Examine the plight of the coral atoll nations and you begin to see why combating global warming is such a fiendishly complex issue on which to negotiate.
No group of countries appears to be more endangered by man-made changes in climate, as worldwide economic and population growth alters the level of heat-trapping gases in the atmosphere.
But it will probably be at least another 10 years before scientists can agree on forecasts for exactly how high sea levels will rise around the world in response to various scenarios for how much fossil fuel humanity consumes.
Even when that is done, it is no simple matter to work out what any given sea level rise means for each atoll. The coral reefs which fringe them may be able to keep pace by growing upwards - they have coped successfully with big, natural changes in sea level in prehistory. In some atolls, however, the coral is already temperature stressed - the warmth of the water is limiting growth.In a warmer world it could slow to zero.
And if the coral can keep pace with the rising sea, there is no guarantee that the dry land, made of dead, eroded coral, will stay dry. That depends on the eroding and beach-building activities of wind, waves and current.
For the inhabitants, the question is not simply whether their home will be permanently flooded or not. The people living on many atolls are already pushing hard at the limits of what nature can provide sustainably from the tiny amount of land and surrounding sea, thanks to rapid population growth.
There is widespread poverty, problems of overfishing, water pollution and over-dependence on foreign aid, not forgetting the radioactive contamination caused by France and the US testing hydrogen bombs.
For some islands tourism is seen as the great hope for the future; American and Australian tourists now scuba dive at Bikini Atoll, although it is still too radioactive to eat food grown there or drink the water.
But growth in tourism depends on long distance air travel, whose contribution to global warming emissions is itself growing fast.
Atolls, like any other densely inhabited part of the world, are already overstressed. In that situation, increasing uncertainty about the fundamentals of nature - climate and sea level - will cut safety margins further.
As well as being the most endangered communities in the global warming stakes, these tiny islands also have the weakest voice because they are small and poor. They can do little more than request and protest on the international stage. Australia has been accused of tying aid to some atoll villages to a promise not to criticise global warming policies.