The reduction in global temperature after the recent El Nino continues though not as swiftly as some predicted. The next few months will be interesting to see if it returns to levels seen before the recent El Nino took place when global annual average temperatures changed little for at least 15 years.
Whilst some are adamant that the “hiatus” does not, and never has existed, many scientists accept it and are seeking explanations. Akira Oka and Masahiro Watanabe, both of Tokyo University, writing in Geophysical Research Letters see two phases in the 1998 – 2012 slowdown. In their model they see warming in the Pacific between 1998 – 2002 and then a hiatus due to the increased heat storage below 700 m in the subtropical Southern Ocean. “The study provides clear evidence that the deeper parts of the Southern Ocean play a critical role in the post-2002 warming slowdown,” they say.
One recent paper I find problematic is by Lu Dong and Michael McPhaden, both from NOAA. They talk of the, “pronounced long-term centennial time scale linear warming trend during the last hundred years due to GHG forcing.” I would not go so far, and neither would the IPCC, in believing that all the warming of the last century or so is down to greenhouse gasses alone. As for the linear trend, one can draw a straight through anything!
They say the 2001 – 2013 hiatus is due to the Interdecadal Pacific Oscillation, saying somewhat presumptively; “The prevailing view is that internally generated variations associated with the Interdecadal Pacific Oscillation (IPO) dominate decadal variations in GMST, while external forcing from greenhouse gases and anthropogenic aerosols dominate the long-term trend in GMST over the last hundred years.”
They add; “The most recent warming hiatus apparent in observations occurred largely through cooling from a negative IPO extreme that overwhelmed the warming from external forcing. An important implication of this work is that when the phase of the IPO turns positive, as it did in 2014, the combination of external forcing and internal variability should lead to accelerated global warming. This accelerated warming appears to be underway, with record high GMST in 2014, 2015, and 2016.”
I beg to differ that the accelerated warming appears to be underway given the recent strong El Nino makes such a detection impossible.
Another paper that falls into the same trap is Shineng Hu and Alexey Federov, both of Yale University, in Geophysical Research Letters called, “The extreme El Nino of 2015-16 and the end of the global warming hiatus.” They say the hiatus was caused by a succession of weak El Ninos.
They maintain that their results confirm that weak El Niño activity, rather than volcanic eruptions, was the cause of the hiatus, while the rapid temperature rise is due to atmospheric heat release during 2014–2016 El Niño conditions concurrent with the continuing global warming trend. This is the same mistake as the previous paper.
Shang-Ping Xie and Yu Kosaka in Current Climate Change Reports also ask what caused the hiatus between 1998 – 2013. They maintain that the tropical Pacific sea surface temperature acts as a pacemaker for the hiatus and claim that the temperature rise after a major El Nino proves it. I think this is a bit of a stretch but at least they recognise the importance of “hiatus” research which they say, “has led to a wide recognition of the importance of internal variability for global mean surface temperature trends over a decade and longer. The strengthened connection between the climate variability and change communities is an important legacy of hiatus research.”
In contrast to Hu and Federov, Monerrie et al in Environment Research Letters, say that since 1998 there has been a “quasi-stagnation” in global temperatures because of volcanic emissions. “Despite a continuous increase in well-mixed greenhouse gases, the global-mean surface temperature has shown a quasi-stabilization since 1998. This muted warming has been linked to the combined effects of internal climate variability and external forcing. The latter includes the impact of recent increase in the volcanic activity and of solar irradiance changes.
They conclude that the observed recent increase in the volcanic activity led to a reduced warming trend (2003-2012) of 0.08°C in ten years. The induced cooling is stronger during the last five-year period (2008–2012), with an annual global mean cooling of 0.04°C (+/−0.04°C). The cooling is similar in summer (0.05°C+/−0.04°C cooling) than in winter (0.03°C+/−0.04°C cooling), but stronger in the Northern Hemisphere than in the Southern Hemisphere
As we have said before, observations are one thing models are another. Comparing climate models with observations Austin Hope (University of Maryland) et al say that climate models warm too quickly, by a factor of two. That will annoy some people as there has been some news stories about a new paper by Iselin Medhaug and colleagues in the May 4 issue of Nature that concludes that climate models are describing the observations exactly.