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Putting politics and blame aside, global warming is occurring and carbon dioxide concentrations in the world are increasing, says a Washington State University researcher. But how does a grower make sense of all the varying predictions?

There are no clear-cut answers or strategies for growers about climate change.

Computerized models used by scientists to forecast changes from global warming are imperfect, said Dr. Mercy Olmstead, WSU Extension viticulture specialist. Olmstead is part of a WSU team that is assisting the University of Washington to better understand the impact of global warming on high-value, specialty crops.

"We’re being asked to predict the future," she said to grape growers and winemakers attending the Washington State Grape Society annual meeting held last November. Several studies about future wine production in the world and the Pacific Northwest have stirred up controversy, she added. "But we’re doing the best we can to show what can happen."

The list of possible changes that grape growers might encounter under a warmer climate is speculative, Olmstead said, emphasizing that they are things that might happen if humans don’t change their ways.

Producing regions

Regions of grape production will likely shift in response to changing temperatures. Europeans are now beginning to plant wine grape varieties in areas that were once considered unsuitable.

One of the more controversial climate models predicts that grape production in the coming century could shift to western Washington
and Oregon, parts of Idaho, Montana, and Colorado, and the New England states. "And in fact, California is almost ‘not’ in terms of grape production," Olmstead said in discussing the model.

The Pacific Northwest will likely benefit most in the cooler areas from climate change, she observed. Cool-climate grape varieties will shift further north, and current warm areas may be able to ripen different warm climate varieties. More consistent ripening could also be a benefit in marginal areas of grape production.

"These changes will probably not affect us, but affect our children and grandchildren," said Olmstead, who is stationed at WSU’s Prosser research center.

She highlighted one change—a rise in the annual minimum average temperature—that is already occurring in Washington State. While maximum annual temperatures in Washington have stayed relatively stable, according to WSU records dating back to 1924, she said the the minimum annual temperature is increasing (see WSU temperature chart).

"This has implications for winter temperatures and implications for acid retention in our grape varieties," Olmstead said. "It’s [global –warming] happening here. This is really interesting for us to look at."

Washington grape impacts

What else can Washington grape growers expect from warmer temperatures?

Grape phenology changes could occur, including shifts in bud break, veraison, and harvest. There could be more damage from heat stress; effects on cold hardiness; and effects on plant processes (decrease in malic acid, increase in pH, and altering of grape volatiles).

She said that already, the average date of the last spring frost in Washington is 24 days earlier, and the average last fall frost is 10 days later than half a century ago. "So you can see that we will be affecting bud break and hardening off in the fall."

Additionally, warmer temperatures could increase sunburn problems in eastern Washington vineyards, bring new pests and diseases, such as Pierce’s disease and grapevine fan leaf disease, change vineyard design by eliminating the need to use the fan system, and increase the need to use rootstocks due to an increase in nematodes.

Irrigation strategies

Although wine grapes are very efficient in terms of irrigation, growers in junior water right districts, like the Roza Irrigation District, will want to consider their irrigation management and strategies in light of potentially reduced water supplies.

Climate change models for Washington do not predict less precipitation for the Cascade Mountain Range in the future, but she noted that the snowpack is expected to melt faster in the spring from warmer temperatures, which could impact water supplies in later summer months when juice grapes need water.

She encouraged Concord growers to consider switching from sprinkler to drip irrigation as a way to conserve water. "WSU has done research on drip in Concords, and those growers already doing it have proven that you can keep tonnage on Concords with less water."

Another irrigation strategy that can reduce water use is partial root-zone drying, a technique developed in Australia that uses two drip lines to alternate irrigation on each side of the vine or tree row.

Stay informed

"We’re going to impact several parts of our vineyard planting decisions, layout of trellising, grape varieties, and sites," Olmstead said. "Some of the sites we didn’t think were suitable in the past could be in the future for wine grape production, but I’m not sure I’d buy land in Colville just yet," she said. Winemaking will also undergo changes.

She urged growers to stay informed. While there are no definitive answers yet about climate change, growers can be better prepared to make changes by staying informed. With so much data about climate change coming from different disciplines, it’s difficult to distill it down to make sense, she said. However, research and governmental groups are working to make it understandable to the general public.