Climate scientists trying to project how quickly the planet may heat up say that "the biggest unknown" they have to plug in to their computer models is not data about carbon or ocean currents or sunlight but rather about "what the humans will do."
Specifically, how quickly they will agree (at summits such as this) to cooperate in slashing greenhouse emissions, and in what amounts.
So in addressing natural and universal human fears, the archbishop was talking of and to a critical element in that "biggest unknown" -- the psychological element that affects resolve and the will to act.
As the service ended and the people, holding lighted candles, spilled out into the darkening afternoon, a bell high in the cathedral tower began to ring steadily, one deep strike after another that would stop only when it had rung 350 times.
Other churches as far away as the western Pacific were reported to be doing the same in a coordinated effort this Sunday to raise awareness of this number.
Several climate scientists, agreeing with the calculations of American climatologist James Hansen of NASA, said that humanity must somehow get the concentration of greenhouse gas CO2 back down to 350 parts per million in the atmosphere if humanity is to avoid an acceleration of global warming that would challenge the stability of basic agriculture and water supplies worldwide.
The atmosphere's concentration of CO2 stands currently at about at 389 parts per million, scientists report.
Many countries have agreed that 450 parts per million is the upper limit to aim for.
Wandering just a few blocks from the Cathedral service, some who had just heard the archbishop joined the bustling crowds exploring an exposition set up in the main square with booths and displays offering ideas about how to fight global warming.
In the middle of the square stood a large Christmas tree strung with lights and surrounded by a dozen stationary bikes.
Each bike was hooked up to generate emission-free electricity for the tree lights.
Three young boys laughed as they tried their best on the bikes -- with only moderate success -- to get the tree lights to do more than flicker intermittently.
Then they got off, and a bearded man in his fifties climbed on and showed them how to do it.
Earnest and smiling, he got the pedals turning, steady and strong, and the tree shone bright against the dark winter sky.