Climate jigsaw puzzle
28 March 2012
Dr Adam Scaife, Head of Monthly to Decadal Prediction at the Met Office, talks about piecing together the UK climate puzzle.
In 2011, we had the warmest spring and the second warmest autumn in records that go back for more than a century. In 2010, the UK's coldest start to winter on record, with heavy snowfalls causing disruption, cost the economy up to £130 million a day. This followed 2009/10 which was another very cold winter. Similarly, our recent summers seem to have been out of sorts, with the last five in a row being wetter than average.
We expect to see fluctuations in our weather and climate from one week, month or year to the next. This is natural variability - chaotic processes that govern our climate which mean we don't get consistency, especially in the UK. But several seasons in a row with similar non-average traits demands a closer look, because the longer it goes on, the more statistically significant that becomes.
It's like tossing a coin and getting several heads in a row - not impossible but you might wonder whether the odds are even. The question for climatologists is whether the last few years are an unlikely series of natural variability events or a trend in our regional climate. If it's a trend, how long will it last? What's causing it? Can we predict it? These are questions at the heart of our work. Working alongside researchers all over the world, we've made huge progress in understanding the processes at work in the Earth's climate system.
Understanding El Niño and La Niña is one example of that progress. This long-term cycle sees variations in tropical Pacific Ocean sea-surface temperatures. Our research helped show that this cycle has impacts all over the world. We now better understand these impacts and reproduce them in our climate models. El Niño years - when Pacific sea-surface temperatures are warmer - tend to drive colder winters in the UK.
This is just one factor affecting regional climate in the UK. Another is Arctic sea-ice, which recently reached its second lowest minimum extent since records began. When sea-ice is at low levels, it may produce more easterly winds over the UK in winter - bringing colder air from the cold Eurasian winter continent and causing freezing temperatures. However, again this is just one factor and it can't explain the past or predict the future in isolation.
Our latest research shows how variations in solar output may affect winters in the UK. If new satellite measurements are correct, periods of low solar activity can also drive more easterly winter flow. Other factors also influence winter climate, such as Atlantic sea-surface temperatures, a cycle in high-altitude equatorial winds called the Quasi-Biennial Oscillation, and volcanic activity.
Greenhouse gases must also be considered to explain European and global climate variations. While in the long-term we expect increasing carbon dioxide to warm all our seasons, it's possible there could be short-term feedbacks from changes these gases are causing. The reduction in Arctic sea-ice extent is one example of a feedback from increasing greenhouse gases that may cause an effect which is opposite to what we might expect.
Each factor exerts its own influence and potentially conflicts with others, so no single factor controls the season ahead in the UK. Unpredictable variability due to chaos in the climate system also has to be thrown into the mix and this makes definitive forecasts impossible. You can't simply look at the indicators and tot up those in favour of, say, a colder winter and those against. Instead, all of these impacts and cycles interact with each other in the equivalent of a global climate jigsaw puzzle.
It's this puzzle that we're piecing together through our research. Some climate cycles are understood well enough to be reproduced in our climate models, so impacts can be included in our forecasting. But they're not all there yet. Our long-term predictions are among the best in the world but we continue to work with the ultimate aim of providing more reliable and skilful forecasts as we already do for our shorter-term forecasts - recognised around the world for their accuracy.
- See a profile on Dr Adam Scaife, Head of Monthly to Decadal Prediction at the Met Office