"Hiatus" in Global Warming: Paradox and Complexity in Climate Science

The strong El Nino event of 1998 marked a transition from the persistent growth in global temperature that had prevailed since the mid-1970s to slower irregular growth since the year 2000. This slow-down has been called a "hiatus." The slowdown occurred even though the growth in greenhouse gas concentrations accelerated after 1998. This apparent paradox has made the hiatus one of the principal public arguments against action on climate change. Two other apparent paradoxes exist. During the hiatus, when the global temperature was close to constant, the Arctic region was warming and sea ice, land ice, and snow cover were retreating faster than before. Furthermore extreme weather events were increasing in number and intensity. In Part 1, this triple paradox is described, as well as the confusion sown in the public mind by the climate's apparently contrary behavior since 1998. In Part 2, the mechanisms to explain the change in surface warming rate that were re-examined and discarded by the climate science community are discussed. The remaining and best explanation is that the oceans began to bury the expected additional warming at depths inaccessible to the atmosphere around the year 2000 (Part 3). Since then, the Pacific Ocean has been in a prolonged cool state a La Nina that suppresses the global temperature, and La Nina-like weather patterns have prevailed around the world. In addition, around 2004 the mid-Atlantic region and the Southern Ocean also began to route ocean heat to depth. Part 4 presents evidence that persistent retreat of Arctic sea ice began when the slowdown in temperature growth did; the retreat of sea ice and snow cover is directly related to the acceleration in the rate of Arctic warming. The rapid local warming has consequences for the global climate, one of which might be changes in the equator-to-pole atmospheric heat transport that can affect the frequency, location, and duration of extreme events at mid-latitudes. Part 5 presents evidence that the frequency of extreme weather events heat waves especially has increased at temperate latitudes since the hiatus started. There is, therefore, circumstantial evidence, if no rigorous proof, that all three paradoxes are related: they are indicators of a general reconfiguration of the climate system. In Part 6, statistical "teleconnections" between weather patterns at the equator and poles are described, as is the global heat flow system that is thought to account for them. Part 7 summarizes the evidence indicating that the climate switched to a new state at hiatus onset. Part 8 explores the idea that the unexpectedly rapid sea ice retreat may be changing the thermal boundary conditions governing the equator-to-pole circulation of heat in the atmosphere, forcing the observed persistent La Nina bias, as well as the cyclone-anticlycone systems that are responsible for extreme events at mid-latitudes. Arctic warming also modifies the boundary conditions governing ocean heat transport and sequestration in the North Atlantic. A new metastable state that balances Arctic and greenhouse warming with forced ocean cooling might have been created. In Part 9, the implications for public communication and policy are addressed, particularly how global temperature is a misleading indicator of climate risk.