As climate changes, world weather agency calls for new baseline

Australia has set many records for unusual heat over the past 18 Peter Hannam, Sydney Morning Herald

The World Meteorological Organisation says it’s time to shift climate baselines because global warming is increasingly setting a new “normal” for weather conditions.

The widely used 1961-90 baseline should be retained as a “stable reference” for climate study but a more current data set – updated every decade – should be adopted to gauge changes in heatwave and rainstorm frequency already under way, the WMO said.

“Rising atmospheric concentrations of greenhouse gases are changing the Earth’s climate much faster than before,” the United Nations agency said in a statement . “As a result, decision-makers in climate-sensitive industries may be basing important decisions on information that may be out of date.”

Under the recommendations, all countries would start using the most recent 30-year period, 1981-2010, as the new baseline. This would be updated to the 1991-2020 period in 2021, and revised each decade afterwards.

Australia’s Bureau of Meteorology supports the two-tier approach, which is likely to be ratified at the WMO congress next May, said Karl Braganza, the bureau’s manager of climate monitoring.

“Due to the long-term warming trend, the climate normals of 1961-1990 are not representative of current climate conditions,” Dr Braganza said, adding the bureau is already using a 1981-2010 reference period for services such as seasonal outlooks.

Australia has set many records for unusual heat over the past 18 months. These include both the warmest calendar year (2013) and the warmest July-June period for the past 12 months.

Climatologists also note a rising frequency of warm days in the past couple of decades.

For instance, over the past 155 years of records for Sydney, the average number of days of 25 degrees or warmer in April is five. However, over the 1995-2014 period, the average of such days has risen to nine.

Of the 22 years with at least 10 days above 25 degrees in April, half have fallen in the past two decades, according to bureau figures.

Monash University Professor Neville Nicholls, who worked at the Bureau of Meteorology for 35 years, says statistics based on the 1961-90 standard “are getting significantly out of date now”.

“If you are basing calculations on it, it is probably getting more and more dangerous to rely on the old climatologies now,” Professor Nicholls said.

This is particularly true for engineers building long-lived assets such as dams, roads and sewage systems that would be subject to more heatwaves and changing frequency of heavy rains, he said.

The bureau’s maps will generally continue to use the 1961-90 period as a baseline but the agency will phase in more interactive options for changing the reference period over the next five years.

Seasonal outlooks already use the 1981-2010 period because they rely in part on ocean data from satellites that only became available in the 1970s. Farmers are also more interested in how the coming planting season is likely to vary from more recent conditions than one based on an average set decades earlier, the bureau said.

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