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"Climate is what we expect, weather is what we get" - Mark Twain
The subject of interest lately is climate change or global warming and this will have some bearing on what goes on in the garden as well as in the wider world. There is an important distinction which must be made between the climate and the weather. The former is the general trend of the climatic conditions whereas the weather is what is happening at the time. So there can be periods when it would appear that the weather is colder and wetter than expected, but taken over the longer term the climate is changing. The main reason for this is an increase in the atmospheric gases which absorb heat and insulate the earth, such as carbon dioxide, methane and water vapour. Since industrialization began the levels of these gases have been increasing much faster due to burning fossil fuels and deforestation. There are many arguments as to whether a change is occurring or what is causing it, but this article is about the effect it is having in the garden.
Many pests and diseases are killed during cold spells so milder winters will lead to an increase in their occurrence. Slugs and their eggs are killed by the frost and the fewer of these around the better.
The winter cull also kills rats which carry diseases such as plague. Recently Anopheles mosquitoes have colonised parts of Lundy Island, most likely by "jumping ship" from a cargo heading up the Bristol Channel. So far they are free from the malaria bug which they carry in other parts of the world, but the potential is there for them to be infected and warmer conditions in the rest of the British Isles could eventually lead to it's spread. Other pests and diseases can be brought due to global trade, and a warmer climate might mean that they become endemic. Many plants, fruit and vegetables are grown all over the world to satisfy the modern taste, so there is great potential for future problems.
Some alien garden pests have moved here from continental Europe in recent years and are moving northward within the British Isles. The Red Lily Beetle was first reported in the south of England in the 1940s and remained there for many years due to the colder northern conditions, but recently it has progressed until it is now here in Northern Ireland. Other insect pests following a similar pattern are the Rosemary Beetle and the Berberis Sawfly.
The types and severity of plant diseases are going to be influenced by the drier conditions expected in summer. Plants will be under stress which makes them more susceptible. Drier roots encourages Powdery Mildew and the waterlogging expected in winter will make root rots more prevelant. On the positive side, Potato Blight will be discouraged by the drier summers.
Seasonal changes have become less defined with spring blending into summer and autumn into winter. Also the growing season is estimated to have increased by about one month since the 1960s. The lengthening of the growing period can have serious effects on the breeding in animals. Hedgehogs have been producing late litters of young which are unable to feed for long enough to store reserves to carry them through hibernation. Also birds which feed their young on caterpillars start their mating process in response to lengthening daylight which is remaining constant - Photoperiodism. If the spring comes earlier due to a rise in temperature, the plant growth and therefore the time when insects reproduce, will be earlier as well, so the food for the nestlings will be gone.
The milder winters have also meant that we can no longer pack the lawnmower away in October as there is still some growth which requires trimming throughout the winter to keep the lawn looking well. This should only be done in drier conditions and when there is no frost, otherwise the grass could be damaged. The spring flowers come earlier so this is exacerbating the problem of pollen allergies and when it is available for insects. There are also more instances of plants blooming out of season, eg. Rhododendrons having some blooms in the autumn.
There will be an increase in winter rainfall which will lead to more waterlogging of soil and many of the less tolerant plants will no longer survive. If plants can be kept relatively dry during the winter they can tolerate fairly low temperatures, but if the soil remains wet for prolonged periods they will rot.
The summers would be expected to be relatively drier so lawns may become brown patches, but there is evidence that the Jet Stream is drifting slightly northward so it is likely that there may be wetter periods, with flash-flooding. This change in the position is thought to be due to the rise in temperatures.
These extremes will reduce the range of plants which can be grown and could change the whole character of the traditional garden. Fruit production has already been affected by the milder winters of the past few years. Blackcurrants have been ripening over longer periods causing problems for commercial growers especially, who require them to ripen together for mass harvesting - also in the garden for jam-making. It is thought that the fruiting buds need a period of cold in the dormant season to keep them all synchronised to mature at the same time.
Weed seeds also take advantage of milder periods, continuing to germinate and grow throughout winter albeit at a reduced rate. If the climate is to get warmer then the amount of garden maintenance will increase.
The change in climate is likely to lead to an increase in the severity of storms and changes in wind direction leading to a different pattern of coastal erosion. For example the South Down coastline is made up of softer rocks so would erode quite fast. There is less likelihood of coastal flooding as the land mass of Northern Ireland is still rising as an after-effect of the last ice age when it sank under the weight of the ice, so possibly cancelling out the effect of melting ice caps. This uplift is known as isostatic rebound or glacial isostatic adjustment.
The melting of ice around Greenland is thought to be slowing down the Gulf Stream which brings warmer water northwards and keeps the climate in the British Isles about five degrees centigrade warmer compared to other places in its latitude. The flow of water is due to a circulation which occurs when the salt water becomes more dense as it cools down and sinks so drawing warmer water from the south. (called thermohaline circulation). The melting fresh water will dilute the salnity so it will be less dense when it cools and the flow will slow. Although records have only been kept since 2005 it has been found with studies of seabed structures that there is some cooling of the Gulf Stream for the first time in 1,600 years. Experts believe this cooling could negate the warming due to climate change to some extent.
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