The Asian monsoon influences a vast area. Understanding how it has changed in the past explains a great deal about climate change, the landscape of the Asian continent, and also cultural adaptation to these environments...
Deposits in a Chinese cave tell the story of the region's climate stretching back more than 200,000 years, well past the last interglacial warm period — an invaluable resource for understanding the Asian monsoon.
In the quest to understand past climate change, and thus to anticipate future trends, records from cave deposits — speleothems — are increasingly taking centre stage. Wang et al.1 present a virtuoso study: a 224,000-year chronicle of the past variability of the East Asian monsoon, recorded in the oxygen isotope ratios of stalagmites built up from the floor of the Sanbao Cave in eastern central China. This is the latest in a series of records2, 3, 4, 5, 6, 7 from Chinese caves that has illuminated the workings of the East Asian monsoon, over timescales ranging from thousands of years to tens of millennia. Much remains to be understood, but the significance of this work extends well beyond the caves and the monsoon that feeds them. It provides yet more backing for a once-daring hypothesis: that wobbles in Earth's passage around the Sun are a prime mover of long-term monsoon variation.
As Earth moves around the Sun, its orbital eccentricity (the deviation of its path from a perfect circle) and its obliquity (the tilt of its rotational axis) vary slowly over time. The axis of its rotation also wobbles like that of a spinning-top, a phenomenon known as precession. These effects combine to induce a 23,000-year quasi-periodicity in the distribution of incoming solar radiation (insolation). At different stages of this slow precessional cycle, insolation at a particular place on Earth's surface may be strongest during winter, summer, or somewhere in between.
In 1981, John Kutzbach recognized8 that the changing seasonal contrast in insolation might have a significant effect on the Asian monsoon, which is driven by different rates of seasonal heating over the continents and the oceans. By taking the values for the amount of radiation hitting Earth 9,000 years ago — when the Northern Hemisphere was closer to the Sun in summer than it is today, and the influence of glacial ice from the preceding ice age had all but disappeared — and plugging them into a climate model, he calculated that the Asian monsoon circulation must have been more intense at that time than it is today. That result matched observations that rainfall was greater in many areas of the tropics between 10,000 and 5,000 years ago. Various data sets have since been used to establish the details of this relationship over many precessional cycles.
Compared with other stalwart proxies of palaeoclimatology — records from tree-rings, sediments, ice cores, corals and the like — speleothems are relative newcomers. Like many proxies, they record the ratios of different oxygen isotopes in material laid down over time. These ratios are sensitively linked to the composition of precipitation, and thus to the prevailing climate. There are valuable examples of speleothem records from most continents, from mid-latitudes into the tropics, but few regions have proved to be as fertile as eastern China. Among the exciting records stored here are several from the Dongge Cave that span the Holocene, the interglacial period from 11,550 years ago to the present3, 6. These complementary records elegantly confirm the orbital theory of monsoon variability, but also reveal decade- to century-scale variability that differs between geological formations (Fig. 1).
Speleothems work best when there are consistent conduits for moisture to work its way through soil and rock into a relatively closed cave where, drip by drip, it contributes to the build-up of stalagmites. As carbon dioxide is lost from this dripwater, calcium carbonate (CaCO3) precipitates. Many factors, including evaporation in soil and the nature of the cave environment, can change the oxygen isotope composition of the water after it leaves the atmosphere9, 10, interfering with the primary climatic signal. The robustness of a speleothem as a climate record thus depends on a large signal-to-noise ratio and on replication and/or calibration to identify clear climate signals.
Wang and colleagues' Sanbao Cave record1 shows that essential aspects of past monsoon variability can be replicated not just within a single cave, but also between widely spaced caves in the same region: the monsoon signal swamps non-climatic and local noise over the long timescales of orbital change. What emerges is a record of monsoon variation unprecedented in its detail and chronology stretching back 224,000 years. The primacy of orbital precession in driving the monsoon with a quasi-periodic beat of approximately 23,000 years is nicely revealed, as are details of millennial monsoon variability that show the influence of changed ocean circulation in the North Atlantic during glacial periods.
These results bring into sharp relief the true power of speleothems: the ability to date their records precisely. This is made possible by measuring the growth of the isotope thorium-230 from the slow radioactive decay of uranium, which is incorporated in trace amounts in the speleothem deposits. This method works for samples hundreds of thousands of years old, far beyond the limit of about 50,000 years that radiocarbon dating allows. Until now, the best well-dated, high-resolution records of climate variability from the Northern Hemisphere have been those from the long cores extracted from the remote Greenland ice cap. These justly famous records extend back only into the last interglacial period, less than 125,000 years ago, and uncertainties in the models used to date the cores remain above the precision possible with uranium–thorium dating11.
The Sanbao Cave record reveals that there is more to monsoon variability than a simple linear response to precessional climate effects. Precession unsurprisingly controls the largest changes in amplitude in the Asian monsoon, by altering the supply of latent heat from the Southern Hemisphere or the amount of heating over the adjacent, 5,000-metre-high Tibetan Plateau, or possibly both8, 12, 13. But the maximum-insolation peak during the last interglacial seems to have produced a weaker monsoon than smaller insolation maxima during the glacial period that preceded it. The monsoon response is also far less uniformly sinusoidal than the precession-induced variation in insolation, making it hard to judge the true nature of the phasing between the two effects. Work to unravel these mysteries will have to tap a variety of proxy sources and elaborate on the mechanisms linking monsoon variability to broader climate variability14, 15.
The smaller-amplitude, higher-frequency variations of the monsoon that occurred during both glacial and interglacial periods are even more of a challenge: here, the discrepancies between individual Sanbao records, just as with the Dongge data, indicate that details may be clouded by the smaller apparent signal-to-noise ratio (Fig. 1). Is the problem related to noise associated with cave processes? Or is it simply that smaller changes in climate forcing yield a monsoon response that varies more from place to place than is supposed? The answer will come from continuing to build up a network of data from different proxy sources, such as speleothems and lake and marine sediments, that covers the most recent glacial cycle, and especially the past 10,000 years.
The foremost goal is, of course, to anticipate how the Asian monsoon might change in the future. One thing seems certain: the monsoon is sensitive to climate changes, and if the future brings a sufficiently large net increase in summer heating of the Tibetan Plateau, its response could be large and relatively homogeneous. That could be good for those living in the shadow of the monsoon who need more rainfall. But a stronger monsoon would be hard on those in parts of south and east Asia already plagued by summer flooding. As sea levels rise along with monsoon floodwaters, the low-lying areas draining monsoon Asia could be especially at risk.