Yesterday I decided to ask Shang-Ping Xie and Yu Kosaka of the widely discussed Nature paper about the slowdown in global temperatures to react on Judith Curry’s blog post. Xie promptly did and with his permission I just post his response here. Curry’s sentences are italic.
Here are some thoughts on her take. I liked Judy’s thoughtful blog about our paper, and agree with her view that natural variability modulates the pace of climate warming over a decade or two.
Compare the temperature increase between 1975-1998 natural internal variability associated accounting for significantly MORE than half of the observed warming.
1975 was a La Nina year, and 1998 followed the strongest El Nino in the instrumental record. My estimate indicates that the El Nino-La Nina difference accounts for 0.2-0.3 C difference of her 0.4 C in POGA-C. So for multi-decadal trend, PDO accounts for only 0.1-0.2 C for the longer period of 1950-2010. El Nino and La Nina are part of the short climate cycle of ENSO, averaged out over several decades. Our paper noted that the warm phase of the tropical Pacific Decadal Oscillation contributed to the fast warming during the 1970s-1990s.
I thought that it might account for at least half of the observed warming, and hence my questioning of the IPCC’s highly confident attribution of ‘most’ to AGW.
I have a different take on this. The IPCC conclusion applies to centennial warming from 1880. Much of the 0.8 C warming since 1900 is indeed due to anthropogenic forcing, because natural variability like PDO and AMO has been averaged out over this long period of time.
Our results concern the effect of tropical Pacific SST on global mean temperature over the past 15 years. It is large enough to offset the anthropogenic warming for this period, but the effect weakens as the period for trend calculation gets longer simply because it is oscillatory and being averaged out.
[Update, short reply by email from Curry on Xie's comments:
Hi Marcel, interesting. You can pick different years than 1975 and 1998 (which were neutral relative to enso), and you still see a strong trend from PDO.With regards to the IPCC statement, the AR4 statement was tied to the latter half of the 20th century, and the 'detection' argument identified the AGW signal since mid 1970's. What the AR5 will eventually say, I don't know (other than what was leaked). Judy [end update]
More comments on her blog:
JC comments: I’m pleased that Xie has responded to what I have written. El Nino and La Nino don’t seem to me to be easily separable from the PDO. Picking ENSO neutral years at the beginning and end of the 30 yr period circa the 1970′s to 2000 still shows a strong increase during the period.
Also, I am interested that Xie seems to refer to the forthcoming attribution statement for the AR5, which apparently refers to the period since 1880. Which is different from the AR4, which referred to the latter half of the 20th century.