Where are we in understanding the early-2000s hiatus of global warming? (Core Science Lecture)

Since 2000 there has been only a very small positive linear trend in globally averaged surface air temperature. This has been termed the hiatus (Meehl et al., Nature Climate Change, 2011) or more generally the “early-2000s hiatus”, and has prompted some to say that global warming has stopped. The hiatus has raised more general and compelling science questions involving the nature of the relationship between internally generated and externally forced climate variability and change, and has prompted an intense research focus on the hiatus. What is known so far is that the small linear trend of globally averaged surface air temperature since 2000 is not unprecedented in either observations or climate model simulations. Analyses from models and observations have indicated that during such hiatus periods when greenhouse gases continue to increase and trap heat in the system, this excess heat is sequestered in the deeper layers of the ocean, and ocean mixing processes in three areas of the world oceans are likely involved (the subtropical Pacific, near Antarctica, and in the North Atlantic). The negative phase of the Interdecadal Pacific Oscillation (IPO) characterizes hiatus periods and drives not only ocean mixing processes in the tropical and subtropical Pacific but also the signal of globally averaged surface air temperature. The negative phase of the IPO can be simulated for the early-2000s hiatus in initialized decadal climate prediction experiments, thus pointing to the possibility that this technique could be used to provide information regarding when the hiatus may end.