THE CLIMATE ADJUSTMENT PROCESS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 156

Given these findings, it seems likely that any changes in the wind climate over
the time horizon of wind project investments, up to 25 years, will be modest. Even a
5% decrease in the mean annual wind speed over 50 years, if it occurred in a linear
fashion, would result in only a 0.5% decrease in the average wind speed over the first
10 years of a wind project's life. More likely, as some studies have indicated, the
pace of change will initially be smaller. For this reason, and for the time being, the
“climate change risk” for wind project investments appears to be small compared to
other sources of uncertainty in wind resource assessment.
12.1.3 Other Factors That May Affect the Local Wind Climate
Other factors besides climate change linked to greenhouse gases could alter the future
wind climate at a project site. Among these are cyclical weather patterns, including
the famous ENSO as well as less-well-known phenomena such as the North Atlantic
Oscillation (NAO) and the Pacific Decadal Oscillation (PDO). ENSO events, which
are especially influential, last from 6 months to 2 years and occur in cycles of roughly
every 4 - 6 years. Although ENSO's root causes are in some dispute, the phenomenon
is closely tied to wind-driven variations, or anomalies, in surface water temperatures
in the eastern Pacific Ocean. These temperature anomalies have a substantial impact
on weather patterns around the Pacific Rim and throughout the world.
To minimize the effects of such cyclical patterns on the outcome of MCP, the
reference station record should span at least two or three oscillation periods. For
ENSO, that means around 10 - 15 years, a feasible time horizon for most MCP studies
in countries with reliable reference data, but a challenging one in other countries.
Because of the problems already noted with older reference data sets, it is far more
difficult to correct for the PDO, with a period of 20 - 30 years, and other very long-
period phenomena. Considering that the period of these oscillations is comparable
to or exceeds the time horizon of wind plant investments, and that their behavior
cannot be reliably forecast, it is questionable whether such corrections should even
be attempted.
Changes in land cover around a site, especially tree growth or clearing, can also
alter the wind climate. Considering only the displacement effect of a forest very near
the turbines, a typical tree growth rate of 0.2 m per year could reduce the mean wind
resource at 80 m height by perhaps 0.5% by the end of the first 10 years of a project's
life. This impact is comparable in magnitude to possible trends from global climate
change, but unlike climate change, it should be easily forecast based on assessments
of the forest condition. Conversely, clearing of 10 m of trees around the turbines could
increase the available wind resource by up to about 2%, depending on the size of the
clearing. Both types of impact could be larger if the changes occurred over a much
wider area, up to several kilometers, surrounding the project.
12.2 REQUIREMENTS FOR ACCURATE MCP
Assuming the wind climate is stable, three key requirements must be met for MCP to
produce a reliable result: