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UK MET: DePreSys – Natural Variability

2011 February 10

So the question has been raised here and here and well LOTS of places – is the ~ 60 year cycle real, a statistcal fluke, or just an illusion from the convergence of aerosols and other factors. Will near-term future warming be moderated by a cooling phase?

James Annan taps quietly on a four year old paper.

It is very likely that the climate will warm over the coming century in response to changes in radiative forcing arising from anthropogenic emissions of greenhouse gases and aerosols (1). There is, however, particular interest in the coming decade, which represents a key planning horizon for infrastructure upgrades, insurance, energy policy, and business development. On this time scale, climate could be dominated by internal variability (2) arising from unforced natural changes in the climate system such as El Niño, fluctuations in the thermohaline circulation, and anomalies of ocean heat content. This could lead to short-term changes, especially regionally, that are quite different from the mean warming (3–5) expected over the next century in response to anthropogenic forcing. Idealized studies (6–12) show that some aspects of internal variability could be predictable several years in advance, but actual predictive skill assessed against real observations has not previously been reported beyond a few seasons (13). Global climate models have been used to make predictions of climate change on decadal (14, 15) or longer time scales (4, 5, 16), but these only accounted for projections of external forcing, neglecting initial condition information needed to predict internal variability. We examined the potential skill of decadal predictions using the newly developed Decadal Climate Prediction System (DePreSys), based on the Hadley Centre Coupled Model, version 3 (HadCM3) (17), a dynamical global climate model (GCM). DePreSys (18) takes into account the observed state of the atmosphere and ocean in order to predict internal variability, together with plausible changes in anthropogenic sources of greenhouse gases and aerosol concentrations (19) and projected changes in solar irradiance and volcanic aerosol (20).

Improved Surface Temperature Prediction for the Coming Decade from a Global Climate Model
Doug M. Smith*, Stephen Cusack, Andrew W. Colman, Chris K. Folland, Glen R. Harris and James M. Murphy

Science 10 August 2007:
Vol. 317 no. 5839 pp. 796-799
DOI: 10.1126/science.1139540

The SI provides some more info on the

Forecasts of global Ts for the coming year are made by multiple linear regression using the following predictors. Interannual variability is captured by the El Niño story Southern Oscillation (ENSO) (measured by the first two ENSO-related empirical orthogonal function (EOF) patterns of high pass filtered SST (S12) for 1911-1995) and volcanic activity (S4) at the end of the previous year. Decadal and longer time-scale variability resulting from human activities is predicted by historical radiative forcing from greenhouse gases, sulphate aerosol and ozone as used in recent Hadley Centre climate predictions (S3). The multi-decadal influence of theoceanic thermohaline circulation (S13) is included by a surrogate index, based on EOF2 of low pass filtered global SST (S12) for 1911-1995. This reflects an interhemispheric contrast in SST anomalies with emphasis on the Northern Hemisphere. Solar variability is included by an index of solar irradiance (S14).

So what is DePreSys?

DePreSys uses the Hadley Centre Coupled Model version 3, HadCM3. Initialisation is achieved by relaxing to full-depth analyses of ocean temperature and salinity (Smith and Murphy, 2007), and atmosphere analyses of winds, temperature and surface pressure. Tests for past cases show that initialisation improves the forecast skill of globally-averaged surface temperature throughout the decade (Smith et al., 2007). Updated decadal forecasts, along with verification of previous forecasts, are available from the experimental seasonal to decadal predictions web page.


But the UK Met sees decadal predictions seem to follow more-or-less the 0.2C per decade rise.

Global average temperature is expected to rise to between 0.32 °C and 0.71 °C (90% confidence range) above the long-term (1971–2000) average during the period 2011–2015, with values most likely to be about 0.51 °C higher than average (see blue curves in the Figure 1 below).

From 2016 to 2020, global temperature is forecast to rise further to between 0.47 °C and 0.94 °C, with most likely values of about 0.71 °C above average. The warmest year in the 160-year Met Office Hadley Centre global temperature record is 1998, with a temperature of 0.40 °C above the long-term average. 2009 had a temperature of 0.32 °C above average. The forecast trend of further global warming is largely driven by increasing levels of greenhouse gases

So …
1998: 0.4C
2009: 0.32C
2011-2015: ~ 0.5C +/- 0.2C
2016-2025: ~ 0.7C +/- 0.2C

Does DePreSys model the ill-defined ~ 60year cycle? Nope, it doesn’t seem to. If it did, then, as I pointed out in the line fitting exercises: If the exp + sine is a realistic model, the global temp anomaly will only see an increase of about 0.1C – 0.2C over the next 20 years.

There is a lead though: following up on Smith’s 2007 reference in the SI for the oceanic thermohaline circulation leads to another paper. Maybe it holds more promise.
A signature of persistent natural thermohaline circulation cycles in observed climate.

But, like usual these days, Tamino (or, in this case, one of his commentators) beats me there. 😆