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Carbon Brief Staff

Carbon Brief Staff

15.01.2013 | 11:00am
ScienceTemperatureAll the reasons why global warming hasn’t stopped
SCIENCE | TEMPERATURE | January 15. 2013. 11:00
All the reasons why global warming hasn’t stopped

Accusations of scientific misconduct flowed from skeptics and some news outlets last week after the Met Office revised downwards its decadal prediction of global temperature rise up to 2017. Although the Met Office has explained why this change to their short-term forecast doesn’t affect their view of the likely long-term warming trend, this didn’t stop the Mail on Sunday resurrecting one of its favourite arguments – that global warming has “stopped”.

This is not a new claim. In the article on Sunday, climate skeptic journalist David Rose claimed the Met Office’s new decadal forecast proves global warming “stopped” 16 years ago – contrary to the Met Office itself. This is a claim he’s been making for well over a year – he dismissed the wave of rebuttals that followed initial claims about the Met Office’s new data as the “Stalinist way the Green Establishment tries to stifle dissent”.

Well, the argument that a slowdown in temperature rise in recent years shows global warming has “stopped” certainly isn’t new – and has been extensively picked apart, discussed, rebutted and critiqued many, many times online. Here, for your amusement, are a selection of responses.

Natural climate fluctuations can slow temperature rise

Climate skeptics often claim that scientists ignore the effect of natural changes on the climate – this is the “The climate has always changed!” argument.

However, the scientific literature is full of discussion of natural fluctuations in the climate – and scientists believe it’s such natural processes which are currently masking the full extent of human-induced warming – making global temperature rise slower than in previous decades. On Tuesday last week, the Met Office said in a statement:

“Small year to year fluctuations such as those that we are seeing in the shorter term five year predictions are expected due to natural variability in the climate system, and have no sustained impact on the long term warming.”

Analysis which strips out known natural influences on global temperature over the last 30 years suggests there is no evidence that human-induced global warming has slowed down – let alone stopped, as this video from blog Skeptical Science shows.

Source: Skeptical Science

Meanwhile, Met Office chief scientist Julia Slingo told the BBC’s Feedback programme on Friday:

“Our latest forecast for the next five years show that earth will continue to be at record warm levels similar to those that we’ve seen over the last decade, and with a fair chance that a new record will be made during that period”.

Making judgements about global temperature usually boils down to how long you measure temperatures for. Others have given helpful explanations of the difference between short-lived natural climate variability and longer term climate change. Scientists Gavin Schmidt and Stefan Rahmstorf said in a post for science blog Real Climate in 2008 that considering only 10-15 years of temperature is like “analysing the temperature observations from 10-17 April to check whether it really gets warmer during spring.”

To demonstrate this, Rahmstorf and Schmidt show the graph below. The red line represents annual global temperature from 1977 to 2007 and each blue line is the average of an eight-year period of data. As the scientists explain,  the trends over short periods are variable; sometimes small, sometimes large, sometimes negative – depending on which year you start with. But over a longer time period, the upward trend in global temperatures is clear.

       Sk S_8_year _versus _long _term _trend

Source: Real Climate

Even prominent US climate skeptic Dr Pat Michaels told an audience that the global warming has stopped argument is “a little bit unfair to the data”. He suggested not using it because it is ultimately easy to disprove – temperatures will go up more in the future – which could undermine credibility.

Finally, need a more facebook-friend illustration of why short-term fluctuations in the weather don’t negate longer term trends in the climate? Try this video about taking a dog for a walk.

Cherry-picking data can flatten out temperatures

Other responses to the “global warming has stopped” argument deal with so-called “cherry-picking” – selectively citing data to support a particular point. In the case of global temperatures, people have been known to suggest 1998 as the year when climate change “stopped”. 1998 was a particularly warm year due to a natural climate cycle known as El Niño. Comparing with this particularly warm year can be used to make subsequent temperature change appear negligible.

In a previous comment on this issue, the Met Office explained why this can be a problem, and why this leads them to talk about temperature change between decades, rather than years:

“[C]hoosing a starting or end point on short-term scales can be very misleading. […] If you use a longer period from [temperature dataset] HadCRUT4 the trend looks very different. […] Looking at successive decades over this period, each decade was warmer than the previous…Eight of the top ten warmest years have occurred in the last decade”.

Or as Dr Myles Allen at Oxford University told the Science Media Centre last week:

“Comparing the expected temperature for 2013-2017 with a single exceptionally warm year (1998), as some reports have done, is just daft.”

Of course, the argument to look at long term trends can also cast over doubt statements about global temperature rise “accelerating” when there are a series of particularly hot years.  Dr Allen points out that just as the current – and temporary – cooling effect of natural factors is not evidence that global warming has slowed, a series of warm years shouldn’t be used to suggest global warming has accelerated. He says:

“[A] lot of people (not the IPCC) were claiming, in the run-up to the Copenhagen 2009 conference, that ‘warming was accelerating and it is all worse than we thought’.  What has happened since then has demonstrated that it is foolish to extrapolate short-term climate trends. We did see unexpectedly fast warming from the mid-1990s to the early-2000s, but the IPCC, quite correctly, did not suggest this was evidence for acceleration.”

Like to look at data more closely? Blogger and climate statistician Tamino took a more  detailed statistical look at temperature data up to 2011. Or if you prefer fewer graphs and more pictures, try here for visual instructions on how to cherry pick data.

Such ‘slowdowns’ are not unprecedented

There is scientific research showing this isn’t the first time in recent history natural fluctuations in the climate have had a masking effect on greenhouse gas warming. What’s more, we’ll probably see similar periods in the future. As the Met Office explained last October:

“Over the last 140 years global surface temperatures have risen by about 0.8ºC. However, within this record there have been several periods lasting a decade or more during which temperatures have risen very slowly or cooled. The current period of reduced warming is not unprecedented and 15 year long periods are not unusual.”

The oceans are a big heat sink

Finally, the vast majority of the warming of the earth’s system since 1955 has gone into the oceans, as these two very simple graphs from the blog Skeptical Science show, here and here.

Earth 's Total Heat Content _SkS                                                                                                                                                                                                                                                                                                    Where Is Global Warming Going _infographic

The vast majority of warming in the last half century has gone into the oceans.  Source:

Atmospheric temperatures are only a small (but important) part of the picture. Professor Chris Rapley of University College London told the Science Media Centre last week:

“90% of the energy imbalance enters the ocean and is not visible to the global mean surface temperature value.”

Oil companies sometimes get a bad rep for not talking enough about the causes and risks of climate change. But David Hone, Shell’s climate change advisor, appears to have gone out of his way to dispel the myth that global warming has stopped, telling the Huffington Post yesterday:

“[R]eal “global warming” is far broader than [atmospheric temperatures] and includes ocean heating (surface and deep ocean) and land ice melting”.

He continues:

“To simply argue that “global warming has stopped” is short sighted. The evidence to support such a claim is not there.”

We wrote a bit more about just how much heat the ocean take up in response to yet another Mail on Sunday article.

New story, old argument

So, we’ve seen another round of “global warming has stopped” stories in certain sections of the media. But as Dr Richard Allan of the University of Reading told the Science Media Centre:

“Nothing in [the Met Office’s] data leads me to think that global warming due to human influence has stopped, or is irrelevant.  It hasn’t, and it isn’t.”


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