The Guardian is reporting that global oil prices are at less than half of the cost of production in Saudi Arabia, contributing to a budget deficit of 20% of GDP.
For the same reason raising fossil fuel prices with a carbon tax is expected to reduce demand, it can be expected that lower fossil fuel prices will lead to more usage. At the same time, low oil prices could be the result of global economic weakness, in which case the amount of fossil fuel consumed may not rise much even with falling prices. In the longer term, low oil prices almost certainly reduce investment in the development of new oilfields, pipelines, refineries, and so on. Given the urgent need to stop investing in such projects to control climate change, low oil prices may be at least partly desirable from a climate change perspective.