Investigating the possible causes for the recent warming hiatus

The IPCC stated in their latest assessment report that the global warming hiatus was attributable in roughly equal measure to natural variability and reduced trends in external forcing. A detailed study involving over 2000 years of model simulations has demonstrated that according to the Norwegian Earth System Model, there is no evidence that forcings errors play a significant role in explaining the hiatus. These results has been addressed in two papers lead by Nansen Center scientists Drs. Stephen Outten and Peter Thorne recently published in Journal of Geophysical Research.

After the exceptional warm El Niño year of 1998 the rate of global temperature increase has slowed – a warming hiatus (IPPC AR5). In spite of this, the most recent decade has been the warmest since the start of instrument records by a large margin. However, the hiatus in global surface warming has continued to be the focus of much debate in the public and scientific communities. The scientific interest is both in the physical cause of the hiatus and in the apparent failure of most of the current generation of climate models to reproduce it.

The IPCC had limited literature available at the time of their report but they attributed the cause of the hiatus in roughly equal parts to natural variability and external forcing1. Much literature has been published since then and the hypotheses concerning the causes of the warming hiatus can be loosely grouped into two main categories: boundary conditions and forcings. The boundary condition hypotheses include the role of sea ice in the Arctic Ocean, which has previously been linked to cooling over Eurasia2, and of variability in Ocean Heat Content and Sea Surface Temperatures, for example through El Niño/Southern Oscillation (ENSO) events3 or Pacific Decadal Oscillation. The forcings hypotheses include changes in greenhouse gases4, long-lived trace gases including ozone depleting substances5, variations in solar forcings, and aerosols – both anthropogenic and volcanic6.

Column mass of volcanic aerosols in kg/m**2 by latitude with the meridionally integrated forcings underneath, for the (top) Reference forcings from the NorESM CMIP5 runs and the (bottom) Sensitivity forcings. These are both based on the work of Sato et al.In the Bjerknes Centre EXPLAIN project - EXamining PotentiaL cAuses for the warmINg hiatus – scientists at the Nansen Center, Uni Research and Meteorological Institute have investigated the role of forcings in explaining the hiatus. In the CMIP5 simulations, used as the basis for the IPCC AR5 report, observations were only used to drive the Historical simulations, which concluded in 2005. Thus no observations were incorporated beyond that year. The project team developed new forcing fields for the Norwegian Earth System Model (NorESM) based on up-to-date observations covering the period from 1998 to 2012, including green house gases, long live trace gases (including ozone depleting substances), solar irradiance, and volcanic (see figure) and anthropogenic aerosols. The net result of these updated forcings was a slightly lower total global forcing of around 0.03 Wm-2 and a slightly greater (more positive) trend over the hiatus period.

Simulations with NorESM were run using the original and the new forcings, creating a new ensemble dataset for future scientific studies in the process. The results were used to analyse the role of external forcings and internal climate system variability in shaping the hiatus independently from other possible causal mechanisms. Comparing the NorESM simulations with the original forcings to those with the new forcings revealed that their surface temperature trends were statistically indistinguishable. Based on this, the authors concluded that changes in the forcing cannot explain the hiatus and that internal climate system variability or ‘natural’ variability is dominant in explaining the hiatus.

The study also showed that NorESM reproduced all the important observed features of the hiatus, including El Niño behaviour in the tropical ocean, anticorrelation between surface temperature changes and the rate of deep ocean heat uptake, seasonal variations in trends in the Northern Hemisphere and in the Arctic e.g. Eurasian wintertime cooling, and the correct spatial scales of trends. While no single simulation captured all of these features simultaneously as they have been observed in the real world, the results suggest that NorESM is capable of correctly reproducing the hiatus. Interestingly, those simulations that exhibited El Niño sea surface temperature trends similar to those observed also exhibited reduced global mean temperature trends, supporting the idea that the El Niño Southern Oscillation (ENSO) has played an important role in the formation of the hiatus. This is inline with the prevailing theory from the latest scientific literature.

The new forcings developed during EXPLAIN are available from the NERSC website, while the 60 simulations are available for scientific use through the national NorStore Research Data Archive infrastructure and the Norwegian Earth System Grid Federation. Meanwhile, the team is focussing on their new project, VOLCANOES4FUTURE, which will study the potential role of volcanoes in shaping global temperatures over the coming century.

 

Figure caption: Column mass of volcanic aerosols in kg/m2 by latitude with the meridionally integrated forcings underneath, for the (top) Reference forcings from the NorESM CMIP5 runs and the (bottom) Sensitivity forcings. These are both based on the work of Sato et al.


Further details are given in the peer-review publications

Outten, S., P. Thorne, I. Bethke, and Ø. Seland: Investigating the recent apparent hiatus in surface temperature increases: Part 1. Construction of two 30-member Earth System Model ensembles. J. Geophys. Res., doi:10.1002/2014JD022805.

Thorne, P., S. Outten, I. Bethke, and Ø. Seland: Investigating the recent apparent hiatus in surface temperature increases: Part 2. Comparison of model ensembles to observational estimates, J. Geophys. Res., doi:10.1002/2015JD023859.

 

Cited references:

  1. Flato, G., J. Marotzke, B. Abiodun, P. Braconnot, S.C. Chou, W. Collins, P. Cox, F. Driouech, S. Emori, V. Eyring, C. Forest, P. Gleckler, E. Guilyardi, C. Jakob, V. Kattsov, C. Reason and M. Rummukainen, 2013: Evaluation of Climate Models. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  2.  Outten, S. and I. Esau, 2012: A link between Arctic sea ice and recent cooling trends over Eurasia, Climatic Change, 110, 1069-1075
  3. Kosaka, Y. and S. Xie, 2013: Recent global-warming hiatus tied to equatorial Pacific surface cooling, Nature, 501, 403–407, doi:10.1038/nature12534
  4. Hansen, J., M. Sato, R. Ruedy, L. Nazarenko, A. Lacis, G.A. Schmidt, G. Russell, I. Aleinov, M. Bauer, S. Bauer, N. Bell, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, M. Kelley, N.Y. Kiang, D. Koch, J. Lean, J. Lerner, K. Lo, S. Menon, R.L. Miller, P. Minnis, T. Novakov, V. Oinas, J.P. Perlwitz, J. Perlwitz, D. Rind, A. Romanou, D. Shindell, P. Stone, S. Sun, N. Tausnev, D. Thresher, B. Wielicki, T. Wong, M. Yao, and S. Zhang, 2005: Efficacy of climate forcings. J. Geophys. Res., 110, D18104, doi:10.1029/2005JD005776.
  5. Estrada, F., P. Perron, and B. Martínez-López, 2013: Statistically derived contributions of diverse human influences to twentieth-century temperature changes, Nature Geosci., 6, 1050-1055, doi:10.1038/ngeo1999
  6. Santer, B.D., C. Bonfils, J.F. Painter, M.D. Zelinka, C. Mears, S. Solomon, G.A. Schmidt, J.C. Fyfe, J.N.S. Cole, L. Nazarenko, K.E. Taylor, and F.J. Wentz, 2014: Volcanic contribution to decadal changes in tropospheric temperature, Nature Geo., 7, 185-189
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