Climate and weather: The predictability of climate
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Professor Iain Stewart explains the differences between weather forecasts and climate predictions. Climate is the long-term average of weather conditions and responds to factors such as seasonal variations in sunlight, fluctuations in the Sun’s output, volcanic eruptions or changing concentrations of greenhouse gases. Unlike weather, which is impossible to predict more than a few weeks in advance, the more slowly varying nature of climate makes it possible to predict some climate trends months, years, even decades into the future.
Climate is the average and range of long-term weather, over a period of at least a decade and generally about 20 to 30 years. While short-term weather is usually highly variable, long-term climate is more stable. Short-term weather is dominated by chaotic effects, making it impossible to predict more than a few weeks into the future. In contrast, climate is far less chaotic and responds predictably to external factors such as seasonal sunlight variations. It may not be possible to predict whether each day in autumn will be cooler than the last, but scientists can predict with greater confidence that there will be a longer-term seasonal cooling trend from autumn to winter.
The Earth’s orbit round the Sun results in a pattern of yearly variation in the distribution and intensity of sunlight across the northern and southern hemisphere. These seasonal variations in sunlight cause progressive changes in temperature and other weather conditions from one month to the next. Whether a particular summer or winter will be warmer or cooler than usual depends on the specific set of climate conditions in a particular year – this is what seasonal forecasting aims to predict. But the overall trends of cooling from summer to winter followed by warming from winter to summer can be predicted with high confidence because the climate responds in broadly stable and well-understood ways to these regularly changing sunlight patterns.
Some large volcanic eruptions eject tiny aerosol particles high into the atmosphere, where they can stay aloft for a year or more. In large quantities, these aerosols can block out sufficient sunlight to have a significant cooling effect on the planet’s surface. The amount of cooling depends on the size and characteristics of the eruption. But scientists can predict the surface cooling trend in the months following certain eruptions, even though the day-to-day weather conditions over that time can’t be forecast. This is because the climate responds in a predictable way to a reduction in sunlight. Climate models have proved capable of correctly simulating the 0.5°C drop in global surface temperature following the Mount Pinatubo eruption in 1991.
Over decades and longer timescales, changes in the levels of atmospheric greenhouse gases such as carbon dioxide can affect the planet’s surface temperature. There are large uncertainties involved in predicting the magnitude of the climate’s response to these changes. However, the general direction of temperature trends can be predicted – warming trends for increasing levels of greenhouse gases and cooling trends for decreasing levels. For example, climate models correctly simulate – within the uncertainties – the long-term warming trend observed from 1970 to the present, despite the fact that year-to-year variations along the way can’t be predicted with the same confidence.
Nick Dunstone is trying to identify the key processes which determine the behaviour of the climate. Using climate models run on powerful supercomputers, he has demonstrated the potential importance of the world’s oceans in driving climate changes many years into the future.
‘Getting good measurements of the ocean’s temperature and saltiness ensures our climate models start with conditions near to reality. This is imperative if we want to predict how the climate could behave over the next decade,’ says Nick. So he uses measurements from the 3000 Argo floats spread across the world’s oceans.
‘It can be a real challenge getting your head around the climate’s complex interactions. But it’s all worth it for those rare moments when you discover something new.’