It's mid-February 2012 and the various groups reporting global surface temperature data have all posted numbers that are on the low side compared to thus far in the 21st century, though still several tenths of a degree above the 20th century average. NASA's Goddard Institute for Space Studies (GISS) in particular, which I've been following and posting guesses/predictions on for several years now, has just posted a January 2012 number of 0.36 as the global land + sea temperature anomaly, 0.15 degrees C below the 2011 annual average of 0.51 and 0.27 degrees below the 2010 (and 2005) record of 0.63 degrees.
So naturally, those who deny the scientific evidence for human-caused warming have been going on about cooling, how warming has stopped for many years, and so forth. Though not as adamantly as they were back in 2008 - that cooling proved very brief; perhaps they have learned to be more cautious. If I was in mind to go with simple linear trends and didn't believe the science on warming myself, the January 2012 number of 0.36 sounds like a pretty good bet for the whole of 2012 at this point, maybe it should be even lower.
But I understand the science and know quite clearly warming will continue (with some minor ups and downs) until we stop with the fossil carbon burning - and it may take a while after that to actually stop. I've plugged some numbers in and come up with a science-based prediction for 2012: 0.65 C (plus or minus about 0.07). That middle value would break all previous records. Even on the low end it would put 2012 at the 5th or 6th warmest year on record (just slightly cooler than the remarkable early warm year of 1998 in the GISS index). Why am I so sure 2012 will be so warm?
|Year||Date of prediction||prediction||GISS - February 2012||Difference|
Now, that is pretty remarkable accuracy, well under 0.1 degree difference every year, though I'm a little sad that my 2011 number was more than twice as far off as the others, as I thought it was the most soundly assessed of all the "predictions" up to that point. My April 2011 post used the fitting technique published last year by Foster and Rahmstorf, which found surface temperatures pretty reliably follow a steady warming trend with variation that can be accounted for by slightly time-shifted multiples of the El Nino-Southern Oscillation (ENSO) index, volcanic aerosol estimates, and the solar cycle (as measured by sunspot counts). The solar cycle is reasonably predictable though the current one has ramped up a couple of years later than originally expected. Volcanoes aren't, except that for the most part the influence is essentially zero except when there is a major eruption. Barring a significant eruption putting sulfates in the stratosphere, we can predict the future influence of those two components reasonably well for a year or so in advance.
The tricky one is ENSO - past behavior is known from the MEI index available for download here. There is a typical two to 5 year cycle of ups and downs, with positive phase representing El Nino (higher surface temperatures) and negative phase La Nina (lower temperature). The fit that I did following Foster and Rahmstorf found an MEI coefficient of 0.073, so an MEI value of +2 (intense El Nino) would mean higher surface temperatures (4 months later) by 0.146 degrees C, and an MEI value of -2 (intense La Nina) would mean lower temperatures by -0.146 C. So if you can't predict the future values of the ENSO index very well, you have a range of about -0.15 to +0.15 of uncertainty in any estimate of future temperatures.
Fortunately, as Foster and Rahmstorf found, the value of temperature next month is most closely related to the ENSO value several months previously - so we can have a few months worth of prediction just based on already measured ENSO values. Furthermore, there are a large number of different models that attempt to predict future ENSO values. Based on the average prediction as of mid-March 2011, it looked like the La Nina that had begun in late 2010 would peter out and conditions would return to ENSO index values of close to neutral or perhaps even slightly positive. So that's what I based my 2011 prediction on.
Unfortunately, most of those models predicting ENSO in late 2011 were wrong. ENSO turned to La Nina conditions late in the year, and has continued down to this past month. As you can see from the image below from the IRI ENSO site, the black line (what really happened) is near the low end of all the model predictions (other colors) from about June to December 2011:
The following graph compares what I used for the value of the ENSO index through the end of 2011 (based on what the models were saying back then) with what actually happened:
The effect of the more-negative-than-expected ENSO conditions was to add about -0.5 to the average MEI term for the year, thus contributing about -0.04 to global average temperature for the year. So if I'd known what ENSO was going to do for the rest of the year that would have made my final prediction close to 0.54, within 0.03 again of what actually happened. Still the ENSO drop doesn't explain the full discrepancy - obviously there are terms affecting global average temperature anomaly that aren't accounted for by the Foster-Rahmstorf approach (extra anthropogenic aerosol contributions, or just some simple variation in weather patterns and cloudiness, might be the explanation).
Forging ahead to 2012, using pretty much the same numbers as for the 2011 prediction but with the latest ENSO model average numbers from the IRI site (and 0 for ENSO values beyond 2012) I get the following graph of GISS temperatures from the fit:
So the model predicts 2012 annual global average temperature anomaly in the GISS index to be 0.65 - despite recent cooling, warmer than any preceding year. That should be good to within about 0.07 degrees (given this past years difference, compounding possible errors in the ENSO estimate with whatever the other source of variance was). For 2013 with no ENSO variation included the model gives 0.72, and 0.75 degrees C for 2014 - both of those numbers have to be given higher error bars of plus or minus about 0.15 degrees thanks to ENSO. Including that uncertainty means there's no guarantee of a new global high temperature record in the next 2 years - but there's certainly a very high likelihood of it, and it is almost certain all three years will be hotter with the GISS index than every year of the 20th century except 1998; 2014 should be hotter than 1998 even with a large La Nina.
Of course there's a caveat here - a large volcanic eruption, or a significant increase in human-caused aerosols or some other effect along those lines could cool things off. But all else equal, it looks like we're in for some more warmer years ahead, despite the slightly cooler start we've had so far this year.