TOKYO/DUBAI -- To achieve net-zero greenhouse gas emissions by 2050, the U.S. and Europe are cutting back on investment in oil and gas production. But it is unclear whether they can promote the adoption of renewable energy quickly enough to slow the growth in demand for fossil fuels, raising concerns over potential energy shortages. Natural gas prices have more than tripled in Europe since the start of the year. While rich countries scramble for natural gas, which produces fewer carbon emissions than other fossil fuels, Russia is curtailing exports as it tries to cut its own reliance on coal-fired electricity.