very strong impact on the climate by altering ocean currents, convection, and overturning. The PDO results in more sequestration of heat in the deep ocean during the negative phase of the PDO; therefore GMST tends to stagnate during this negative PDO phase, but increases during the positive phase. Indeed, observations and models show that the PDO is a key player in the two recent hiatus periods. Some other examples of causes behind natural variation include El Niño, volcanic activity, and decreased water vapor in the stratosphere. These natural variations are strong enough to mask steady background warming at any point in time, Trenberth argues. As researchers develop and test climate change models, it's important to expect these variations and plan for them.
Article #7: "Has there been a hiatus?," by K.E. Trenberth at National Center for Atmospheric Research in Boulder, CO.