This further complicates the relationship between future CO 2 emissions and global warming.Ĭarbon budgets nevertheless have become a powerful tool for communicating the challenges we face in aiming to hold warming to 1.5 ☌ and to well below 2 ☌-the limits of global average temperature increase set out in the United Nations Paris Agreement 14, 15, 16, 17, 18. Estimating the remaining carbon budget thus also implies making assumptions about these non-CO 2 contributions. Other greenhouse gases (such as methane, fluorinated gases or nitrous oxide) and aerosols and their precursors (including soot or sulphur dioxide) affect global temperatures. Moreover, global warming is not driven by emissions of CO 2 only. These additional processes add uncertainty and may change our understanding of this linear relationship. However, additional processes that influence and are influenced by future warming, such as the thawing of permafrost, have recently been included in models that simulate the Earth system. Once established, the appeal of this concept became immediately evident: the possibility that the response of an enormously complex system-such as the response of planet Earth to our emissions of CO 2-could potentially be reduced to a roughly linear relationship would allow scientists to infer clear and easy-to-communicate implications. This literature has allowed scientists to define the linear relationship between warming and cumulative CO 2 emissions as the transient climate response to cumulative emissions of CO 2 (TCRE). A series of studies over the past decade has clarified and quantified why the rise in global average temperature increase is roughly proportional to the total cumulative amount of CO 2 emissions produced by human activities since the industrial revolution 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The idea of a remaining carbon budget is grounded in well established climate science. This is not to be confused with another concept, the historical carbon budget, which describes estimates of all major past and contemporary carbon fluxes in the Earth system 3. We here define the remaining carbon budget as the finite total amount of CO 2 that can be emitted into the atmosphere by human activities while still holding global warming to a desired temperature limit. Since the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) 1, the concept of a carbon budget has risen to prominence as a tool in guiding climate policy 2. We propose that application of this framework may help to reconcile differences between estimates of the remaining carbon budget and may provide a basis for reducing uncertainty in the range of future estimates. Here we present a framework that enables us to track estimates of the remaining carbon budget and to understand how these estimates can improve over time as scientific knowledge advances. However, a wide range of estimates for the remaining carbon budget has been reported, reducing the effectiveness of the remaining carbon budget as a means of setting emission reduction targets that are consistent with the Paris Agreement. This makes it possible to estimate the remaining carbon budget: the total amount of anthropogenic carbon dioxide that can still be emitted into the atmosphere while holding the global average temperature increase to the limit set by the Paris Agreement. Research reported during the past decade has shown that global warming is roughly proportional to the total amount of carbon dioxide released into the atmosphere.
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