Judith Curry, a professional climate scientist who writes the Climate Etc. blog, is apparently blown away by a recent paper in Nature. The paper is by Yu Kosaka and Shang-Ping Xie and is called Recent global-warming hiatus tied to equatorial Pacific surface cooling. The paper seems to be suggesting that the recent “hiatus” in surface warming is due to variations in the sea surface temperature of the central to eastern tropical Pacific. It seems that Judith Curry is blown away because the paper seems to be suggesting that natural variability could be partly responsible for global warming.
I don’t think that that is what this paper is actually suggesting. Judith is, however, a professional climate scientist so I’m more than willing to accept that I could be the one who is wrong. The paper is really only talking about how the sea surface temperature in the Pacific can modulate the rate at which the surface (sea surface, land and atmosphere) warms. Global warming is a consequence of rising atmospheric greenhouse gas concentrations that act to trap outgoing long-wavelength radiation and increase the amount of energy in the climate system. This energy is then distributed across the climate system, with the majority (90% or more) going into the oceans. How this energy is distributed across the climate system will, however, depend on the properties of the different parts of the system. It seems reasonable that there will be times when a larger fraction goes into the oceans and other times when this fraction reduces, increasing the fraction heating the surface and atmosphere.
All that this paper is showing, I think, is how the sea surface temperature in the Pacific can influence how the excess energy associated with global warming is distributed within the climate system. The paper actually ends with
We conclude that the recent cooling of the tropical Pacific and hence the current hiatus are probably due to natural internal variability rather than a forced response. If so, the hiatus is temporary, and global warming will return when the tropical Pacific swings back to a warm state. Similar hiatus events may occur in the future and are difficult to predict several years in advance owing to the limited predictability of tropical Pacific SST.
I think the above conclusion from the paper is probably consistent with how I’ve interpreted their work. Again, happy to be corrected by those who know more than me.
Having said the above, the paper doesn’t seem to do a particularly good job of distinguishing between the warming of the surface and overall global warming. I can see how some might be confused by what the paper is presenting. A comment by Bernard J over at HotWhopper explains this quite nicely, in my opinion. Bernard says (and I hope he and Sou don’t mind me quoting part of it here)
To be blunt I think that the paper suffers from poor editing.
The term “hiatus” as used implies a physical pause in the warming of the planet, but all it really is is a statistical variation in the overall surface temperature trajectory – one that results from the complex interaction of physical phenomena. There remains, however, a similarly physical top-of-atmosphere energy imbalance and hence heat content below is still increasing, irrespective of whether it is masked in the surface temperature record by the interaction of heat-transporting phenomena.
I, like Bernard, was also surprised by some of the terminology used in the paper. I would have expected people working in the field to realise that papers like the one being discussed here could be mis-interpreted by those who don’t realise that surface temperatures are not a particularly good indicator of global warming. I would have expected authors of papers like this to have been a little more careful in how they described the work so as to not introduce further confusion. I appreciate that there was a period (maybe 10 or more years ago) when surface temperatures were typically associated with global warming. This was, I believe, because the surface temperature record was the longest and most reliable record available. Now we have an extensive and reliable record of ocean heat content which shows that most of the excess energy associated with global warming goes into the oceans and that this indicates that global warming has continued despite the “hiatus” in surface warming. The ocean heat content is therefore not only more representative of global warming, but is also a better indicator of global warming than the surface temperature data.
Anyway, this seems to be a very interesting paper which is probably consistent with what many have expected (the hiatus in surface warming is simply a natural variation associated with ocean cycles, and not an indicator that global warming has stopped). I’m just a little surprised that the author, reviewers and editor didn’t put more effort into distinguishing between global warming and warming of the surface, given that many would probably (and some have) interpreted the paper as suggesting that natural variability can be playing a significant role in global warming. Just to be clear, I’m not suggesting that it doesn’t play a very important role in determining how the excess energy is distributed through the climate system. I’m simply pointing out that the reason we’re undergoing global warming is not because of natural variability. It’s because we keep adding CO2 to the atmosphere through our continued use of fossil fuels.