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World getting warmer and wetter, new dataset shows

  • 28 Jan 2013, 11:15
  • Roz Pidcock

The world has got warmer and generally wetter since the beginning of the 20th century, according to new data just released by the Met Office and a global team of experts. With an extra 50 years worth of observations, the new data tracks how high temperatures and heavy rainfall extremes are becoming more frequent due to climate change.

The UK's Met Office has just published observations from over 6000 temperature and 11,000 precipitation stations around the world, which look specifically at how extreme events have changed between 1901 and 2010.

The new dataset - called HadEX2 - is an update to a previous one that only covered the second half of the 20th century. The analysis is published online in the Journal of Geophysical Research Atmospheres.

Warmer

Extreme temperature or rainfall events are roughly classed as anything that falls outside normal fluctuation around a long term average.

The scientists look at global temperature data to spot extremes, such as particularly hot days. They also measure cold extremes, like how the minimum night time temperature has changed over time.

In all 17 parameters the scientists measure, they find a shift towards warmer temperatures over the 20th century.

Averaged across the globe, the number of cool nights - when the temperature dropped a certain amount below the local average - halved in the last 60 years to 18 days per year. The number of warm nights increased by 55 per cent - to 20 days per year.

The number of anomalously warm days also increased over the 60-year period, but with more variation according to location.

The researchers call any more than six days in a row of anomalously warm or cold days a spell. The plots below show that, on average, cold spells across the world are four days shorter now than in 1950. Warm spells have got longer by eight days, with most of the change occurring since 1980.

 Cold _spell _warm _spell

Cold spells are, on average, four days shorter than in 1950. Warm spells are eight days longer. Source: Donat et al., (2013)

Wetter

The research also finds a global trend trend towards generally wetter conditions across 12 different parameters - although the pattern varied more with location than temperature.

For example, the number of days with heavy rainfall - which means at least 10 mm - and the amount of rain that falls during such events, have both increased in the last 60 years.

There is a caveat to the new analysis, warn the scientists. As the IPCC noted in its 2012 Special Report on Extreme Events ( SREX), there are large data gaps in some areas of the world, particularly Africa and northern South America. According to the new study, this means that the coverage is "still insufficient to provide a truly global picture of changes in extremes".

But Dr Markus Donat of the University of New South Wales and lead author of the study told Carbon Brief:

"[W]ithin the limitations e.g. of incomplete spatial coverage we find larger areas with significant trends towards heavier (i.e. wetter) extreme precipitation than areas with significant trends to less strong (i.e. drier) extreme precipitation."

Historical context

Before this new analysis, scientists only had data available for the last half of the 20th century. Even so, they were still able to spot a trend towards warmer and wetter conditions.

According to the IPCC's SREX (p133 to 149):

"Since 1950 it is very likely that that there has been an overall decrease in the number of cold days and nights and an overall increase in the number of warm days and nights at the global scale ... [I]t is likely that there have been statistically significant increases in the number of heavy precipitation events...but there are strong regional and subregional variations in the trends."

The additional 50 years in the new analysis agrees with those trends, and shows they extend right back to the start of the 20th century. The scientists also note that all trends in temperature and rainfall extremes were more pronounced between 1951 and 2010 than the longer period between 1901 and 2010. As Donat told us:

"This newly produced data set hence increases our confidence in the robustness of trends which were already identified before. Extending the time period covered (110 years) allows to put the changes in a broader historical context."

The human link

The main point of the new paper is to present the new data, not to discuss the causes of the trends. However, Donat told us that a combination of observations and physical reasoning has led scientists to infer a clear link between the frequency of extreme events and human activity. He says:

"A number of previous studies have attributed changes in extreme temperatures and extreme precipitation to increasing greenhouse gas concentrations in the atmosphere (many of these studies used the previous HadEX dataset)."

For example, scientists estimate that the chance of experiencing record-breaking high monthly temperatures is now, on average, five times higher than expected in a climate with no long-term warming trend.

Detecting global changes in the climate boils down to being able to spot a signal above and beyond the noise caused by natural variability in the climate system. These observations, showing an increase in the frequency and intensity of extreme temperature and rainfall events right back to the start of the 20th century, are an important part of understanding what's happening to the planet's weather.

 

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