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Why the Met Office’s revised forecast still doesn’t show global warming has stopped

  • 09 Jan 2013, 10:00
  • Roz Pidcock

There have been claims in the papers today and yesterday that new figures from the Met Office show global warming is "at a standstill" - and that this is set to continue for the next few years. But while the new figures do suggest the recent slower rate of temperature rise may continue for a few years, this doesn't mean that global warming has stopped - as a statement released by the Met Office underlines.

The Telegraph's article is based on the Met Office's latest temperature forecast, issued at the end of last year. The new forecast says that by the period 2013 to 2017, global temperatures will have risen to about 0.43 degrees above the long term average.

This is 0.11 degrees lower than the Met Office's last round of predictions for temperature rise over roughly the same period, released in 2007.

It appears the Telegraph story was prompted by a blog post on climate skeptic campaign group the Global Warming Policy Foundation (GWPF)'s website, which claimed:
"[T]his is a forecast of no increase in global temperatures above current levels".

The BBC covered the revised forecast on yesterday's Today programme. Both the Times and the Daily Mail swiftly took up the story, taking a similar line to the Telegraph piece.

All the attention prompted the Met Office to issue a statement pointing out that a slowdown in temperature rise does not mean global warming has stopped - a misconception that we've written about before.

What does the new forecast say?

The Met Office predicts global temperature between 2013 and 2017 is most likely to be about 0.43 degrees Celsius higher than the long term average, measured between 1971 and 2000. The prediction includes some room for variation - temperatures could be as much as 0.59 degrees above average, or as little as 0.28 degrees above the average.

 Met Office _Dec 2012_Decadal Predictions

Global temperature rise above the long term average. Observations are in black and the revised Met Office forecast is in blue. Source: Met Office Hadley Centre

Temperature projections are lower in the new forecast than in the Met Office's previous forecast, issued in 2007. The old forecast predicted that by the period 2012 to 2016, global temperature would most likely reach about 0.54 degrees above the long term average with a likely range of 0.36 to 0.72 degrees. So the new projection is about 0.1 degrees Celsius cooler.

To put the new forecast in context with current global temperatures, the warmest 12-month period in the Met Office Hadley Centre global temperature record occured in 1998, with a global temperature of 0.40 degrees above the long term average. This is 0.03 degrees lower than the best estimate for temperature rise by 2017 given in the new forecast.

Natural forces

So why has the Met Office prediction changed? These projections are based on computer modelling of the climate, and the revised forecast is based on the Met Office's new model, HadGem3, which has been updated.

The main difference, according to the Met Office, is the new model better represents recent natural fluctuations in the climate system, such as ocean circulation patterns. Such natural climate fluctuations can significantly affect global temperatures from one year to the next, and scientists are working to include them more completely in computer models. The Met Office said in its statement:

"The Met Office is actively researching potential causes of the recent slowdown in global warming, including natural variability, the recent deep solar minimum, the influence of forcing from short-lived species, such as sulphate aerosol emissions, and the climate response to these forcings."

Global warming hasn't stopped

The Telegraph piece claims that since the projected temperature for 2013 to 2017 is only "a little higher" than the warmest year so far in 1998 this means:

"global warming will have stalled in the intervening two-decade period".

But as we've written before, the fact that temperatures are currently rising slowly compared to the rapid warming from the 1970s until the late 1990s doesn't mean global warming has stopped, or stalled. Rather, natural fluctuations in the climate system are currently having a combined cooling effect on atmospheric temperatures that's damping the full extent of human-caused temperature rise - as the Met Office explained in a response to a similar claim by journalist David Rose last year.

The fact that natural variability can affect global temperature from year to year is why scientists are reluctant to draw general conclusions from less than several decades worth of data. The Met Office's previous rebuttal to David Rose explains further:

"Looking at successive decades over this period, each decade was warmer than the previous - so the 1990s were warmer than the 1980s, and the 2000s were warmer than both. Eight of the top ten warmest years have occurred in the last decade"

Signal or noise?

As Dr Peter Stott, Head of Climate Monitoring and Attribution at the Met Office, said last year,  such periods of slower warming are not unprecedented in the temperature record, a point that the Telegraph notes in its piece. As Stott put it:

"This variability in global temperatures is not unusual, with several periods lasting a decade or more with little or no warming since the instrumental record began".

Natural variability is noise around the long term trend in global temperatures - which are increasing. As the Met Office said yesterday, the new forecasts "have not changed the overall warming signal of about 0.75 °C since 1900."

Meanwhile, the atmosphere is only one part of the climate. The global ocean has been warming considerably during the period that the rise in atmospheric temperatures has slowed, which is causing sea level to rise substantially. The fact that ice sheets are melting in both the Arctic and the Antarctic is another stark indicator of global warming.

As the Telegraph notes with another quote from Stott, the current period of slower warming is likely to reverse at some point soon, which means that "global warming could speed up again at any time". So suggesting that global warming has stopped further up in the text is pretty misleading. This all demonstrates why we need to look at at the full picture when considering the likely course of climate change, not short lived events predicted over just a few years.

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Updated 9th Jan 12:25pm

A sentence in the Met Office's revised global temperature forecast reads "The warmest year in the 160-year Met Office Hadley Centre global temperature record in 1998, with a temperature of 0.40°C above long-term average". This was quoted by several newspapers.

We queried this with the Met Office, as according to a separate Met Office statement released in December last year, the latest HadCrut4 temperature dataset puts 2010 as the warmest year, followed by 2005, then 1998.

The Met Office told us that the 0.40 degrees figure is "based on a 12-month period which isn't synchronised with the calendar year - in which case 1998 is the warmest on record". But they do allow that this isn't particularly clear, and the reference is apparently going to be updated to be in line with the HadCRUT4 records.

It's worth noting that the temperature averages in the 2013 Met Office annual forecast and the figures in the revised decadal forecast are comparisons with different long term averages (1961 to 1990 and 1971 to 2000, respectively) - which adds an extra level of complication to comparisons.

 

 

 

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