There's been quite a lot of excitement in the media and on Twitter following announcements by NASA and the Met Office that 2015 was the hottest year ever recorded 'by some measure'. In particular, the debate has centred around how much El Nino contributed to the 'record hot year' (not evident at all in the lower troposphere satellite data) and how much man-made global warming was responsible. Given that global mean temperatures usually show a delay of several months after El Nino peaks, it seems reasonable to suppose that 2016 will peak higher than 2015.
So, we might expect that in 2015 the contribution from El Nino would be modest. The satellite data confirms this: we see temperatures rising steadily throughout the year but failing to exceed 2010 or 1998.
It is very likely that RSS/UAH will continue to rise sharply in the first few months of 2016. The million dollar question of course is: will 2016 exceed 1998 to become the hottest year in the satellite data series? The multi-million dollar question is: will El Nino 2015/16 'step up' global mean temperatures to a new level or will the inevitable follow-on La Nina be very deep and 'step down' global temperatures? For it should be obvious by now that ENSO is the vehicle whereby global warming/cooling is expressed - not the driver of warming/cooling, I hasten to add.
So anyway, logic tells us that, for 2015 at least, El Nino might only have contributed to a modest rise in GMST. Peter Stott of the Met Office says that El Nino contributed a "small amount on top", although it's not clear if this 'small amount' is to the 1C total rise since pre-industrial times or just to the spike in temperatures that the surface datasets show for 2015. I think most sceptics are interested in what the contribution might be just to the 'spurt' in global temperatures between 2014 and 2015, which, according to the Met Office, amounted to 0.18C. According to NOAA, the increase was a staggering 0.29C and NASA put the figure slightly less at 0.23C. Of course, we can rely upon that all-knowing Oracle of Global Warming, the Guardian to give us the lowdown on this vital question:
"A strong El Niño event is peaking at the moment, putting the “icing on the cake” of high global temperatures. El Niño is a natural cycle of warming in the Pacific Ocean which has a global impact on weather. But scientists are clear that the vast majority of the warming seen in 2015 was due to the emissions from human activity.
“Even without an El Niño, this would have been the warmest year on record,” said Prof Gavin Schmidt, director at Nasa’s Goddard Institute for Space Studies. He said he expected the long trend of rising global temperatures to continue because its principal cause – fossil fuel burning – was also continuing.
“It is clear that human influence is driving our climate into uncharted territory,” said Prof Phil Jones, from the University of East Anglia’s Climatic Research Unit, which produces the temperature record – called HadCRUT4 – with the Met Office."
I agree. Even without El Nino, 2015 would probably have been a record breaking year in the surface datasets. What I am more sceptical about is whether this contribution to record breaking warmth has come from man-made global warming or man-made temperature adjustments (courtesy of Karl et al for NASA/NOAA)!
Let us abandon the flights of fancy of the Guardian and Gavin and come back down to earth with the realisation that ENSO does contribute to global warming. the Met Office tells us that El Nino 1997/98 elevated global mean temperatures by 'at least 0.2C':
"The major El Niño of 1997/98 elevated the global mean surface temperature by at least 0.2C. Since then the increase in global mean surface temperatures has been small and this has been linked in part to decadal changes in the circulation of the Pacific Ocean".
What is important to note is that this was after the La Nina which followed. 0.2C was a measure of how much ENSO 'stepped up' global temperatures. This is very different from what sceptics and AGW convinced scientists are squabbling about on Twitter, i.e. the transient contribution to GMST from an El Nino happening now. In 2016 the transient contribution to GMST and LT temperatures may be very significant - much more than a few tenths of a degree perhaps - but the question is how will the global climate system setlle down after El Nino/La Nina? That is the really interesting question. Will we see a new 'high' established, from which point presumably global warming will take off again in earnest, or will we indeed witness global cooling in late 2016, on into 2017/18?