New study suggests solar variability has a regional – rather than global – influence on climate
Release Date 13 October 2010
Scientists from the University of Reading have found a link between the blocking of the jet stream which results in colder winters and variations in the ‘activity' of the Sun, as measured by alterations in its magnetic field.
These findings suggest that solar variability could provide a valuable source of data for seasonal forecasts predicting the severity of European winters.
The study, led by Dr Tim Woollings of the Department of Meteorology at Reading, suggests that variations in solar activity can have an effect on regional temperatures that is much stronger than their effect on global temperatures.
Dr Woollings said: "There are essentially two ways that solar variability can influence climate. One is known as ‘bottom-up', which is due to changes in the incoming solar radiation which directly warms the Earth's surface, and so heats the atmosphere from below. The second is ‘top-down', in which changes in solar radiation affect temperatures in the upper atmosphere and these changes propagate down to influence the jet streams.
"The bottom-up effect can in principle alter global average temperature, but the top-down effect is inherently regional, in that it affects the wind patterns which move warm and cold air masses around. When these wind patterns change, some regions get warmer while others get colder.
"Our work is in agreement with recent satellite observations suggesting that the top-down, and hence regional effect, may be stronger than previously thought. However, the bottom-up effect, and hence the solar influence on global average temperature, may be weaker."
The research suggests that the top-down effect is particularly large in Northern European winter and is associated with changes in ‘blocking'. This occurs when the jet stream bringing warm air from the west to Northern Europe is blocked by north-easterly winds from the Arctic. Blocking episodes can persist for several weeks, leading to extended cold periods in winter. The research team found that in the last 50 years there have been about twice as many days of blocking during a winter when solar activity was low than when it was high.
Dr Woollings added: "Seasonal forecasting is notoriously difficult, especially over Europe. However, there are a few factors which sometimes have a predictable influence on European winters, and this work suggests that solar variability is one of them."
Solar radiation of different wavelengths has different influences on climate. The top-down effect is particularly associated with UV radiation, whereas the bottom-up effect arises from longer wavelengths. This study used variations in the sun's magnetic field to characterize solar variability, and this measure is more closely linked to UV than the measures which are been conventionally used.
The paper, ‘Enhanced signature of solar variability in Eurasian winter climate', by T. Woollings, M. Lockwood, G. Masato, C. Bell and L. Gray, is published in Geophysical Research Letters on 22 October, paper reference: 2010gl04460. Currently in press.
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Press release extra notes: Q and A
Q: What does the paper actually show?
A. The paper shows that since 1957 there is a statistical link between solar variability and weather patterns over Europe in winter. In particular, blocking weather systems have occurred more often during solar minima than solar maxima. This link is stronger when a measure of solar variability is used that better represents changes in UV radiation. The effect is strongest in Europe, probably because the Atlantic jet is particularly susceptible to blocking.
Q. What are "blocking events"?
A. A blocking event is a large, stable weather pattern, generally an anticyclone, which can block the path of the westerly (i.e. eastward-flowing) jet. The jet stream is responsible for the mild south-westerly prevailing winds, and during wintertime blocking these are replaced by north easterlies that originate in the Arctic or the cold Eurasian interior. Regions under the blocking anticyclone are further cooled under clear cloudless winter skies. The duration and number of the cold snaps that blocking events cause is a major factor determining the average temperature for the winter as a whole.
Q. How big is the effect?
A. The average winter temperature in the UK has been about half a degree lower when the solar activity was low than when it was high. The probability of a blocking event occurring was up to twice as high when solar activity was low than when it was high.
Q. How certain are the results?
A. Statistical tests show that there is less than a 5% probability that the result was obtained by chance.
Q. Are there any practical consequences of the work?
Seasonal forecasting is notoriously difficult, especially over Europe. However, there are a few factors which sometimes have a predictable influence on European winters, and this work suggests that solar variability is one of them. Consideration of the solar forcing may therefore lead to an improvement in seasonal forecasts.
Q. Was the cold winter of 2009/10 due to the solar minimum?
The cold weather of last winter was associated with several blocking events, and so it is consistent with the general behaviour we found. However, we can't say that the cold winter was caused by the sun. Other factors such as the El Nino event may also have been important and individual seasons of severe weather can occur purely by chance, with no obvious external cause.
Q. Does this mean the Sun is responsible for global warming?
A. No, the effect is purely regional. This is because the effect is a change in the atmospheric circulation that moves warm and cold air masses around - while one region is warmer than usual another region is colder. In this case if Europe has a cold winter, then Greenland has a mild winter.
Q. What about the recently reported satellite observations?
Recent work led by Prof. Haigh from Imperial College showed that in new satellite measurements the UV radiation decreased more strongly than expected in the approach to the current solar minimum. In contrast, some visible and longer wavelengths actually increased over the same period. Since it is the UV which leads to the top-down (regional) effect, this is consistent with our findings.
Q. Doesn't this show that scientists don't understand the solar contribution to warming at all?
No. The change in the total solar irradiance as the sun's activity varies is of order 0.1%, and this is not disputed. What is being debated is the spectral variation - i.e. how radiation of different wavelengths contributes to the variations. The solar contribution to the observed global warming is conventionally estimated to be of the order of 10 times smaller than the anthropogenic contribution. None of the recent work challenges this.
Q. Does this mean that climate models will need to be re-written?
No. The majority of the climate models which contributed to the last IPCC report included time-varying solar forcing in their simulations of the 20th century. If the recent satellite observations are validated there will be a case for a change in the solar forcing at different wavelengths. For the bottom-up effect this would be a simple change in the values of some parameters which have a very small influence on global temperature. Further modifications may improve the representation of the top-down (regional) effect, in particular improving the modelling of the relationship between UV radiation and stratospheric ozone.
Q. What does it mean for the future?
A. The Sun has been unusually active for the past few decades and now seems to be declining towards average or even low activity levels. If this is true then the anthropogenic warming may be partially offset, but only in Europe and only in winter. For example, it is thought that European winters were about a degree colder during the Maunder Minimum of low solar activity around 1700. Projections of anthropogenic warming in European winter are larger than this, generally in the range of 2-4 degrees by the end of this century.