Each spring, farmers look forward to the thaw, a natural heating up of frozen ground as it turns soft under balmy climate conditions. When the ice melts away, and the soil warms, spring growing season has officially begun. In recent years, however, spring thaw dates have come earlier, making them harder to forecast.
NASA scientists have recorded an earlier regional thawing trend across northern high latitudes, advancing almost one day a year, since 1988. This trend, a likely result of global warming, leads to a longer growing season and supplies more time to harvest, which on the surface can be seen as positive. Some new studies, though, warn that this situation could actually increase the effects of climate change in the long term.
Why? Early thaw has the potential to alter the cycle of atmospheric carbon dioxide intake and release. A longer growing season promotes more carbon uptake, which is then stored in seasonally frozen and permafrost soils. But when permafrost soils thaw and dry out, higher temperatures in the fall promote release of the stored carbon back into the atmosphere. This process is projected to increase over time at an accelerated rate, sending carbon dioxide levels soaring, and further warming the planet.
“Global warming leads to earlier thaw dates and delays the first frost in the fall,” says Dr. Lianhong Gu, a Research and Development Staff Scientist for Oak Ridge National Laboratory in Oak Ridge, Tennessee. “A recent study found that carbon release due to warming in the fall more than cancels the increased carbon intake in the spring.”
The “Easter Freeze” of 2007, which devastated many crops in the eastern U.S., showcases another alarming trend – the occurrence of late-season frosts following an earlier thaw date. Thaw and freeze is a process that occurs when temperatures change above and below the freezing point, with plenty of moisture in place. Repeated thaw and freeze can cause damage to buildings, bridges and archaeological sites. It can also create problems for wildlife, natural plants and crops.
Gu says climate models project that future winters will be warmer and moister, and that means we can expect more thaw and freeze cycles. In addition, we can expect more crop and natural plant damages in winters and early springs. Early springs make plants particularly vulnerable to late-season frosts, and the resulting young, tender plant tissues tend to be prematurely developed. And, as plants start to grow earlier in the spring, they also stop growing earlier in the fall – even though temperatures are still high.
This process affects a plant’s carbon uptake abilities too. As a result, the 2007 freeze is liable to have lasting effects on carbon balance in the region, even well into 2008. In terms of calculating a similar thaw and freeze scenario for this year, Gu says that year-to-year variations are tough to foresee, and that what happened in 2007 may or may not repeat this year.
One scenario that is easier to predict relates to crop productivity. Since plants cannot re-absorb nutrients from dead tissue, nutrients will become less available to plants that experience a thaw and freeze cycle. If plants have difficulty absorbing nutrients they will not grow well. In fact, they may even die, and in turn, produce less food.
Gu suggests that farmers prepare for longer growing seasons by planting earlier and planting alternative crops. They should also take late-season frost into consideration when making farming decisions. Native species fared much better than introduced, cultivated species during the 2007 freeze, and farmers can learn a lot from those results. For retailers, says Gu, variety is key.
“There is a need for a balanced combination of products produced from both frost tolerant crops and vegetables and opportunistic crops and vegetables,” says Gu. “Retailers – and farmers – will need both to ensure safe supply and to take advantage of longer growing seasons in the years to come.”