Troy's Scratchpad

March 22, 2011

Ensemble Member Early Trends vs. Later Trends

Filed under: Uncategorized — troyca @ 6:21 pm

There’s been a lot of work done with respect to evaluating GCM projected temperature trends vs. observed trends, particularly in determining whether individual models or the ensemble mean can be rejected based on these trends. 

What I’ve been a bit curious about is whether the model runs with lower trends in this first decade of comparison (2001-2010) generally correspond to lower overall projected temperature increases for the remainder of the century (2011-2099).  No, it’s not an incredibly deep question, and has probably been answered elsewhere, but this gives me an excuse to work with some time series stuff in R and dive into model data.

I got the 54 ensemble members for SRES A1B from Climate Explorer. 

I’ve also included the data, along with my R code, in the link here.

Anyhow, here is a histogram of the global temperature trends for the ensemble members in the first 10 years, starting in 2001:

Next, here is what the model run trends look like for the remainder of the century:

As is clear, the variability in the first 10 years in much greater than in the remaining 90, which would be consistent with the idea of weather noise dominating the short-term trends while having the GHG forcings take over in the long haul.  14 of the 54 runs seem to show less than an 0.1 C rise in the first decade, whereas none showed a trend this low continuing over the the remainder of the century.

So, back to the original question — do early trends in these model runs give us an indicator of what the remainder of the century for that run is going to look like?

From this chart, it would appear that no, it does not. 

Of course, it may be that it is better to look at individual models instead of the ensemble members, since two runs from the same model may have different early trends but are likely to eventually converge (and hence we would not gain anything by looking at the separate runs in the early stages), whereas two different models are more likely to have different underlying processes whereby differing early trends may theoretically suggest a difference in climate sensitivity.

I’m also curious about how well these individual runs compare to the 20th century observations vs. the multi-model mean.

1 Comment »

  1. […] Like my last post, this one gives me a chance to dive more into the CMIP3 models and work on my time series programming in R.  Once again, I’ll be looking at the 54 ensemble members for SRES A1B that I got from Climate Explorer.  The code and data can all be downloaded here.  The R file itself will read the HadCRUT data over the internet.  I was planning on including the ability to use additional surface temperature datasets for the 20th century, at least GISS, but the format is a bit more difficult to parse.  I may need to borrow one of Kelly O’Day’s scripts to help with that processing in the future. […]

    Pingback by CMIP3 Multi-Model Mean vs. Individual Runs in 20th Century Hindcast « Troy's Scratchpad — April 22, 2011 @ 5:57 pm

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