The question is not whether the Earth’s climate will change in response to human activities, but when, where and by how much. Human activities are changing the Earth’s climate and further human-induced climate change is inevitable. Indeed the climate of the next few decades will be governed by past emissions. The most adverse consequences of human-induced climate change will be in developing countries and poor people within them. Climate change threatens to bring more suffering to the one billion people who already go to bed hungry every night and the approximately 2 billion people exposed to insect-borne diseases and water scarcity. Sea level rise threatens to displace tens of millions of people in deltaic areas and low-lying small island states. Climate change will undermine the ability of many poor people to escape poverty and the long-term sustainable economic development of some countries. Hence, climate change is not only an environmental issue, but a development and security issue. The challenge is to limit the magnitude and rate of human-induced climate change, and simultaneously reduce the vulnerability of socio-economic sectors, ecological systems and human health to current and projected climate variability by integrating climate concerns into local and national economic planning. Technological options for reducing greenhouse gas emissions cost-effectively over the next few decades already exist. However, the required transition to a very low carbon economy (a reduction in global emissions by at least 50% by 2050) will require a technological evolution in the production and use of energy, energy sector reform, appropriate pricing policies and behavior change, coupled with a more sustainable agricultural sector and reduced deforestation. This transition to a low-carbon economy must be achieved while improving access to affordable energy in developing countries, which is critical for economic growth and poverty alleviation, and while ensuring adequate affordable and nutritious food. The challenge is to negotiate a long-term (up to 2050) global regulatory framework that is equitable with common but differentiated responsibilities and has intermediate targets that can reduce greenhouse emissions to a level that limits the increase in global mean surface temperature to 2C above pre-industrial levels. While this goal has been widely accepted, the current rate of growth in emissions globally, coupled with a failure in Copenhagen to agree to stringent targets to reduce emissions, makes this goal extremely difficult, hence the world needs to be prepared to adapt to a 4C warmer world.