Eh, again, I'm no expert, and it's been a year or three (ahem) since I cracked a book on it, but "exploding" seems a bit over-enthusiastic a word. Or, perhaps, any growth compared to almost none is "exploding".
Okay, bring out the big guns...
[20 minutes of web research later]
Okay, the modern horse collar (which I was admittedly hazy on) showed up somewhere well before the millenium:
The Franks invented (or at least perfected) the horse collar, the wheeled moldboard plow, three-field crop rotation, the stirrup, and the water wheel.
(The Franks run was from about the 5th century to the 8th) So, it wasn't that - all that helped spark the so-called "growth" mentioned below...
(But I knew a plow was in there somewhere...)
Standardized Latin and Medieval Economic Growth
University of Montreal
hss.caltech.edu/media/from-c ... er/360.pdf
[i]Between 1000 and the Black Death of the fourteenth century, an acceleration in economic growth permitted Western Europe to double its population. Traditional explanations attribute this upswing to favorable shocks -- an end to external invasions, an autonomous rise in trade or the establishment of feudal institutions. Implicitly, once the effects of these shocks had worn off, the region returned to its pre-industrial Malthusian equilibrium. However, Jones (1987) and Maddison (2001) have suggested that Western Europeâ€™s per-capita income rose considerably between 1000 and 1500. To the extent that these estimates are accurate, one must look elsewhere for the sources of growth in the High Middle Ages.
Innis (1950) suggested that the key to understanding the medieval European economy was a new information technology (IT) ... More recently, Wright (1982, 1991) ... standardized forms of both written and spoken forms of a vehicular language -- medieval Latin -- that allowed people of different dialects to communicate with one another...
This paper presented an argument based on transaction costs that captures this IT hypothesis. The transactions-cost model predicts that by reducing the cost of contracting, a standardization of easily decoded information technology will lead first, to a collapse of hierarchical structures and second, to an increase in the marginal productivity of human capital. During the High Middle Ages, urban growth in Western Europe appears to have followed this pattern. Rapid economic growth was concentrated within a ring that stretched from the North Sea through northern and southeastern France into northern Italy. Cities outside the ring grew to the extent that they supplied the ring cities with human capital.[/i]
Meh, not the most on-point citation, but it seems to support my main point - that relatively nothing was "exploding" at this time (tho' perhaps was becoming poised for it). Now, I was wrong that it was "stagnant", which it had been pre 1000, but the blossom of the renaissance was centuries away.
So, the main growth seems to have been urban, and it took some 400 years for Europe to double. My calc isn't what it used to be, but that's somewhere below 5%/year - quite possibly below .5%.
Reading the paper fully also touches on the concept of innovation, or lack thereof. That, in the absence of impetus from a lord of some sort (be that noble or eclisiastic), a population base tended to keep doing exactly what it had been, and at the same scale.
So, I restate my assertion, that nothing was much growing at this time. Doubling in 400 years is not growth, except in comparison to the absolute stagnation of the first millenia. Even tho' compared to centuries of nothing, or worse in the case of some Roman territories, it was a friggin' explosion.
(Of course, if someone actually ~knows~ what they're talking about, please jump right in and slap some authority into this discussion, one way or another!)