Introduction of hydrogen in global energy system can lead to lower CO2 emissions (high end-use efficiency; low-carbon production of hydrogen) but might also increase CO2 emissions (producing hydrogen from coal). We used the long-term energy model TIMER 2.0, to study the use and production of hydrogen and its influence on global CO2 emissions. This is done using a set of scenarios with assumptions on technology development, infrastructural barriers and climate policy (CP). We found that even under optimistic assumptions hydrogen plays a minor role in the global energy system until the mid-21st century due to the system inertia-but could become a dominant secondary energy carrier in the second half of the century. Hydrogen is mainly produced from coal and natural gas. Hence, hydrogen-rich scenarios without climate policy increase CO2 emissions up to 15% by 2100 compared to the baseline. However, if climate policy is assumed, CO2 from fossil feedstock-based hydrogen production is captured and sequestrated, which indicates that an energy system that includes hydrogen is much more flexible in responding to climate policy. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.