Warmest year on record: what does it mean?
Temperature data released by the NOAA’s National Climatic Data Center (NCDC) and NASA’s Goddard Institute for Space Studies (GISS) agree that global surface temperatures for 2010 tied with those in 2005 for the warmest ever recorded.
The Met Office’s Hadley Centre – which administers a different temperature dataset – are yet to release their final figures, but estimated a month ago that 2010 was ” on track to become first or second warmest in the instrumental record.”
The announcement of average temperatures from the three main datasets is an annual event, following the collection of data from 4-6,000 weather stations around the globe. It is accompanied by a scattering of media coverage around the world, and plenty of climate sceptics attacking the organisations responsible for the datasets, particularly the Met Office and NASA.
But what’s the significance of saying that a year is the first, second or third hottest?
Clearly, the annual temperature matters enough for scientific bodies to measure and report on it every year. But although it is tempting to fixate on a ‘new record high’, it’s really only the long-term trend in temperature – found by building up annual measurements – that gives an accurate picture of what’s going on.
Comparing single years – whether 2010 was marginally warmer or cooler than 2007, 2003 or 2009 – doesn’t give as reliable a picture because the difference between each year is so slight it falls well within the range of natural variation.
As Dr Gavin Schmidt from NASA put it last year:
“for any individual year, the ranking isn’t particularly meaningful. The ranking between the second warmest and sixth warmest years, for example, is trivial.”
Natural variability in the climate means that temperatures fluctuate up and down over short amounts of time – like year to year, or even over a decade. Take a look at this graph comparing the world’s temperature datasets from 1880 onward:
Image - Datasets graphs (note)
You can see that looking at individual years doesn’t give a very clear picture of what’s going on – and indeed, there are periods when warming stops or reverses for a period of time. However, the long term trends show warming.
This is why scientists tend to talk about the world getting warmer decade-on-decade – looking at the change in decadal temperatures over longer timeframes filters out short term variation.
A recent paper by scientists at NASA found that:
In the past four decades, relative to the 1951-1980 base period, successive decades warmed by 0.17°C�Global temperature in the past decade was about 0.8°C warmer than at the beginning of the 20th century (1880-1920 mean). Two-thirds of the warming has occurred since 1975.
Of course, rising temperatures are not the only sign that the planet is warming. NOAA itself publishes an annual ‘State of the Climate’ report, which listed ’10 indicators of a warming world’ looking at key indicators like sea ice, sea level, sea surface temperatures, snow cover and glacier melt. They wrote:
‘Seven of these indicators would be expected to increase in a warming world and observations show that they are, in fact, increasing.
Three would be expected to decrease and they are, in fact, decreasing.’
The Met Office is currently predicting that 2011 will be a cooler year as a result of La Nina. Unfortunately, this will not mean that climate change has stopped.