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Page 1 of 2 Vineyards in Scotland and the gradual disappearance of the English cottage garden are just two possible scenarios that could result from climate change, according to a study by Richard Bisgrove and Paul Hadley of the University of Reading.
Perhaps the most visible impacts of extreme climatic events are disasters such as the prolonged drought of 1976, the storms of 1987 and 1990 that felled millions of trees across the UK and torrential rains and flooding in 2000 and 2001.
Experts such as Drs Richard Bisgrove and Paul Hadley of the University of Reading describe these events as "a wake-up call from the weather". In their report Gardening in the Global Greenhouse - Climatic Change and Gardens, they conclude that as well as general rise in temperature, the frequency of floods, droughts and perhaps storms seems set to increase.
However the effects of global warming are not exclusively negative. Among the report's conclusions, drawn from a careful study of the available evidence on how plants and garden components may respond to an altered climate, are that milder winters will allow a greater range of more tender species to be grown outdoors.
The growing season will lengthen steadily and the number of flowers and yields of many fruit and vegetables are likely to increase. "Grape cultivation could extend as far north as Scotland in the second half of the 21st century," the authors conclude.
Unquestionably, however, climate change will present gardeners with problems, some quite easy to deal with by changes to garden management, but others more difficult to address. Given that predicting the future is inherently inexact, what are the effects of global warming likely to be on the British climate? How will garden plants respond to them? And in what ways should gardeners change their horticultural practices to minimise the impact on their own plots?
Nothing newClimate change is not a new phenomenon; it has occurred many times in the earth's history. In fact, records show that the current phase is part of a gradual warming that has been occurring for the last 150 years. The primary cause is thought to be the release of so-called greenhouse gases into the atmosphere as a result of human industrial activity.
The most important gas is carbon dioxide (CO2), produced primarily by the burning of fossil fuels. CO2 has little effect on the amount of sunlight reaching the earth's surface, but traps the heat energy this sunlight produces, keeping it within the atmosphere and preventing its re-radiation out into space. What is different about the present warming is the speed at which it is happening, making it much more difficult for plants and animals to adapt to the changing conditions.
It is hard to predict how much warming will occur as so many factors are involved. A study by the UK Climate Impacts Programme, which involved the RHS, National Trust, Anglian Water, DEFRA, English Heritage, Forestry Commission and Notcutts Nurseries, into how the British climate may change in the 21st century concluded that average global temperatures are likely to rise by 2-4.5°C (3.6-8°F) by 2080. This takes into account rapid economic growth, particularly in developing countries, and continued reliance on fossil fuels - the 'high emissions scenario'.
Even if globally we adopt less polluting technologies and limit human population growth - the 'low emissions scenario' - average temperatures will rise by 1-2.5°C (1.8-4.5°F) by 2080 at the present rate of 0.1.3°C (0.18-0.5°F) per decade. These rises may seem relatively small, but changes of fractions of a degree can have a major impact on weather and climate.
The amount of warming is predicted to vary substantially across the globe. A number of mathematical models predict that the maximum amount of warming will be felt in temperate areas of the northern hemisphere, such as the UK and much of North America.
In both low and high emission scenarios, further climate change is inevitable and the likely effects on the UK climate include: - Higher annual temperatures producing a longer growing season;
- Greater warming in summer but milder winters with higher minimum temperatures, less of a temperature range and few frosts;
- An increase in rainfall of up to 10 per cent, but with 1-3 per cent more in winter and 20-55 per cent less in summer, leading to an overall reduction in soil moisture.
- More summer droughts and drier autumns.
- More erratic weather patterns, with a greater frequency of torrential rain, temperature extremes and possibly storms.
Within these overall patterns for the UK, there are likely to be pronounced regional differences with the lowest rainfall and highest temperatures in the South East. The predicted rise in sea level will also be felt most in South East England where land is already subsiding and more vulnerable to increased coastal erosion and flooding.
Climatic effects on plantsExperimental evidence shows that increases in temperature and levels of carbon dioxide - one raw material for photosynthesis - both increase the growth of agricultural crops and forest trees, but the majority of garden plants have not been studied. Carbon dioxide in the atmosphere is currently 350 parts per million (ppm) or 0.035 per cent, and is predicted to rise to anything between 525-810ppm (0.05-0.08 per cent).
Experimental evidence suggests that plant growth increases by as much as 50 per cent with a doubling of CO2. Plants tend to use water more efficiently with more CO2, as their stomata do not need to be open as much and bud burst, flowering and fruiting are all hastened by elevated CO2 levels.
Rising temperatures also enhance growth rates, and hasten germination, bud burst leaf expansion and flowering. Grape bud-burst and maturity dates may advance by up to 50 days and yields may increase by 25 per cent.
However, there is also a downside that we are already experiencing. Spring arrives two to six days earlier per decade at present. Results have been precocious spring flowering, with more frequent frost damage and extended lawn growth making year-round mowing necessary in some areas.
The combination of rising temperatures and an increase in CO2 levels has a mutually reinforcing effect on many plants as higher temperatures make them respond more vigorously to enhanced CO2 levels. However, this may not automatically be reflected in higher yields.
Benefits are likely to be most marked in vegetables harvested relatively early in their life cycle, such as carrots, rather than onions, cauliflowers and others harvested towards the end. Warmer temperatures can actually shorten the growth duration of onions and some brassicas, or cause them to 'bolt', making yields fall.
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