Precipitation is also likely to be affected by climate change, with important implications for viticulture, although it is more difficult to project for the future than temperature. The recent assessment from the IPCC points toward wetter conditions in high latitudes and decreased precipitation in subtropical latitudes such as the southern U.S. and southern Europe.

California may experience a little of both, with a drier south and little change over much of the state. The state may also experience changes in fog patterns. Some climate models predict increased ocean upwelling from climate change, which might intensify fog development and summer onshore flow, though better models of fog pattern are needed.

Warming, however, has additional effects on water availability and demand, independent of changes in precipitation. Warmer temperatures lead to more precipitation falling as rain rather than snow, reducing the valuable storage and gradual release of water that snowpack provides (particularly in California, where the state relies on stored water from melting winter snow to meet its peak water demands during the dry summer).

Even for growers who do not rely on snowpack to irrigate vineyards, increasing future demands for overallocated water resources are likely to add stress for winegrowers. In addition, warming affects soil moisture through evaporation. Warmer temperatures lead to drier soils that, in the absence of increased rainfall, increase irrigation demand.

WHY CARE ABOUT SOMETHING THAT MIGHT HAPPEN FAR OUT IN THE FUTURE? Climate change is not something that will happen in the distant future. It is something that is occuring now and can already be felt, and the emissions pathway we follow over the next few decades will set the trajectory for big differences at the end of the century.

The effects of greenhouse gases are cumulative. The greenhouse gases from our drive to work today will continue to exert their full climate-warming impact for up to a century. This means that the our climate for the next several decades is likely to experience similar amounts of warming, under every emissions scenario, because differences in emissions haven't had a chance to take full effect yet. (Note the overlap in projected temperatures for all the scenarios in Figure IV until about 2040.)

The differences in climate as a result of different scenarios of human greenhouse gas emissions begin to emerge after that time, and these differences amplify over time, leading to the large range of projections by the end of the current century (year 2100), see Figure IV. Further extending the time period over which greenhouse gas impacts are felt is the long time it takes for the vast ocean to respond to changes in the atmosphere.

In California, temperatures are projected to warm an additional 0.6°C (1°F) by 2020; during this time, climate conditions will remain within the range of variability experienced in the past.9 After that time, the warming will be more than we have experienced with historical variability. This means that we would be experiencing essentially new climates in familiar places, climates that behave differently than they have in the past.

WHAT MIGHT CLIMATE CHANGE MEAN FOR VITICULTURE? We know that both the average growing-season climate and temperature extremes such as very hot days can affect vine and fruit development. Generally, prices are substantially higher for fruit from even moderately cooler regions. For example, Cabernet Sauvignon from the Fresno district sold for about $260/ton in 2006, compared with fruit from the Napa district, which sold for over $4,100/ton. This 15-fold difference in price was accompanied by only a 2.7°C (5°F) cooler average annual temperature.

Up to this point, it appears that the modest warming experienced has been positive for grapegrowing in many regions. For example, the incidence of frost in Napa and Sonoma has been substantially reduced with the warming experienced over the last 50 years, and cooler winegrowing regions such as the Mosel Valley in Germany have had more good vintages.

However, further warming is poised to be a risk for many regions. For example, D.B. Lobell et al. showed that wine grapes in California are already growing within 1°C (1.8°F) of a modeled optimumfor springtime temperatures, after which yields were projected to flatten out and then decrease.

ISN'T THE WINE INDUSTRY PROTECTED FROM CLIMATE CHANGE DAMAGE? The wine industry has some advantages in dealing with climate change. Much of coastal winegrowing in California, for example, does not use water from the Sierra snowpack, which is projected to decrease substantially under climate change. Because of the relatively high crop value, many growers can afford to invest in adaptation resources that might not be feasible for less valuable crops.