Questions related to natural variability. What it is and how it can be addressed.
What is natural internal climate variability and why is it important?
A range of processes in the climate system, including interaction between its different parts, give raise to a number of long-term variations that occur naturally without any external forcing. Depending on the phases and future variations in these, and other variations, climate features vary over years and decades. As climate projections are generally not in phase with the observed climate such natural variability can impose relatively large differences between climate projections and the actual observed climate. This is particularly a problem for shorter time periods and in a regional to local perspective. Evaluating long time periods can partially circumvent this problem and is a reason for why climate scientists often refer to 30-year periods or more when discussing climate change.
As the initial state in a global climate model differs from the real state of the climate system there will necessarily be a mismatch between various modes of variability in the real system and in the climate integration. Such modes of variability include, for instance, changes in sea surface conditions in the central Pacific Ocean associated with El Nino and La Nina episodes, or phases with stronger or weaker NAO (North Atlantic Oscillation) indices, of strong relevance for the European climate. Mismatches like this can influence the performance of climate models at scales covering years to decades and are also important sources of uncertainty in climate change projections, especially in a regional to local perspective (e.g. Hawkins and Sutton, 2009).
For European climate change this means that for some variables – for which natural variability is large – long-term trends may not be detected until far into the future. For others, with relatively smaller variability, and larger signal, trends may already be discernable (e.g. Kjellström et al., 2013).
How can internal variability be handled?
The best tool for assessing the internal variability is to use large ensembles of climate model projections. By this approach, the probability of having some, or several, projections close to the actual evolution increases.
Similarly, as in weather prediction, large initial-value single model ensembles have been put forward from climate modelling groups during the last decades. Recently, a number of such large ensembles have been assessed by Deser et al. (2020). As pointed out in that study it can be important to realize that the actual evolution of the climate system is just one realization. Consequently, it may be difficult to interpret climate change aspects as result from changing forcing conditions based only on observations as what is observed may be a feature of such variability.
Most CORDEX RCMs have been used to downscale single member GCM simulations. However, for EURO-CORDEX some models have been downscaling three different ensemble members (Documentation for the role of internal variability over Europe). There are also a few examples of regional climate models that have been used for downscaling large ensembles (Addor and Fischer 2015; Aalbers et al. 2016).
References
Aalbers EE, Lenderink G, van Meijgaard E, and van den Hurk B (2016) To what extent is climate change detection at the local scale 'clouded' by internal variability? In EGU General Assembly Conference Abstracts. 18, s. 10121.
Addor N and Fischer EM (2015) The influence of natural variability and interpolation errors on bias characterization in RCM simulations. J. Geophys. Res. Atmos. 120. doi:10.1002/2014JD022824.
Deser C, Lehner F, Rodgers KB et al. (2020) Insights from Earth system model initial-condition large ensembles and future prospects. Nat. Clim. Chang. 10, 277–286, https://doi.org/10.1038/s41558-020-0731-2
Hawkins E and Sutton R (2009) The potential to narrow uncertainty in regional climate predictions Bulletin of the American Meteorological Society, 90, pp. 1095-1107, 10.1175/2009BAMS2607.1
Kjellström E, Thejll P, Rummukainen M, Christensen JH, Boberg F, Christensen OB, Fox Maule C (2013) Emerging regional climate change signals for Europe under varying large-scale circulation conditions, Clim. Res., 56, 103–119, DOI: 10.3354/cr01146.
