Has global warming stopped?

Please note, this page has been archived as of Feb 2013 and will not be updated. 

The argument that global warming has stopped is typified by this quote from Benny Peiser, Director of the Global Warming Policy Foundation, in the Daily Telegraph last December:

"The data on 2010 confirms that since 1998 there has been no overall warming… Climate theory is at a loss to explain this."

The assertion that "global warming has stopped" over the last 10-15 years is  frequently  stated by climate skeptics.

It has been featured by the UK media - for example, in a  Daily Mail article subtitled "The truth is global warming has halted", in the  Daily Telegraph, the  New Statesman and the  Spectator. In  Parliament Lord Lawson suggested that "there has been no warming so far this century at all," and it is even alluded to in the masthead graph of the Global Warming Policy Foundation's website.

The most recent example comes from the Daily Mail - where author David Rose created a graph to show that "global warming stopped 16 years ago". We, and many others, looked into exactly why headlines like this are misleading.

Temperature rise

Those using argument that 'global warming has stopped' point to where the global average temperature measurements conducted by scientists show temperatures falling over time. Skeptics often point to the year 1998 as a turning point, after which global temperatures have fallen.

However, this argument relies on considering global temperature over very short periods of time - ten or fifteen years. There are good reasons to not draw conclusions from such short time periods.

The world's temperature varies in response to natural cycles, in particular the three-seven year  El Nino/ La Nina cycle and the eleven year solar cycle, as well as occasional one-off events like volcanic eruptions. For example, 1998 was hot in part because of a particularly strong El Nino effect.

Over ten or fifteen year timescales natural variation can dominate temperature behaviour, masking the longer-term trend in rising temperatures.

This is evident from the header on the Global Warming Policy Foundation's website, which charts global temperature data over just 10 years:

GWPF_graph

Over ten years - a brief snapshot of climate history - there's no clear up or down trend. However, by increasing the amount of data they consider, scientists are able to see clear trends and exclude the "noise" of natural variablity.

The past hundred years have seen significant periods of time when temperatures have not risen. But by looking at the bigger picture we get a more informed view of what's going on.

Temp _rise

Global surface temperature as measured by the four main datasets since 1880. Source:NASA earth observatory

The Met Office itself has  specifically indicated that short-term observations of its data do not indicate that 'warming has stopped':

Over the last ten years, global temperatures have warmed more slowly than the long-term trend. But this does not mean that global warming has slowed down or even stopped. It is entirely consistent with our understanding of natural fluctuations of the climate within a trend of continued long-term warming.

As one scientist   put it, 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."

Statistically flawed

Because it relies on looking at temperature behaviour over only short periods of time, the argument that "global warming has stopped" has been labelled statistically flawed.

In 2008, the Associated Press (AP) gave four statistical experts global temperature data which had been 'anonymised' so the analysts wouldn't know what the data represented.

According to AP:

Statisticians who analyzed the data found a distinct decades-long upward trend in the numbers, but could not find a significant drop in the past 10 years in either data set. The ups and downs during the last decade repeat random variability in data as far back as 1880.

One of the experts, a professor of statistics at the University of South Carolina, concluded:

"If you look at the data and sort of cherry-pick a microtrend within a bigger trend, that technique is particularly suspect."

This argument has even been rejected by Dr Pat Michaels, one of the most prominent US climate skeptics. In this video, he urges the audience at a skeptic conference not to use the argument that global warming has stopped, because it is so easy to disprove it undermines their credibility.

Global temperatures

The data shows that temperatures have been rising since the 1880s. There has been roughly 0.75°C warming since the beginning of the 20th Century. The period 2000-2010 was the   hottest decade on record. 

There has been a slow-down in the rate of warming over the last decade. Whilst the average temperature rose at about 0.16-0.17 degrees per decade since the late 1970s, the temperature rise through the 2000s has been between 0.05-0.13 degrees per decade. This slow-down has been attributed to natural variability.

The lower end of this estimates comes from the Met Office. The other main datasets which use surface temperature measurements (held by NASA and the NCDC) show greater temperature rise over the last ten years.

The difference may be because the Met Office leaves areas of the Arctic ocean out of their calculations, whilst the other two datasets do not. The rate of Arctic sea ice loss indicates that the Arctic climate is changing rapidly, and would support suggestions that the Met Office method may underestimate temperature rise in the Arctic. Climate skeptics usually ignore the data from the other datasets.

Finally, it's worth pointing a wider point: arguing 'global warming has stopped' based on only the temperature record would be ignoring all of the other indicators of warming.

To support their claim that 'global warming has stopped' climate skeptics cherry-pick a small sample of years from a single dataset. This approach has been labelled 'particularly suspect' by independent statistical experts, and rejected by the scientific bodies which produce the temperature datasets.

Please note - This material was last updated on 11/5/11 and has since been archived.

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