Given how rapidly global temperature was rising prior to 1998, what’s the most surprising thing about global temperature since 1998?
Most who call themselves “skeptics” of global warming would probably say “No global warming since 1998!” Under the name “hiatus” or “pause,” it features prominently in public discussion and even in senate testimony (e.g. from Judith Curry). In truth, such a “pause” or “hiatus” is not that surprising, neither from a statistical point of view nor based on climate model output. But there is one thing about post-1998 temperatures, compared to the pre-1998 temperatures, that is quite a surprise.
It has — quite rightly — been pointed out that surface air temperature (SAT) isn’t all there is to global climate or global warming. Since 1998 we’ve witness sizeable warming of the oceans, including the deep ocean. We’ve seen a staggering decline of Arctic sea ice and the continued dwindling of most of the world’s glaciers. Sea level has continued to rise at a rate much faster than the 20th-century average (which itself was much higher than the average over the last several thousand years). It has been emphasized that a lack of “statistically significant” warming is not the same as a lack of warming. It has also been pointed out that the “pause” in SAT is not inconsistent with climate model simulations, that in fact climate models show episodes like we’ve observed “since 1998″ even in a still-warming world. And it has been shown (as climate scientists knew all along) that greenhouse gases aren’t the only factor influencing temperature, that “since 1998″ we’ve seen the most prominent known non-greenhouse factors (el Nino southern oscillation, volcanic aerosols, and solar variations) conspire to lower global temperature. It’s obvious to those whose eyes are open that without continued greenhouse-gas warming to offset these natural factors, we would have seen a notable decline in global temperature “since 1998.”
But, let’s put all those perfectly valid considerations aside. Let’s ignore the oceans, the ice, the known natural factors, all of it, and look at nothing but global average air temperature (at the surface and in the lower troposphere).
Riddle me this: if we had been told by an unimpeachable source on January 1st, 1998 that there would be no statistically significant temperature increase over the period from the beginning of 1998 through the end of 2013, what would we have predicted? How would that compare to what has actually happened?
Let’s use temperature data starting in 1979 (so we can include satellite data for the lower troposphere) and ending with 1997 to predict what we would have expected over the next 16 years, then compare that to what happened. We’ll even make two predictions: 1) Based on continued warming; 2) based on certain knowledge that there would be no statistically significant warming from 1998 through 2013.
We’ll start with the HadCRUT4 data set from the Hadley Centre/Climate Research Unit in the U.K. Taking the data from 1979 through 1997, we’ll compute a linear regression line, then extrapolate that line through to 2013 to construct our “still-warming” prediction. We’ll also compute the standard deviation of the residuals from our linear regression so we can add two lines to the graph, one of which is two standard deviations above our forecast, the other two standard deviations below, in order to delineate the range in which we would expect most of the future data to be.
We’ll also take the final value of the linear regression line (not the slope) as our estimate of what we would expect if we had been given certain knowledge of no statistically significant warming from 1998 through 2013, and we’ll add extra lines, two standard deviations above and below, to mark out the expected range.
If all we know is data before 1998 then here is what these simple approaches predict (“still-warming” prediction in red, “no-warming” prediction in blue):
It’s clear that if we expected a pause, we would expect most of the following years’ temperatures to be below the red forecast line, but about half above and half below the blue forecast line. On the other hand, if we expected continued warming, we would expect most of the following years’ temperatures to be above the blue forecast line, but about half above and half below the red forecast line.
So … how did it turn out? Were subsequent years about half above and half below the red (warming) forecast, or the blue (no warming) forecast? The answer is: neither.
What actually happened is that, according to the HadCRUT4 data, most of the data are above both forecasts. Twelve of sixteen were hotter than expected even according to the still-warming prediction, and all sixteen were above the no-warming prediction:
When it comes to global temperature, the HadCRUT4 data set isn’t the only game in town. There’s also data from NCDC (the National Climate Data Center):
Once again, twelve of sixteen years were hotter than expected even according to the still-warming prediction, and all sixteen were above the no-warming prediction.
Well, how about the data from NASA GISS?
Using these data, thirteen of sixteen years were hotter than expected even according to the still-warming prediction, and all sixteen were above the no-warming prediction.
Maybe we should include the new data set from Cowtan & Way, which uses the best (in my opinion) method for estimating unobserved areas:
Now fourteen of sixteen years were hotter than expected even according to the still-warming prediction, and all sixteen were above the no-warming prediction.
Do the satellite data give a different result? Here’s what we get using the TLT (lower-troposphere) data from RSS (Remote Sensing Systems):
Once again, fourteen of sixteen years were hotter than expected even according to the still-warming prediction, and all sixteen were above the no-warming prediction (although one is just barely so).
Finally, here’s what we get using the TLT (lower-troposphere) data from UAH (Univ. of Alabama at Huntsville):
All sixteen years were hotter than expected even according to the still-warming prediction, so of course they also were above the no-warming prediction.
Given how fast global temperature was rising prior to 1998, the real surprise which followed is not that temperatures slowed or stopped their increase … the real surprise is that temperatures rose so far so fast and were so damn hot. Even allowing for the existing trend.
Does this mean we need to launch a massive research effort to divine the reason for this sudden and pronounced warming? No.
If we had chosen the 1998 boundary because we had been visited by an omniscient alien in January of 1998, then maybe we would. But I didn’t pick 1998 because of that. I picked it because so-called “skeptics” picked it, and they chose it because of the result it gives. Which makes it cherry picking.
That kind of cherry-picking requires a large compensation to statistical analysis, one which makes it clear that there’s no justification for a massive research project to investigate the post-1998 blazing heat. It also makes clear that there’s no justification for running off at the mouth about the so-called “pause.”
As for “skeptics” about global warming, I love ’em. I welcome their valuable, even essential contribution to the field. Unfortunately, most of those who call themselves skeptics, espcially those who blather on and on about the end of global warming while swallowing the most ludicrous ideas hook line and sinker, would be better described as fake skeptics.
Arctic vs Antarctic sea ice decline:
Just the smoothed versions: