Stronger regional differences due to large-scale atmospheric flow.

A new paper by Deser et al. (2012) (free access) is likely to have repercussions on discussions of local climate change adaptation. I think it caught some people by surprise, even if the results perhaps should not be so surprising. The range of possible local and regional climate outcomes may turn out to be larger than expected for regions such as North America and Europe.

Deser et al. imply that information about the future regional climate is more blurred than previously anticipated because of large-scale atmospheric flow responsible for variations in regional climates. They found that regional temperatures and precipitation for the next 50 years may be less predictable due to the chaotic nature of the large-scale atmospheric flow. This has implications for climate change downscaling and climate change adaptation, and suggests a need to anticipate a wider range of situations in climate risk analyses.

Although it has long been recognised that large-scale circulation regimes affect seasonal, inter-annual climate, and decadal variations, the expectations have been that anthropogenic climate changes will dominate on time scales longer than 50 years. For instance, an influential analysis by Hawking & Sutton (2009) (link to figures) has suggested that internal climate variability account for only about 20% of the variance over the British isles on a 50-year time scale.

I believe Deser et al.‘s results are important and a wake-up-call, because climate change projections used for the studies of climate change impacts have usually been based on a limited amount of regional climate model (RCM) and global climate model (GCM) simulations. Thus, they may not have sufficiently acknowledged the wide range due to internal variability.

Past research projects such as ENSEMBLES (25 runs with a combinations of RCMs/GCMs, thereof 13 independent GCMs simulations – link & final report), NARCCAP (5 independent simulations with GCMs – link), the UKCIP (11-member ensemble of RCMs), and PRUDENCE (included four different simulations with GCMs – assumed to reflect internal variability of climate (Beniston et al., 2007)) may not have — if Deser et al. are correct — accounted for the wide range of outcomes associated with different large-scale atmospheric flow.

The reason is that – separately – they imply a statistical sample (determined by the number of independent GCM simulations) of different large-scale atmospheric flows that may be too small to represent the actual range of possibilities.

Deser et al.‘s analysis was based on simulations carried out with one model (CCSM3), where they carried out nearly identical simulations 40 times which only differed by using slightly different starting points. Simulations carried out with one model, where the set-up is slightly different, is known in the climate science world as an single-model ensemble simulation.

The purpose of such ensemble runs is to explore the sensitivity of the predictions to different descriptions of the atmosphere at the start of the simulations. For instance the location and strengths of low- and high-pressure systems affect the flow of heat and moisture and the subsequent climatic evolution.

The importance of Deser et al.‘s findings can be seen in their figures (one reproduced below), where they highlighted the model simulations giving the smallest and greatest local response.

Figure 1 from Deser et al. (2012)Figure 1 from Deser et al. (2012): The maps show the average response from the entire ensemble as well as the simulations which gave the warmest and coldest future. The middle panel shows the time evolution of the temperature at a selection of locations and averaged over areas (red curves mark the warmest simulations and blue the coldest). The right panel shows how many simulations in the ensemble that gave temperature trends of different values.

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  1. C. Deser, R. Knutti, S. Solomon, and A.S. Phillips, "Communication of the role of natural variability in future North American climate", Nature Climate Change, vol. 2, pp. 775-779, 2012.
  2. E. Hawkins, and R. Sutton, "The Potential to Narrow Uncertainty in Regional Climate Predictions", Bulletin of the American Meteorological Society, vol. 90, pp. 1095-1107, 2009.
  3. M. Beniston, D.B. Stephenson, O.B. Christensen, C.A.T. Ferro, C. Frei, S. Goyette, K. Halsnaes, T. Holt, K. Jylhä, B. Koffi, J. Palutikof, R. Schöll, T. Semmler, and K. Woth, "Future extreme events in European climate: an exploration of regional climate model projections", Climatic Change, vol. 81, pp. 71-95, 2007.