Tar sands development is something that finds at odds some "depletionists" (that is, those worried over peak oil or other fossil fuels) and virtually all "emissionists" (that is, those most worried over the effect of carbon emissions on warming and climate change):
"Other low-grade fossil fuels, such as tar sands, oil shale, and heavy oil are also problematic from both the depletion and emissions perspectives. Some depletion analysts recommend full-speed development of these resources. However, the energetic extraction costs for these are usually quite high compared to the energy payoff from the resource extracted (also known as the energy returned on energy invested, or EROEI). Their already-low energy profit ratio would be compromised still further by efforts to capture and sequester carbon, since, as with coal, these low-grade fuels have a high carbon content as compared to natural gas or conventional oil. Currently, natural gas is used in the processing of tar sands and heavy oil; from an emissions point of view, this is rather like turning gold into lead. Many depletionists point out that, while the total resource base for these substances is enormous, the rate of extraction for each is likely to remain limited by physical factors (such as the availability of natural gas and fresh water needed for processing), so that synthetic liquid fuels from such substances may not help much in dealing with the problem of oil depletion in any case."