Emissions of greenhouse gases (GHGs) from human activities disturb the radiative energy balance of the earth-atmosphere system. They exacerbate the natural greenhouse effect, leading to temperature changes and other disruption of the earth's climate. Land use changes and forestry also play a role by altering the amount of greenhouse gases captured or released by carbon sinks. Carbon dioxide (CO2) from the combustion of fossil fuels and deforestation is a major contributor to greenhouse gases. CO2 makes up the largest share of greenhouse gases and is a key factor in countries’ ability to mitigate climate change. National emissions are also affected by changes in global demand and supply patterns with increasing trade flows and the displacement of carbon-intensive production abroad. Reductions in domestic emissions can thus be partially or wholly offset elsewhere in the world.