My background is in Maritime Southeast Asia (SEA), so this answer will be biassed towards that leg of the route. Also, the Silk Road is out of my area of expertise so I unfortunately cannot do a direct comparison.

The trade route between China and the Middle East and beyond saw periods of rise and decline, but was generally very popular at least from the 7th century onwards. It was in existence even before that, however we don't yet know quite how well used it was. There are no exact figures on total trade volume but we have enough evidence to know that it was significant and sophisticated.

First, some context: perhaps the most famous study of the trade route and its effects on Southeast Asia was historian Anthony Reid's 2 volume series 'Southeast Asia in the Age of Commerce: 1450-1680', published in the late 1980s. During the 1990s, the archaeologist and historian Jan Wisseman Christie wrote about a 'sea trade boom' from around 900 to 1300, which historian Geoff Wade termed an 'earlier age of commerce' in 2009. In the late 1990s, Wisseman Christie also began to tentatively outline another Asian sea trade boom beginning around the 7th century and ending in the 9th.

The efforts of these and many other scholars changed the way we look at maritime trade in Southeast Asia. Previously, trade was assumed to have moved through periods of booms and busts, with each 'age of commerce' followed by an 'age of depression'. However, the periods of high trading volume being proposed were so long that we are now starting to see an 'age of commerce' as the default for Maritime SEA and the rest of the route, with occasional dips in trading volume.

CHANGES IN TRADING VOLUME

What caused these dips? In most cases it was some kind of political disruption in one or more of the major trading partners. For example, for a long while the Chola Dynasty was responsible for much of the trade coming from India, but this trade was significantly reduced as the dynasty faced war and political chaos from around 1250.

The greatest influence on the route, though, was exerted by China, sometimes in counterintuitive ways. The more obvious reason for this was that China was the biggest single source of supply and demand along the route.

The less obvious reason was that China was also the end point of the main competitor to the maritime route - the famous 'Silk Road'. Thus, when events in China encouraged trade via land, the maritime trade route suffered. Ironically, wars and invasions in China could often stimulate the maritime trade route, as if a dynasty lost access to northern China where the land routes terminated, it might encourage trade through the southern ports instead. Of course, if or when the war reached the south and the ports turned into battlegrounds, the maritime trade route would be badly affected. A change of dynasties, with its accompanying chaos, often led to a temporary reduction in trade.

Some examples of Chinese policies affecting the maritime trade route include:

The Tang dynasty (618-907) discovered that paying Chinese silver for Indian Ocean luxury goods (pearls, fine textiles, coral, aromatic woods) was unsustainable. They thus encouraged the export of Chinese ceramics to balance the trade deficit. Transporting ceramics overland was difficult as they were bulky and fragile, so the Tang encouraged the use of the maritime route, leading to its increased popularity.

750-1250 saw a population boom in southeast China where the ports were located. There were 286,000 households in 742, 654,000 in 980 and 1,537,000 in 1080. The growth in population led to increased maritime trade which in turn stimulated the economy leading to further population increases.

The expansionary monetary policies of the northern Song dynasty (960-1127) led to the minting of 8m copper coins per year, more than double the maximum output of 3m+ of the Tang. This rose to 18m coins in 1007 before settling at about 10m per year in 1021. At the same time, neighbouring economies such as Korea, Japan and Southeast Asia were seeing increased monetisation, however these economies produced limited money of their own. This stimulated the maritime trade route as Southeast Asian merchants brought large amounts of goods, including silver, to China to trade for copper coins.

The southern Song (1127-1279) withdrew support for maritime trade, for example, by banning the export of copper coins and instituting quotas for imported goods. This led to a large decrease in trading volume along the maritime route in the 1200s.

And an example of a counterintuitive effect: The Ming dynasty instituted a ban on foreign trade in 1371. This led to decreased trade along the China-SEA leg of the route but stimulated intra-SEA trade, as local craftsmen ramped up production to make up for the lack of Chinese products.

That last example raises an interesting point, which is that the trade route was not merely between China and the Middle East with the territories in between acting as stopovers. The trade route was more like a trade web, with everyone trading with everyone else. Spices from the Banda Islands in SEA, for example, were shipped within SEA as well as to China, India and the Middle East by merchants from all these places. On the one hand, this meant that disruption of any part of the route could cause disruptions across the whole thing. On the other hand, the route was sufficiently diversified that no one polity could kill the entire thing completely. Even in the second half of the 13th century, which saw the fall of both the Chola and the Song, the trade route continued to exist, albeit in a much diminished form.

THE SIGNIFICANCE OF THE TRADE ROUTE

As mentioned, we don’t have hard numbers for the overall volume of trade conducted via the maritime route. We do, however, have a lot of evidence that it was very significant and sophisticated indeed. Some examples:

ASBESTOS FIBRE

Asbestos textiles, known as 火浣布 (huohuan bu, fire cleansing cloth) were traded through the overland route from the Roman Orient to China in the first few centuries CE. Accounts of its import regularly appear in early Chinese dynastic histories, including the 汉书 (Han shu, History of the Former Han), compiled before 116CE. In the 5th century, however, we have reference to its import by sea in the 刘宋书 (Former Song History). The passage mentions that the dynasty has been cut off from the Central Asian overland trade routes. It also mentions asbestos as one of the rare commodities desired by its rulers, that came from the seas and were brought by ship ‘in continuous stream’. Other Chinese sources from around the same period describe asbestos coming from various ports in SEA, and thus we may assume that maritime trade between the Roman Empire and China was already present in some form by the 400s… right?

Not quite! Because in 2006, a burial site was discovered in Java dating between the 1st century BCE and the 2nd century CE. In this, minute remains of asbestos fibres were discovered, presumably also imported from the Roman Orient. This exciting find speaks of the existence of a maritime trade route several hundred years before the 400s, though we do not yet know what form it took or just how popular it was.

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