a biosphere out of balance?

Canto: There are so many issues around global warming, so many questions to ask. I want to begin with an article I read in a November issue of New Scientist. It was written by Alun Anderson, a former research biologist and editor-in-chief of the magazine. The blurb at the front of the magazine described this piece as 'dispatches from a collapsing ecosystem', but that is very misleading. In fact, Anderson describes a thriving, resurgent ecosystem, that of the Arctic. Polar bears and narwhals and other creatures at the top of the food chain are in trouble there, but killer whales are thriving and taking over at the top. The ecosystem isn't so much collapsing as transforming:

In purely biological terms, the new Arctic will be more productive than the old, because there is more water, open to sunlight for longer, with more plankton growing in it, and more food supports more life. The first signs are already there. After the sudden collapse of the sea ice in 2007, a satellite-borne sensor, measuring the water's 'greenness', showed that the total productivity of the Arctic seas leapt by 40%. That is a big increase. 

These observations underline one particular theme of mine. The 'save the planet' mantra is bullshit. A few degrees of warming is not a threat to our planet. What it does, of course is threaten the biosphere as we know it. It will transform the biosphere, not necessarily for the worse, from a non-human perspective. It will disrupt human activity, though it would be a gross exaggeration, I think, to suggest that it threatens our species' survival.
Jacinta: Yet there are so many unknowns. They say the oceans are becoming more acidic. I don't know if that's directly linked to global warming, but it would presumably be life-threatening to many species. How will this interact with the burgeoning of species, especially in colder regions, due to global warming? But let's get back to the initial question you asked, in the last post. How do we know that greenhouse gas emissions, particularly carbon dioxide, lead to global warming?
Canto: I know that carbon dioxide levels in the atmosphere are continuing to climb, with current levels being at their highest for 650,000 years, but what precisely is the connection between these levels and temperature?
Jacinta: Carbon dioxide levels have remained fairly steady for a long time, with a balance between emissions and absorption helping to maintain relatively stable climatic conditions. As you know, the current level is around 390 ppm, and that's well up on the range in the half million years or more before the twentieth century,  which, according to evidence from ice cores, has been between 180 and 300 ppm. The concern is that emissions are now outstripping absorptions.
Canto: So what, generally, emits carbon dioxide and what absorbs it?
Jacinta: Emissions are measured in gigatonnes. Some 440 Gt of carbon dioxide is emitted annually, about half by plant respiration, and half by consumption of vegetation by microbes and animals. These figures are, incredibly, balanced by the enormous amount of carbon dioxide absorbed annually via photosynthesis. Now, we must look at human emissions...
Canto: Next time.