The US stock market reacted swiftly and negatively to Pattern Energy’s announcement in April that the lowest wind speeds recorded in 47 years in California and Texas during the year’s first quarter had caused a 5% loss of its expected annual electricity generation. And when yieldco NRG Yield announced on 4 August that the second quarter's "historically low" winds had hit earnings and revenues, its shares dropped 11% that morning.
Concerns that climate change might be impacting long-term wind speeds and trends perhaps fuelled the over-reaction, but the primary cause in this case was a high-pressure ridge that formed over the northern Pacific prompting above-average temperatures due to the polar jet stream being further north than usual.
That spelt bad news for western US and Texas in terms of wind power generation, but good news for central Canada and some border US states with wind speeds up to 20% higher than usual.
A trend of long-term change in weather linked to carbon emissions is clear, but it is hard to tease out the natural variability of weather and climate, especially since wind patterns are harder to predict than temperature changes.
Preliminary evidence indicates the impact of climate change on wind is likely to be mild over a current project’s 20-25 year lifetime.
"Climate change doesn’t factor into most wind investment or lending decisions at the moment," said Dr Michael Brower, president and CTO of renewable engineering consultancy AWS Truepower. But, he added, "there are hints in the models of modest change" over 40 to 50 years.
Dr Pascal Storck, global manager of energy services at weather monitoring firm Vaisala, largely agrees. He points to a number of studies that have examined the trends either historically, using data, or looking to the future with climate model projections.
"The take-away from these studies is that there are probably trends in wind speed connected to climate change, but they are small, they vary regionally, and are not consistently of the same sign," he said. However, he added: "We are starting to see more extreme records broken, for example in terms of temperature, precipitation and snowpack. I have no expectation that wind will differ."
For the past three or four years AWS Truepower has added climate change risk (or uncertainty) to its wind resource assessments. It already factors in an 8% uncertainty ratio for ten-year projections, and 7.9% for 20-year projections. Adding climate change uncertainty lifts these risk ratios to 8.1% and 8.5% respectively.
For the financial community, AWS Truepower expresses the uncertainty as a P50:P90 ratio (the standard ratio used in wind power financing). P50 or P90 means there is 50% or 90% probability that the predicted energy will be met or exceeded.
P50 is the mean, and therefore more likely to occur, while P90 is the high-confidence measure. Excluding climate change risk, the company expresses the uncertainty as 90.7% for ten years and 90.8% for 20. When climate change risk is factored in, the ratio falls marginally — to 90.6% for ten years and 90.2% for 20.
A research paper published in 2010 by atmospheric scientist Diandong Ren, then of the University of Texas in Austin, compared various climate projections and found that, on average, higher temperatures would reduce wind power density at turbine hub height in China by about 14% by 2100, equivalent to a 5% reduction in wind speed. However, that is not enough of a reduction to affect long-range wind development planning, said Brower.
A few years ago, Vaisala summarised the research this way: on an annual basis, climate change is predicted to cause stronger surface wind speed values across the boreal regions of the northern hemisphere, including much of Canada, Siberia and northern Europe, and in tropical and subtropical regions in Africa, Central and South America.
However, Greenland, southern Europe, China, India, southern Australia, and much of the west coast of South America are expected to experience decreasing surface wind speeds.
According to atmospheric scientist Sara Pryor, global climate change may alter either storm dynamics or storm tracks or both, particularly in the mid-latitudes and thus may impact the wind resource or the context in which the industry operates at specific locations. But, she cautioned, there has been very limited research so far, especially in Asia and South America.
As temperatures rise, icing on turbines could become less problematic, opening up more sites for wind turbine deployment. Similarly, more ice-free areas of sea would alter the accessibility of project sites, while drifting ice changes the loads on turbine foundations.
Less permafrost would also have a huge positive impact on road construction and repair at high elevations, opening up new avenues for wind development.
A higher sea level may make foundations in coastal areas more prone to flooding, and more salinity can increase corrosion damage. And a change in waves — higher in the North Sea and lower in the Mediterranean — could increase or decrease the loading on offshore turbines, say the researchers.
Component materials fare differently in different temperatures. Rubber seals can become more brittle in the cold, and different lubricants are needed if temperatures rise.