There has been some discussion recently about the possibility of an El Nino starting later this year and if it will restart global warming. Certainly if the usual effect of an El Nino – warming of the surface waters of the equatorial Pacific – happens and the global annual average surface temperature reaches a new record because of it, perhaps only by a few thousandths of a degree, then it will be hailed by some as a “resumption” of global warming.
Anyone who has turned a globe of the Earth so that they are facing the Pacific Ocean will be in no doubt that it is perhaps the major feature of our planet’s surface. Looking at the globe that way it is hardly possible to see any land at all. When the Sun shines down on the Pacific it has a powerful effect warming the surface waters. Trade winds blow the warmed surface waters to one side of the Pacific where the warm water accumulates in a warm pool storing heat. When those winds reduce in strength the warm pool sloshes back across the Pacific releasing energy, changing current directions and strengths and wind directions that can be felt all over the world. El Ninos happen every few years and are a way of reducing the heat content of the Pacific and distributing it worldwide.
The biggest El Nino on record occurred in 1997-98. It catapulted the world to then record surface temperatures. Unfortunately, its onset was not predicted at the time as well as many thought it would be with most predictions only suggesting a weak event six months ahead of time.
No one knows how an El Nino starts, some say its quasi-periodic nature points to an unstable mode of ocean-atmosphere coupling. Others believe it is related to the behaviour of the thermocline – the interface between warm water at the surface and the cold water below about 100 metres.
When the 1998 El Nino occurred it was a record breaker. In Nasa Giss (current values) it was 0.2 deg C warmer than the years either side of it. In many respects it is one of the dominant features in the global temperature record over the past 40 years. In the 1980s and early 1990s there was little significant increase in global annual average surface temperature. Looking at the surface temperature record it is clear that the 1997-8 El Nino is positioned at a step-change in global surface temperatures from one 15-year period of little warming to another 15-year plus period of the same though at a more elevated temperature. Indeed it was the 1997-8 El Nino’s boost to global surface temperatures that helped the decadal rate of surface temperature increase given by the IPCC, 0.2 deg C, to be “validated.”
Since 1998 El Ninos have not made any statistically significant impact on the global surface temperature. They have raised it slightly causing alarmist claims that global warming has restarted but one year of statistically insignificant increase does not a restart make.
Today, after many post hoc corrections to the temperature data gathered at the time, in Nasa Giss 1998 is the third warmest year behind 2005 and 2010 (other El Nino years) although when one allows for the errors of measurement 1998 is statistically indistinguishable from 2013, 2012, 2011, 2010, 2009, 2007, 2006, 2005, 2003, and 2002. Technically the annual temperature of 2010 was equal to 2005 and only 0.03 deg C above 2007 – 2010 and 2005 were El Nino years, 2007 was a cool La Nina year – though not in statistical terms. The 2010 warmth was not yearlong being confined to two very warm months in March and June. The other ten months were at average or less than average temperature, as defined by the post-1997 surface standstill.
Some, such as Kevin Trenberth, are making a big deal of the putative 2014 El Nino, “there are some things going on in the tropical Pacific Ocean that we haven’t seen since the 1997, 1998 El Nino event…the question is how large it is going to be?”
While an El Nino might nudge temperatures up slightly, which considering the 2014 global surface temperatures seen in the first third of the year is probably the most that can happen this year, I don’t think that it will be a record breaker because there is less heat stored in the Pacific now than there was in the years preceding the 1997-8 event.
Looking at the surface temperature record the way the 1997-8 El Nino changed things is obvious. What will the next super El Nino do, if one is possible in the elevated temperatures of the past 16 years. Will it cause another step up?