Heavier daily doses of rainfall do suggest a change in the world’s climate

Marlborough’s weatherman, Eric Gilbert, puts last week’s downpour into a broader context.

They’re holding a climate change summit next week in New York.  And they will be all too aware of the World Meteorological Organisation’s recent statement: “It’s not too late to prevent dangerous climate change and preserve the planet for future generations, but it may soon be”.

After the wettest September day I have recorded since 1984 – 40.6mm of rain fell during the night of September 18 – it is interesting to analyse the data on heavy daily rainfall over this extended period.

There is a rising trend for the incidence of daily rainfall in excess of 10mm. In the 1980s it averaged around 22 days per year. The average has now risen to approximately 29 days a year – with the greatest number occurring in 2012 when 45 days saw more than 10mm of rain.

Investigating higher daily rainfall totals reinforces the suggestion that our climate is changing. Taking the last thirty years in 10-year blocks, the incidence of heavy daily rainfall of 30mm and above was six occasions between 1985 and 1994, nine in the intermediate ten years and 14 in the last ten years, with three months still to go in 2014.

Researching the occasions when the daily rainfall is equal to or exceeds 40mm highlights the change. There have been six occasions when I have recorded daily rainfall above 40mm and they have all occurred since October 2000. The record was set in 2013 when  46.4mm was measured on December 23.

The average local temperature has also been rising over this 30-year period. In the 1980s to mid 1990s the average was 9.1C, but the rising trend since then brings the current average annual temperature to 9.7C.

With warmer air holding greater quantities of moisture it is not surprising that the incidence of heavy, localised daily rainfall is rising.

However, scientists have struggled to explain the so-called pause in global warming that began in 1999 – despite ever increasing levels of carbon dioxide in the atmosphere.

The latest theory is that a naturally occurring, current-based 30-year cycle in the Atlantic Ocean is behind the slowdown. The researchers say this slow-moving current could continue to divert heat into the deep seas for another decade.

Details of local weather since 1984 can be found at my website.