The IPCC Working Group I stated in chapter 2 of the Fourth Assessment Report that there are "large uncertainties in the current bottom-up estimates of components of the global source", and the balance between sources and sinks is not yet well known.
O Grupo de Traballo I do IPCC afirma no capítulo 2 do Informe da Cuarta Estimación que hai "grandes incertezas na estimación de abaixo a arriba actual dos compoñentes das fontes globais", e o equilibrio entre as fontes e os sumidoiros non está tampouco claro.