RealClimate has published their latest 2012 Updates to model-observation comparisons. I’ll take a different twist. Instead of comparing observations to computer model projections, I’ll compare them to very simple statistical projections.
In a year gone by I posted about what kind of betting terms I might consider appropriate for the reality of global warming (incidentally, that post contained an error which was corrected in an update, but the archived copy doesn’t include the update). The idea is to take annual average data (for global temperature) from 1975 through the end of 1999, then fit a trend line by linear regression. If the trend continues, then future data should probably be within two standard deviations of the extrapolated trend line. This is the “projected range” according to the existing trend. The “projected range” according to the not-still-warming theory is that future values should be within two standard deviations of the existing average (in that case, from 2001 through 2007).
I also mentioned that since it would be unlikely but far from shocking if a single future value were outside either range, I would require two (not necessarily consecutive) future years outside the range to decided against either claim — if I were a betting man.
Since the post was made (early 2008), subsequent global temperature data have fallen within the projected range, for both the “still-warming” and “not-still-warming” claims.
Other things have changed too. We have more data now, and not just since that post was made. Many temperature stations report late, and the methods of calculation for global averages keep getting better. So even data for past years isn’t quite identical to what it used to be. Also, that post looked only at global temperature, mainly from NASA GISS. But the method itself — compare the 1975-through-1999 trend with what followed — is a good way to visualize whether or not the existing trend is likely to have continued or not. It’s not a rigorous statistical test, but it gives a good visual impression. So … let’s do it again, for global temperature from all 5 main sources, and for a few other variables too.
First up is global temperature, from NASA GISS, for which all the post-2000 data are within the projected range:
Then there’s global temperature according to HadCRUT4. Again all the post-2000 data are within the projected range, although it’s on the low end for the last couple of years:
The same can be said for global temperature from NCDC:
For satellite data on global temperature in the lower atmosphere from RSS, everything has proceeded according to projection.
For satellite data on global temperature in the lower atmosphere from UAH, the last decade has actually been running a bit hot but still well within the projected range:
It turns out that all 5 major global temperature records have remained within the projected range since 2000.
Although it’s not a very good indicator of global warming, a lot of people are interested in temperature in the USA (the 48 contiguous states). It too is within the projected range:
If however we had projected that 1975-through-1999 temperature would persist into the 21st century, then we have definitely gone outside that projected range:
Of course there are other data related to man-made climate change, and it’s interesting to see whether or not they have strayed outside the projected range according to this method. First up is Arctic sea ice extent during its minimum month of September:
Clearly this has strayed outside the projected range, but the divergence is on the low end which would indicate that global warming has proceeded faster than projected. For March the post-2000 data has remained within the projected range:
For the annual average, again post-2000 data has exceeded the projected range on the low end, indicating faster warming than projected:
If we look at annual average Arctic sea ice volume rather than extent, again we have strayed outside the projected range on the low end, indicating faster-than-expected warming. In fact we’ve been below the projected range now for 8 years running:
Annual average snow cover in the northern hemisphere is within the projected range:
It is also within the projected range if we had simply extended the 1975-through-1999 average value:
For snow cover during December, we are within the projected range from extending the trend, and from extending the 1975-through-1999 average:
For snow cover in the month of June, we are well outside the extension of the 1975-through-1999 average:
but well within the extension of the 1975-through-1999 trend:
Sea level has been a bit above the projected range:
Why have these global warming indicators continued or exceeded their existing trends? Because of this: