In a word, feedback, explains the Guardian.
Dramatic flips in the way ecosystems absorb carbon dioxide will see oceans and vast swaths of land falter in their ability to draw up the greenhouse gas, allowing it to build up in the atmosphere and cause more warming. The phenomenon is known as a positive feedback - where global warming drives changes in ecosystems that themselves cause more heating.
The warning came in a major report on climate change published yesterday that suggests average temperatures could rise more than expected - by as much as 6.4C by 2100, unless greenhouse gas emissions are reined in. The report, from the UN's Intergovernmental Panel on Climate Change, has upgraded its 2001 estimate that temperatures would rise by at most 5.8C, because at the time the feedback mechanisms were either unknown or poorly understood.
The latest report states that the predicted temperature rise for 2100 was raised because "the broader range of models now available suggests stronger climate-carbon cycle feedbacks".
Early climate change predictions were calculated predominantly by anticipating levels of carbon dioxide and other greenhouse gases in the atmosphere. The gases allow radiation from the sun to warm the planet but block it as it is radiated back off the surface, forming a virtual blanket around the globe.
But scientists have steadily uncovered ecological feedback mechanisms driven by climate change that complicate the outcome. In some cases global warming triggers feedbacks that act to cool the planet, but others exacerbate the warming.
One of the earliest feedback mechanisms identified was the melting of ice sheets and sea ice. The vast sheets of bright white ice reflect nearly 80% of sunlight that falls on them. But as they melt they reveal dark waters or soils beneath that absorb sunlight, warm up and cause yet more melting.
The latest IPCC report for the first time includes climate models that take into account two other ecological feedback mechanisms that accelerate global warming: the ability of the oceans and land to absorb carbon.