It is now all but certain that 2015 will be a record hot year. Past evidence from El Nino events strongly suggests that next year will be, too.
That will definitively end a recent “hiatus” of sustained, but not increased, surface global warming from around 1998 to 2013, which had led to some renewed questioning, at least among non-expert circles, about the strength of the global warming trend.
Climate scientists have long warned not to read too much into temperature variation from year to year, when weighing evidence for human-caused climate change.
Most emphasise a steady decadal warming, where every decade since the 1960s has been warmer than the one before, with 2000 to 2009 by far the warmest decade on record (see figure below).
Various natural and human factors have been proposed for the recent hiatus, including a weaker than average solar cycle, transfer of heat into the deep oceans which continued to warm, and record coal consumption in China, leading to greater atmospheric pollution which can scatter and dim sunlight.
The latest data (updated to August 2015) underline the influence of many factors in driving temperature variation from year to year. Except this time natural factors are driving a sudden peak in warming.
El Nino is the warm phase in a natural cycle of changing sea surface temperatures in the eastern and central Pacific Ocean. Besides hotter sea surface temperatures, it is linked with milder than average winters across most of North America, and a greater likelihood of drought and heat waves in the western Pacific.
The recent temperature record shows how El Nino usually leads to global average warming both at its onset, in the first year, and in the following year.
Recent El Ninos began in the following years: 1986, 1991, 1994, 1997, 2002, 2004, 2006 and 2009. Reviewing global average surface temperature data, these El Ninos saw global average warming in the first year in five out of the eight events, and further warming again in the succeeding year in all but two cases, in 1991 and 2006.
The present El Nino was officially declared in March, and has since become a powerful event predicted to peak this northern hemisphere winter, and gradually weaken through the spring of 2016.
Of course, we shouldn’t forget that El Nino is often – but not always – followed by a cold phase, called La Nina. As a result, 2017 may well be a cooler year, continuing the recent saw-tooth pattern around a continuing global warming trend. But 2017 is far off for a weather forecast.