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Fruit growers who depend on irrigation are likely to feel serious impacts from global warming, scientists say.

The average temperature during the first 20 years of this century is expected to be 2°F higher than the average for the last 30 years of the last century, and 3°F higher by 2040, according to a study entitled "Impacts of Climate Change on Washington’s Economy" compiled for the Washington State Department of Ecology.

The warmer temperatures are attributed to an increase in the so-called greenhouse gases in the earth’s atmosphere (such as water vapor, carbon dioxide, nitrous oxide, and methane) that allow incoming solar radiation to pass through the earth’s atmosphere, but prevent most of the outgoing radiation from escaping into space. While this is natural and necessary to keep the earth’s temperature in the range that sustains life, burning of fossil fuels, such as gasoline, has increased the amount of carbon dioxide in the atmosphere, raising the temperature.

Dr. Greg Jones, a climatologist at Southern Oregon University, who contributed to the Washington report, said estimated temperature increases vary, depending on whether it’s assumed that the use of fossil fuels, which emit carbon dioxide, continues to increase, remains stable, or decreases.

"If we’re just going on our merry way, we’ll have one response. If we reduce our emissions, we’ll have a different response," he said.

Either way, temperatures will still increase, because the carbon dioxide being emitted today will stay in the atmosphere for a time. "It’s not just going to go away," Jones said. "It’s ramping up. We’re probably going to continue warming, even if we were to stop tomorrow. That doesn’t mean we should not cut back or lower emissions. That’s what governments are looking at—ways we can limit carbon use so we can limit the type of temperature change that will occur in the future."

Water shortages

Scientists say that climate change will affect agriculture in a number of ways. Some might be positive—such as a longer growing season—and others negative, such as lower water supplies.

The report’s authors expect no significant impacts on yields of crops such as apples and other tree fruits, assuming that enough water is still available for irrigation. Higher temperatures, which generally reduce yields, are likely to be offset by higher carbon dioxide levels in the atmosphere, which generally increase yields.

Global warming is not likely to change total precipitation, but more winter precipitation will fall as rain. In addition, the snow pack is expected to melt earlier in the spring. Jones said runoff in the Pacific Northwest might begin as early as February or March, instead of April and May, which could leave less water available later in the season and put pressure on already scarce water resources.

Water shortages in the Yakima River Basin, for example, are likely to become more common. The scientists figure that with a warming of 3.6°F, junior water-rights holders will be rationed to 50 percent of their water every other year by midcentury, instead of once every seven years as they are now.

Though total precipitation might not change, the timing and frequency will, and there are indications that there will be fewer, but more severe rainstorms that might result in flooding.

Sunburn

If temperatures rise, sunburn of apples and other crops is likely to be more widespread, assumes Dr. Larry Schrader, horticulturist with Washington State University in Wenatchee.

However, sunburn is related not just to high air temperatures.

"I’ve seen days when it was 95°F when we didn’t get sunburn and days when it was 86°F when we did get it," he said.

A number of other factors influence the temperature of the fruit surface, including ultraviolet-B rays.

Evaporative cooling reduces the temperature in the orchard, but doesn’t filter out the ultraviolet rays, and so most cooling systems don’t give total protection from sunburn, he said. Growers will need to use other methods, such as reflective products sprayed on the trees.

Although Schrader is not aware of an increasing amount of sunburn over the years so far, there have been reports of more sunburn late in the season. He thinks that the threshold temperature for sunburn might differ as the fruit matures and the pigments in the skin change, and susceptibility might differ by variety.

Sunburn appears to be a greater problem in New Zealand orchards than in Washington orchards, even though the temperature rarely rises above 28°C (82°F) in New Zealand, Schrader noted. That might be because of depletion of the ozone layer, which screens out ultraviolet-B. The ozone hole is caused by the release of ozone-depleting chemicals such as chlorofluorocarbons and methyl bromide into the atmosphere, and is not directly caused by global warming.

Schrader does not think that global warming will lead to higher ultraviolet light levels, but said he’s personally very concerned about the potential rise in temperatures. "I see things happening that don’t bode well for us in the future if we keep going this way," he said.