In phenology, season creep refers to observed changes in the timing of the seasons, such as earlier indications of spring widely observed in temperate areas across the Northern Hemisphere. Phenological records analyzed by climate scientists have shown significant temporal trends in the observed time of seasonal events, from the end of the 20th century and continuing into the 21st century. In Europe, season creep has been associated with the arrival of spring moving up by approximately one week in a recent 30-year period. Other studies have put the rate of season creep measured by plant phenology in the range of 2–3 days per decade advancement in spring, and 0.3–1.6 days per decade delay in autumn, over the past 30–80 years. Observable changes in nature related to season creep include birds laying their eggs earlier and buds appearing on some trees in late winter. In addition to advanced budding, flowering trees have been blooming earlier, for example the culturally-important cherry blossoms in Japan, and Washington, D.C. Northern hardwood forests have been trending toward leafing out sooner, and retaining their green canopies longer. The agricultural growing season has also expanded by 10–20 days over the last few decades. The effects of season creep have been noted by non-scientists as well, including gardeners who have advanced their spring planting times, and experimented with plantings of less hardy warmer climate varieties of non-native plants. While summer growing seasons are expanding, winters are getting warmer and shorter, resulting in reduced winter ice cover on bodies of water, earlier ice-out, earlier melt water flows, and earlier spring lake level peaks. Some spring events, or "phenophases", have become intermittent or unobservable; for example, bodies of water that once froze regularly most winters now freeze less frequently, and formerly migratory birds are now seen year-round in some areas. Relationship to global warming The full impact of global warming is forecast to happen in the future, but climate scientists have cited season creep as an easily observable effect of climate change that has already occurred and continues to occur. A large systematic phenological examination of data on 542 plant species in 21 European countries from 1971–2000 showed that 78% of all leafing, flowering, and fruiting records advanced while only 3% were significantly delayed, and these observations were consistent with measurements of observed warming. Similar changes in the phenology of plants and animals are occurring across marine, freshwater, and terrestrial