Global Chilling - Materials & Methods

A detailed description of the methodology can be found here:
Luedeling E, Girvetz EH, Semenov MA and Brown PH. Climate change affects winter chill for temperate fruit and nut trees. PloS ONE 6 (5), e20155.

Winter chill projections were derived from observations of temperature and precipitation collected at 4293 weather stations globally, which were obtained from the National Climatic Data Center of the National Oceanic and Atmospheric Administration of the United States ( Data from all stations between 1973 and 2002 were used to calibrate a weather generator. This weather generator was then used to generate 101 years of weather for each of 20 climate scenarios (1975, 2000 and 28 future scenarios). Since winter chill models require hourly temperatures as input data, idealized daily temperature curves were used for temporal donwscaling. Based on the resulting hourly weather records, seasonal winter chill totals for 100 winters were calculated (101 year for winters in the southern hemisphere).

Safe Winter Chill

It is not sufficient for a grower of tree crops to fulfill the chilling requirement of his/her trees, only occasionally, or in an average year. To ensure high productivity, requirements must be met every year, or at least in most years. In this study, the amount of chilling that is exceeded every year with a 90% probability was interpreted as Safe Winter Chill. That means, that if a grower grows a tree cultivar, whose chilling requirement equals the Safe Winter Chill value of his growing region, the tree's requirements will be met with 90% probability in a given climate scenario. Mathematically, Safe Winter Chill is the 10% quantile of the population of 100 seasonal chill totals. All maps available on this website show Safe Winter Chill.

Chilling Model

Many growers do not use the Dynamic Model for assessing winter chill. Instead, they use older models, such as Chilling Hours or the Utah Model. We regret that at present this may restrict the usefulness of much of the information presented here for many growers. The reason for this is simply that the older models, particularly the Chilling Hours model, have been shown to be inaccurate under warm winter conditions, such as those prevailing in California. In addition to that, our own work has shown that these models are very sensitive to increases in temperature. This means, that if the Chilling Hours model were used in the same way as the Dynamic Model here, predicted winter chill losses would be a lot higher than those shown on this website. Given an increasing amount of evidence that the Chilling Hours is not reliable in warm climates, and unsuitable for climate change analysis, we therefore refrained from putting projections with this model up on this site. The purpose of this website is to provide information on the quantitative extent of expected changes, and we did not want to publish information, which we believe to be inaccurate exaggerations of expected changes. In the long term, we hope that the Dynamic Model, which appears to be the most useful model existing at present, will be adopted by more and more growers and extension services. In the short term, we can only apologize for this mismatch between methods used here and established grower practice. Because the Dynamic Model is still unfamiliar to many, to provide a frame of reference we have compiled a list of chilling requirements for many temperature tree crops from the literature. View table here.

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