The report provides a comprehensive assessment of sea level rise, and its causes, over the past few decades. It also estimates cumulative CO 2 emissions since pre-industrial times and provides a CO 2 budget for future emissions to limit warming to less than 2°C. About half of this maximum amount was already emitted by 2011.
There is alarming evidence that important tipping points, leading to irreversible changes in major ecosystems and the planetary climate system, may already have been reached or passed. Ecosystems as diverse as the Amazon rainforest and the Arctic tundra, may be approaching thresholds of dramatic change through warming and drying. Mountain glaciers are in alarming retreat and the downstream effects of reduced water supply in the driest months will have repercussions that transcend generations.