New Two-Tier approach on “climate normals”

New Two-Tier approach on “climate normals”



1 June 2015

Congress recognizes urgent need to incorporate climate change in decision making

The World Meteorological Organization will introduce a new a two-tier approach to the 30-year baselines for climate data to take into account the rapid pace of climate change as well as the operational needs for up to date climate information.

Because the climate varies naturally from year to year, climatologists use standard 30-year averages of temperatures, precipitation and other variables to put, for example, the magnitude of a current heatwave or rainstorm into historical context. These 30-year historical averages are called “climate normals” and can be calculated at the local, national or global levels.

Climate normals are presently updated once every 30 years, and the current official climate normal period is 1961-1990. The resulting averaged data are called WMO Climatological Standard Normals. However, rising atmospheric concentrations of greenhouse gases are changing the Earth’s climate much faster than before. As a result, decision-makers in climate-sensitive sectors and industries such as water management, energy, agriculture and viticulture may be basing important decisions on information that may be out of date.

The World Meteorological Congress, WMO’s top decision-making body on Standards, approved a resolution that WMO will update the climatological Standard Normals for operational purposes every 10 years and will use 1981-2010 as the current base period. However, it will retain 1961-1990 as the historical base period for the sake of supporting long-term climate change assessments.

“In a world in which the climate is changing rapidly, we need to update the climate normals more frequently than we did in the past to keep them useful,” said Thomas C. Peterson, President of the WMO Commission for Climatology and Principal Scientist of NOAA's National Centers for Environmental Information.

“But at the same time, we need to keep the historical baseline for the sake of public and scientific understanding about the rate of climate change.”

Many national weather services have already started to use the more recent 30-year period of 1981-2010 for operational services, such as forecasts of peak energy load and recommendations on crop selection and planting times. One consequence of this is that different researchers and weather services are using different baselines, which results in inconsistent comparisons.

The move to two-tier baselines will help harmonize and standardize the differing national approaches and facilitate international comparisons.

The new technical regulation on “Calculating Climatological Standard Normals approved by Congress means that all countries will start using the period 1981-2010. This period would be updated every 10 years, so that the 30-year climate normal to be used in the 2020s would be 1991-2020. But the 1961-1990 baseline for assessing climate change will be kept until there is a scientifically compelling reason for changing it.

Today’s increasingly powerful computers and climate data management systems now make it much easier to conduct more frequent updates, which involve analyzing massive amounts of climate data. Another advantage of decadal updates is that they will make it possible to incorporate data from newly established weather stations into the normals more rapidly.

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