The challenge of reducing global carbon emissions by 50-85 per cent by the year 2050, which is suggested by the Intergovernmental Panel on Climate Change (2007a) as a target compatible with limiting the risk of a more-than-2∫C temperature increase, clearly conflicts with existing patterns of economic growth, which are heavily dependent on increased use of fossil fuel energy. While it is theoretically possible to conceive of economic growth being ìdelinkedî from fossil fuel consumption, any such delinking would represent a drastic change from economic patterns of the last 150 years. Current macroeconomic theory is heavily oriented towards an assumption of continuous, exponential growth in GDP. The historical record shows GDP growth is strongly correlated with a parallel record of increasing fossil energy use and CO2 emissions. A path of reduced carbon emissions would require major modifications in economic growth patterns. Climate change is part of an inter-related group of environmental issues associated with growth limits. These include population growth, agricultural production, water supplies, and species loss. To achieve a low-carbon path requires population stabilization, limited consumption, and major investments in environmental protection and social priorities such as public health, nutrition, and education. Macroeconomic theory must be adapted to reflect these new realities. A reclassification of macroeconomic aggregates is proposed to distinguish between those categories of goods and services that can expand over time, and those that must be limited to reduce carbon emissions. This reformulation makes it clear that there are many possibilities for environmentally beneficial economic expansion. New forms of Keynesian policy oriented towards ecological sustainability, provision of basic social needs such as education and health care, and distributional equity can provide a basis for a rapid reduction in carbon emissions while promoting investment in human and natural capital.