The recent apparent hiatus in warming of global mean surface temperatures continues to be the subject of much debate. Various explanations have been proposed, with at least one recent study suggesting it is an artefact of the interpolation methods [Cowtan and Way, QJRMS, 2013]. Changes in various natural and anthropogenic forcing factors in a way that differs from CMIP-5 prescribed forcings have been variously posited [Estrada et al, Nature Geo., 2013; Solomon et al, Science, 2011]. Alternatively, the strong El Niño event of 97/98, and the subsequent cooling over the equatorial Pacific, can explain part of this hiatus [Kosaka and Xie, Nature, 2013]. However, it is clear that ENSO behaviour does not explain the winter time cooling trend observed over Eurasia, which plays a significant role. Despite numerous publications demonstrating the relationship between Arctic sea ice loss and the Eurasian cooling [Outten and Esau, Climatic Change 2012; Petoukhov and Semenov, JGR, 2010; Honda et al, GRL, 2009], the new IPCC report does not discuss sea ice as a potential factor in explaining the temperature hiatus in their Box 9.2 (replicated in the Technical summary). To date what is sorely lacking is a holistic assessment of these various posited internal and external system drivers and feedbacks as potential explanators in an encompassing testing framework.

The project focussed upon producing two 30-member ensembles using the Norwegian Earth System model (NorESM) which covered the period of 1980 to 2012. This period was selected since it covers the hiatus period of 1998-2012, with plenty of time for the model to spin up in advance. The first of the ensembles served as a Reference ensemble with forcings from CMIP5 Historical and RCP8.5 experiments. The second ensemble was the Sensitivity ensemble, where many of the climatic forcings were replaced with more up to date observations. The forcings changed included Greenhouse Gases (GHGs), long-lived trace gases, volcanic and tropospheric aerosols, and solar irradiance. 

The project has yielded two publications in the Journal of Geophysical Research (JGR). The first paper describes the construction of the ensembles, detailing the observations used and the experimental set up. The second paper details the analysis of global and regional near surface temperature in the two ensmebles as compared to various observational datasets. 

Project Report:

1. EXPLAIN Final Project Report

Publications:

1. Investigating the recent apparent hiatus in surface temperature increases: Part 1. Construction of two 30-member Earth System Model ensembles
2. Investigating the recent apparent hiatus in surface temperature increases: Part 2. Comparison of model ensembles to observational estimates

Data:

1. Code for analysis and figures in Part 2 publication