This paper discusses sustainability problems related to socioeconomic energy flows based upon the societal metabolism approach. Contrary to conventional energy statistics that only include energy used in technical devices, this approach considers all kinds of energy flows related to human societies, including nutritional energy flows of humans and domesticated animals. Based upon human population data and data on the pro capite energy metabolism of hunter-gatherers and agricultural societies as well as on statistical data on industrial energy flows a time series of the global socioeconomic energetic metabolism for the last 106 years and a scenario for the next 50 years is derived. These estimates show that the total energy input of mankind has risen by several orders of magnitude since the Neolithic revolution about 10,000 years ago. Whereas the energy input of agricultural societies prior to the advent of industrial societies about 200-300 years ago did not exceed 5% of global terrestrial net primary productivity (NPP), humanity's energy input currently amounts to about 30% of global terrestrial NPP and is likely to surpass 50% in about 2050. This shows that the sheer magnitude of human-induced flows is historically unprecedented and poses at least two closely interrelated sustainability challenges: (1) a reduction of energy available to ecosystem processes that can be assessed using the concept of 'human appropriation of net primary productivity' and (2) the changes in the global carbon cycle resulting from land-use change and fossil-energy combustion. (c) 2004 Elsevier Ltd. All rights reserved.