Climate scientists are abandoning their old estimates about greenhouse gas emissions, saying there are too many uncertainties to predict likely future levels. A new draft report on emissions by the U.N.’s Intergovernmental Panel on Climate Change, which encompasses the views of hundreds of leading scientists and economists, does not present just one likely scenario, as past IPCC reports have. Instead, the report, due to be published in March, lays out 40 potential scenarios for future emissions based on four plausible assumptions about world population, economic growth, and technical advances. While the old central scenario estimated that carbon dioxide emissions in 2100 would be around 18 billion tons, or three times current levels, the new report has predictions ranging from 4.3 billion tons to 36.7 billion.