Abstract
The Intergovernmental Panel on Climate Change (IPCC) noted a recent 15-year period (1998–2012) when the rate of surface global warming was a factor of 4 smaller than the mean of the state-of-art climate model projections and than that observed in the previous three decades. When updated to include 2014 by Karl et al. using the new version of NOAA data, the observed warming trend is higher, but is still half or less, depending on dataset used, that of previous decades and the multi-model mean projections. This period is called a surface warming slowdown. Intense community efforts devoted to understanding this puzzling phenomenon—puzzling because atmospheric greenhouse gas accumulation has not abated while surface warming slowed—have yielded insights on our climate system, and this may be an opportune time to take stock of what we have learned. Proposed explanations differ on whether it is forced by counteracting agents (such as volcanic and pollution aerosols and stratospheric water vapor) or is an internal variability, and if the latter, on which ocean basin is responsible (Pacific, Indian, or Atlantic Ocean). Here we critically review the observational records, their analyses and interpretations, and offer interpretations of model simulations, with emphasis on sorting through the rather confusing signals at the ocean’s surface, and reconciling them with the subsurface signals.
Highlights
Which ocean should we look at to find the subsurface signal?
The Pacific extension of this low-frequency pattern in Figure 3c is very weak in comparison and is consistent with the view that interdecadal components of ENSO-cycle mode (ENSO) should fall in the red-noise continuum of the spectrum [26,27]
The pioneering work of Meehl et al [55,56] supports (2) as a possible explanation. They found in a coupled model that the surface hiatus periods are associated with enhanced storage of heat in the intermediate layers of the global oceans
Summary
IPCC’s Fifth Assessment Report (AR5) [1] noted “a much smaller increasing trend” of 0.05 ◦ C per decade of the global-mean surface temperature over the 15-year period 1998–2012, than the previous. We show that the observed global-mean surface temperature, including its error bars, remains near the lower bound of the projected warming by Climate 2018, 6, 82; doi:10.3390/cli6040082 www.mdpi.com/journal/climate. Warming by 42 climatean models assessed by IPCC, and significantly below the multi-model mean, It changed in the. El Niño year, when the observed warming caught up to the up to 2014 It changed in 2015–2016, an extreme El Niño year, when the observed warming multi-model mean projection, before declining in 2017. Caught up to the multi-model mean projection, before declining in 2017. 1. Modeled and observed annual-meanglobal-mean global-mean surface temperature relative to 1986–2005. Modeled and observed annual-mean surface temperature relative to 1986–2005 mean. The grey curves are the maximum and minimum envelopes of 107 ensemble members
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