According to the global surface temperate data set compiled by Kevin Cowtan and Robert Way, which achieves the best coverage of the rapidly-warming Arctic by filling in data gaps between temperature stations using a statistical method called kriging, 2013 was the 5th-hottest year on record (since 1850). The top three hottest years (2010, 2005, and 2007) were influenced by El Niño events, which cause short-term warming of the Earth's lower atmosphere.

Over the past decade, we've seen less warming at the surface and more warming in the oceans. This has been in large part due to a change in Pacific Ocean cycles. We're currently in a cycle that tends to produce more La Niña than El Niño events, which has resulted in the oceans accumulating more heat, leaving less energy than normal to warm the atmosphere. This in turn has led to the widespread myth that the slowed rate of increase of global surface temperatures means we no longer have to worry about global warming, or that its consequences won't be as bad as expected.

The fundamental flaw in this argument is that it neglects a key fact: cycles are cyclical. In the '80s and '90s when the Pacific Ocean was in the previous phase of this cycle, we saw more El Niño events and more warming of global surface temperatures than the average of climate models projected. However, we can separate out the short-term El Niño and La Niña influences from the human-caused global warming component in the simple manner first suggested by Texas state climatologist John Nielsen-Gammon, shown in this animated graphic:

Global surface temperature data from Cowtan & Way, separated into El Niño (red), La Niña (blue), and Neutral (black) years for 1966–2013, with linear trends plotted for each category.

The El Niño/Neutral/La Niña years here are categorized using a slightly modified approach from the one described in this post last year. In essence, a year with a significant surface cooling influence from a La Niña event is put in the La Niña category, the same for El Niño, and a year with no significant influence is put in the Neutral category.

For each of these three categories, the linear global surface warming trend for 1966–2013 is 0.16°C per decade. That is our long-term underlying global surface warming trend, caused almost entirely by human influences. Note that the colored data points tend to fall close to each of their respective trend lines. This tells us that, for example, an El Niño year today is about 0.6–0.7°C hotter than an El Niño year in the 1970s, and the same is true of Neutral and La Niña years.

What's also interesting is that despite being a Neutral year, 2013 was hotter than 1998, which saw one of the strongest El Niño events on record. This tells us that humans have caused as much global warming over the past 15 years as a powerful El Niño event. The difference is that an El Niño is a temporary event, while human-caused global warming is permanent, unless we can quickly pull a lot of carbon dioxide out of the atmosphere.

Cowtan & Way global surface temperature data, with the powerful El Niño year of 1998 in red and the Neutral year of 2013 in blue.

Due to the current phase of the Pacific Ocean cycles, of the past six years, four have been cooled by La Niñas. Seven of the past 15 years have seen La Niñas, compared to just four El Niños. Conversely, in the previous Pacific Ocean phase, the 1990s saw seven El Niño years to just two La Niñas. Thus natural ocean cycles amplified human-caused global warming at the Earth's surface in the 1990s, but have dampened it since 1999.

It's really interesting to compare the reactions to these two cycles. Today, climate contrarians are arguing that the slowed rise in surface temperatures means global warming is nothing to worry about – they're confusing short-term cycles with something meaningful in the long-term. Just a few years ago, mainstream climate scientists looked at the accelerated rise in global surface temperatures. A 2007 paper by Stefan Rahmstorf and colleagues concluded,

"The global mean surface temperature increase ... is 0.33°C for the 16 years since 1990, which is in the upper part of the range projected by the IPCC. Given the relatively short 16-year time period considered, it will be difficult to establish the reasons for this relatively rapid warming, although there are only a few likely possibilities. The first candidate reason is intrinsic variability within the climate system."

Climate scientists didn't panic and decide the short-term acceleration in rising surface temperatures meant that climate models were underestimating global warming, or that it would be worse than expected. They correctly suggested that it was probably just due to short-term noise from natural cycles. On the other hand, climate contrarians have overreacted to and misinterpreted the current short-term noise, incorrectly declaring that it means climate models are overestimating global warming and we have nothing to worry about.

What this really means is that climate models may underestimate the internal variability of the climate system due to these types of ocean cycles, but they still get the long-term global warming trend right.

2013 also saw an incredible amount of heat accumulate in the oceans – equivalent to 390 million Hiroshima atomic bomb detonations, or over 12 atomic bomb detonations per second. 2013 tied 2006 as the year with the most energy accumulating in the oceans since the Argo buoy network began producing much-improved estimates of ocean temperatures to depths of about 2,000 meters. This is significantly higher than the average of about 4 atomic bomb detonations per second over the past decade, and also much higher than 2009. This helps explain why 2009 had higher surface temperatures than 2013 – less heat went into the oceans and more into the atmosphere that year.

Annual ocean heat content accumulation to 2,000 meters in units of atomic bomb detonations per second, with standard error shown. Data from NODC.

The National Oceanographic Data Center (NODC) provides five-year average estimates of deep ocean heat content data to 1957. The five-year period ending in 2013 saw the 5th-largest ocean heat accumulation on record, equivalent to 7.3 atomic bomb detonations per second. Since 1998, when global warming is supposed to have paused, we've seen over 2 billion atomic bomb detonations worth of global warming.

Overall, 2013 was another year consistent with the long-term human-caused global warming trend. It was the 2nd-hottest year on record without an El Niño event, hotter than the powerful El Niño year of 1998, and saw a global heat buildup equivalent to more than 12 atomic bomb detonations per second. The oceans warmed so much in 2013, NODC is going to need a bigger graph.

2013 was further evidence that those who invoke the myth of the global warming pause to argue against taking action to address the climate threat are fooling themselves and failing to grasp the basic concepts of risk management.