This article has been cross-posted from Carbon Brief.

A new paper published on June 4 says the much-discussed “slowdown” in warming at Earth’s surface may not exist after all.

The study, published in the journal Science, says it is likely to be largely a figment of the way temperature records have been pieced together over time.

Scientists from the US National Oceanic and Atmospheric Administration (NOAA) reanalysed temperature records and concluded that surface warming over the past 15 years is higher than reported by the Intergovernmental Panel on Climate Change (IPCC), the UN body set up to assess global warming. Temperatures are rising at least as fast as they were in the second half of the 20th century, say the authors.

Given the interest in the topic, this new finding is likely to generate headlines. But it’s probably not the last word on this complex topic, scientists tell Carbon Brief.

To coin a phrase

In its latest report, the IPCC calculated that the rate of warming from 1998-2012 was 0.05C per decade. Accounting for the uncertainty, this is 30-50% slower than the 0.12C per decade rise over the longer period of 1951-2012. The new paper says:

“The apparent slowdown was termed a ‘hiatus’, and inspired a suite of physical explanations for its cause.”

But the authors say that once you account for improvements to the historical temperature record and a couple more years’ of temperature data to take us up to 2014, the pace of warming in the first 15 years of the 21st century hasn’t slowed after all.

Points of clarity

Before we move on to the hows and whys of the new study, here are some points of clarity.

The so-called “hiatus” has never been about a reduction in the speed of “global” warming. It relates only to the temperature at the Earth’s surface. When you look at all the components of the climate system – land and vegetation, ice cover and the ocean – scientists have no doubt that the planet as a whole is warming up.

^{Source: Rosamund Pearce, Carbon Brief}

Surface temperature rise has not “paused” or “stopped” during the period dubbed the “hiatus”, but the speed of temperature rise is thought to have slowed slightly. As the new paper puts it, the period since 1998 has seen an “apparent decrease in the upward trend of global surface temperatures”.

Natural ups and downs

Since the IPCC report, a lot of research has been published looking at how natural variability could be contributing to slower surface warming by reshuffling heat between the atmosphere and the oceans.

On top of the long-term warming trend from greenhouse gases, these natural fluctuations in the climate system can temporarily boost or dampen the speed of warming, causing global temperatures to temporarily rise above or below the long-term average.  

As the new paper notes, the IPCC acknowledged that the 1998-2012 period starts with an extremely strong El Niño, which gave a boost to global temperature.

The new study revisits temperature trends in the early 21st century with the benefit of an extra couple of years’ data. This includes 2014, which is likely to be the hottest year on record.

For the period 1998-2014, the authors calculate a warming trend that’s 0.02C per decade higher than the trend over the 1998-2012 period that the IPCC report talks about.

Similarly, the trend for the period 2000-2014 is 0.03C per decade higher than the trend over the 1998-2012 period. The paper notes:

“In other words, changing the start and end date by two years does, in fact, have a notable impact on the assessment of the rate of warming.”

This is already well-understood. The IPCC makes this point in the last report, and demonstrates it by calculating temperature trends for different 15-year periods. It says:

“Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”

But there’s another difference in the new paper. This is where it gets a bit more complicated.

Data quality issues

Scientists use thousands of temperatures measurements from ships, drifting buoys and weather stations on land to construct records of global temperature.

The new paper addresses some well-known issues with how the data is all pieced together.

First, is how to combine traditional ship-based measurements of sea surface temperature with those collected by the ARGO network of free-floating buoys, operating since the early 2000s.

Another issue is how to account for changes over time in the way ships collect temperature data while on the move. There is also the well-known problem of having little data available in the Arctic.

Scientists have developed various ways to “correct” for all these issues. The new paper uses an approach developed by NOAA scientists – some of whom are authors of the new paper.

While the authors apply their corrections to the full temperature record stretching back to 1880, the biggest impact is on the rate of warming in recent decades, say the authors.

Global average surface temperature with the new corrections (black) compared to an earier version of the dataset (red) and compared to with no corrections at all (green). Source: Karl et al. ( 2015)

No pause, after all

The upshot of the analysis is that the pace of warming over the last 15 years is not, after all, very different to that observed since the 1950s. The paper says:

“There is no discernible (statistical or otherwise) decrease in the rate of warming between the second half of the 20th century and the first 15 years of the 21st century.”

The authors calculate a warming trend from 2000-2014 of 0.116C per decade, compared to 0.113C per decade for the period 1950-1999. Mark Maslin, professor of climatology at University College London, says:

“This important reanalysis suggests there never was a global warming hiatus; if anything, temperatures are warming faster in the last 15 years than in the last 65 years …The weight of evidence for anthropogenic climate change is overwhelming and this new study shows that the global warming hiatus was just wishful thinking.”

The new paper concludes that the IPCC‘s statement that the past 15 years shows a much smaller warming trend than the past 30-60 years “is no longer valid”.

The new study finds the rate of surface warming has continued uninterrupted in the last 15 years, despite much discussion of a “slowdown”. Source: Karl et al. ( 2015)

Was the IPCC wrong?

The results of the new study don’t mean the IPCC was “wrong”, says Tim Osborn, professor of climate science at the University of East Anglia. But its assessment may have been different had the IPCC had the new data available at the time, he says.

Piers Forster, professor of Climate Change at the University of Leeds and one of the scientists involved in the section on the ‘hiatus” in the last IPCC report, tells Carbon Brief:

“This study has not magicked the hiatus away or somehow corrected the IPCC…I strongly dispute that the IPCC report got it wrong on the hiatus…The IPCC made a very cautious and preliminary assessment of the hiatus acknowledging that the change wasn’t significant.”

But the new study shows the importance of refining the way temperature data is collected and handled, Prof Richard Allan, climate scientist at the University of Reading, tells Carbon Brief:

“This study highlights the care that is required in turning measurements into a credible climate record.”

Dr Ed Hawkins, also a climate scientist at the University of Reading, echoes this point, saying:

“The process is never finished. Climate scientists continue to refine our understanding of past temperature changes.”

Hawkins makes another interesting point, which is that this means estimates of climate sensitivity based on recent temperatures “may need a slight upward revision”.

Not the last word

Issues of data quality aside, what role has natural variability played?

Prof Sir Brian Hoskins, chair of the Grantham Institute at Imperial College London, says:

“[I]t had been thought that the reduction in surface warming must be due to natural variation in the heat exchanged between the atmosphere and ocean. Now it appears that any such exchange of heat between the atmosphere and ocean has not been large enough to obscure the global warming trend.”

But we shouldn’t dismiss the presence of a “slowdown” in surface warming just yet, warns Osborn. He says:

“There are other datasets that still support a slowdown over some recent period of time, and there are intriguing geographical patterns such as cooling in large parts of the Pacific Ocean that were used to support explanations for the warming slowdown.”

Neither should scientists stop seeking to understand the role of natural decadal variability in influencing short-term trends in climate, Osborn says. Dr Peter Stott, head of climate monitoring and attribution at the Met Office Hadley Centre, echoes this point, saying:

“[N]atural variability in the climate system or other external factors have still had an influence and it’s important we continue research to fully understand all the processes at work.”

On the whole, scientists seem to welcome the new study in terms of its contribution to fine-tuning the global surface temperature record. But the so-called “hiatus” – its causes, consequences and even its very existence – is a multi-faceted topic. Forster predicts:

“I still don’t think this study will be the last word on this complex subject.”

Karl et al. (2015) Possible artifacts of data biases in the recent global surface warming hiatus. Science. Doi: 10.1126/science.aaa5632