WASHINGTON • Nations around the world are trying to get their greenhouse gas emissions under control - to see them inch down, percentage point by percentage point, from where they stood earlier in the century.
If all countries get on board and shave off enough of those percentage points, the world may be able to get on a trajectory to prevent the planet from warming more than 2 deg C above the temperature prior to industrialisation.
But if a new study is correct, there is a problem: There might be more greenhouse gases going into the atmosphere than believed, increasing the need for cuts.
The new paper, to be published next week in BioScience, confirms that a significant volume of greenhouse gas emissions is coming from a little-considered place: man-made reservoirs, held behind about one million dams around the world and created for the purposes of electricity generation, irrigation and meeting other human needs.
In the study, 10 authors from US, Canadian, Chinese, Brazilian and Dutch universities and institutions have synthesised a considerable body of existing research on the subject to conclude that these reservoirs may be emitting just under a gigatonne, or one billion tonnes, of annual carbon dioxide equivalents. That would mean that they contribute 1.3 per cent of the global total.
Moreover, the emissions are largely in the form of methane, a greenhouse gas with a relatively short life in the atmosphere but with very strong short-term warming effects.
Flooding large areas of the Earth can set off new chemical processes as tiny micro-organisms break down organic matter in the water, sometimes doing so in the absence of oxygen - a process that leads to methane as a by-product.
One reason this happens is that the flooded areas initially contain lots of organic life in the form of trees and grasses.
Meanwhile, as nutrients such as nitrogen and phosphorous flow into reservoirs from rivers - being poured in by human agriculture and waste streams - these can further drive algal growth in reservoirs, giving micro-organisms even more material to break down.
The study found that for these reasons, reservoirs emit more methane than "natural lakes, ponds, rivers or wetlands".
The research complicates the idea that hydropower is a carbon-neutral source of energy, with the authors arguing that the greenhouse gas calculus should be included in evaluating such projects.