How does climate change affect seasonal weather?
Over the past four decades there has been a gradual trend toward higher sea surface temperatures and surface land temperatures across the globe. Increasing greenhouse gas concentrations have resulted in regional temperature biases in the order of +0.8°C (1.4°F) over the UK and Ireland compared to the 1961 - 1990 average temperatures. As temperatures rise further, the climate impacts are likely to become more pronounced across the nation.
The warm bias incorporated into our seasonal weather prediction model has the following impacts:
The inter-annual variability in winter air temperatures over the British Isles are around ±1.5°C (2.7°F) - so the odds of seasonal temperatures being "above average" are now ~34% (one third) greater than during the 1961 - 1990 base period. Evidence from recent studies, published by a attributions group led by Dr James Hansen of the NASA Goddard Institute for Space Studies in New York, indicate that the standard deviation (variability) in climate is increasing. Our seasonal forecasts, based on a shifted Gaussian distribution, indicate that in our current climate 1 in every 3 seasons are below the long term average temperature - a value that is expected to decrease to 1 in every 8 by the year 2050.
Rainfall intensity is now 6 to 7% greater. With a fixed relative humidity, a warmer atmosphere can hold more water vapour, so there is a greater potential for heavier rainfall events than in the past or when the seasonal prediction indicates average or below average UK temperatures.
Flooding due to sporadic rainfall events (thundery showers) are now much more frequent and extremes of wet and dry weather are now more common - an impact that can either be dampened or amplified on a regional scale by a strong blocking pattern in the upper atmosphere. Examples of extreme weather patterns induced by strong jet stream deflections include the droughts in Russia and flooding in Pakistan during summer 2010.
The number of rainfall days has remained largely constant, however there is a steady trend of wetter summers in the north and hotter and drier summer in the south of the British Isles. This is due to a gradual northward shift of the polar jet stream and sub-tropical highs as the world warms. As a long term average, there is generally the same cloud fraction as in the past. Clouds are located at higher altitudes and are more likely to carry a tiny bit more water or ice particles that can fall as precipitation.
Heat waves are between 5 and 30% more frequent during the summer seasons. The higher temperatures dramatically increase the average cost of air conditioning bills by 40 to 60% for home and business*.
The number of days of snowfall in the UK have decreased by 15 to 25%.
The energy demand for household heating in spring has decreased by 10 to 20%*.
*These figures assume that customer behaviour in energy usage and energy efficiency has remained unchanged. In general there has been a steady trend toward a higher temperature comfort zone, which has been offset by improved household loft and roof insulation and through the increased use of double glazing. Air conditioning is now used in households much more frequently and is also considered a desired component of the work place'.