Precious Time

The Twelfth Hour: Paying close attention to time series can help us to solve some of our most pressing issues.

‘Time and tide wait for no man’. That’s one ancient proverb that never ceases to ring true. Some of our most fascinating insights about a universe in which the only true constant is change itself rely upon this very tendency for everything to vary. The processes we want to understand nowadays often take place far away in time or space. Perhaps all we know about them is a set of sparse observations. Astronomers may only have a few years’ worth of data from many different sources and with large gaps to tell them about other galaxies. When we’re looking back through human records to understand our ancestors’ behaviour, we may only have a few teeth or bones to go on. But from these scraps of knowledge, beautiful edifices of understanding can be assembled if only we look carefully at how things change over time.

Earth’s climate is changing. We can tell that from observations of the world around us. But we also know, thanks to ice-cores, that the climate has been far from stationary in the past. So the big question is, what drives this climatic change, and what relevance does it have for us today? The answer lies in time-series analysis. We know that seasons are caused by the axis about which Earth spins being tilted, so that the Northern Hemisphere leans towards the sun and experiences longer days for half the year (our summer) and the Southern Hemisphere leans towards it for the other half (our winter).

In the early twentieth century, Milutin Milankovich constructed a radical theory: perhaps it’s Earth’s orbit that determines longer-term climate changes as well. He suggested that changes in how rounded Earth’s orbit is, the exact angle of Earth’s tilt and the place in the orbit where each hemisphere leans inwards would all be involved, on different timescales. These timescales interfere to produce a messy trend in temperature changes. But when we look for overlapping patterns in the temperature data from the ice-cores, we can pick out trends on just the timescales he predicted. This is evidence that his model is likely to be correct.

At all forefronts of science, the ability to pick out patterns and relationships in how things change over time remains crucial. For example, it turns out that temperature and carbon dioxide closely followed one another over the course of the ice-core record, with carbon dioxide lagging slightly behind temperature. This gives us insights into how Earth switched so rapidly between warm periods like the present and much colder ‘glacial’ conditions: when the climate warms, the oceans must release more carbon dioxide, amplifying the warming.

Recent work has also compared the sun’s 11-year activity cycle to sea surface temperatures and pressures. There’s a strong correlation, but with a lag of three to four years, which should tell us more about how the atmosphere and oceans exchange heat. Meanwhile, geologists can date the lava accumulated in large ‘provinces’ by ancient eruptions by comparing it to records of how Earth’s magnetic field has switched over time. This tells us whether the lava came all at once from a sudden eruption that could explain a mass-extinction or gradually in wimpy dribs and drabs. And archaeologists may even be able to tell more about how our ancestors lived and why civilisations rose and fell from what they ate, by looking for trends in the carbon and nitrogen isotopes in their remains. It may never wait for us, but as all these cases illustrate, we can learn a lot by paying careful attention to the steady march of time.