Researchers expect emissions to exceed any of these remaining budgets. To meet budget limits, they estimate that CO will have to be captured from the atmosphere and stored in products, in the environment or underground. A 2015 study estimated that carbon budgets can only be met through carbon capture, "in all but the most optimistic cases, we also found negative emissions requirements that have not yet been shown to be achievable."[25]​.

Scientists widely agree that this research is necessary. The IPCC says: "All pathways that limit global warming to 1.5°C with limited or no overshoot project carbon dioxide removal (CDR) use on the order of 100-1000 GtCO over the century. CDR would be used to offset residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C after a peak (high confidence)."

Even for the less stringent target of 2°C warming, carbon capture is needed. The IPCC has only one Representative Concentration Pathway (RCP) that limits warming to 2°C: "RCP2.6 is representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures. Most models indicate that scenarios meeting forcing levels similar to RCP2.6 are characterized by substantial net negative emissions by 2100, on average around 2 GtCO/ year"[21].

In light of the many differences between nations, including, but not limited to, population, level of industrialization, national emissions histories, and mitigation capabilities, scientists have attempted to allocate global carbon budgets among countries using methods that follow various equity principles.[26] Allocating national emissions budgets is comparable to sharing the effort to reduce global emissions, underlined by some assumptions of state responsibility for climate change. Many authors have conducted quantitative analyzes that allocate emissions budgets,[27]​[28][29][4]​ often simultaneously addressing disparities in historical GHG emissions between nations. "Paris Agreement-compliant" national emissions budgets have also been calculated that quantify the discrepancy between emissions reductions resulting from current national mitigation pathways and those needed to meet the temperature and equity commitments enshrined in the Paris Agreement.[27]​.

A common principle that has been used to allocate global emissions budgets to nations is the principle of common but differentiated responsibilities.[26] This principle recognizes the cumulative historical contributions of nations to global emissions. Countries with higher emissions over a set period of time (for example, from the pre-industrial era to the present) have more responsibilities for mitigating emissions. Their national emissions budgets should therefore be smaller than those that have polluted less in the past. The concept of historical responsibility for climate change has been prevalent in the literature since the early 1990s[30]​[31]​ and is a fundamental part of the architecture of international agreements on climate change (the UNFCCC, the Kyoto Protocol and the Paris Agreement). Consequently, countries that have quantified the accumulated historical emissions of the states have greater responsibility in taking the most forceful actions[32]​ and in helping developing countries in their mitigation and adaptation efforts to climate change. This principle is recognized in international agreements and is part of the diplomatic strategies of developing countries, which need a larger emissions budget[33]​ to reduce inequality and achieve sustainable development.

Another common equity principle for calculating national emissions budgets is the "egalitarian" principle. This principle stipulates that people should have equal rights to pollute and therefore emissions budgets should be distributed proportionally according to each country's populations.[26]​ Therefore, some scientists have reasoned the use of national emissions per capita in national emissions budget calculations.[28]​[29][34]​ This principle may be favored by nations with larger or rapidly growing populations.[33]​.

A third equity principle that has been used in national budget calculations considers national sovereignty.[26] The "sovereignty" principle highlights the equal right of nations to pollute.[26] The grandfathering method of calculating national emissions budgets uses this principle. Grandfathering allocates these budgets proportionally according to emissions in a particular base year,[34]​ and has been used under international regimes such as the Kyoto Protocol[35]​ and the initial phase of the European Union Emissions Trading Scheme (EU ETS).[36]​ This principle is often favored by developed countries, as it allocates them larger emissions budgets.[33]​.

Researchers expect emissions to exceed any of these remaining budgets. To meet budget limits, they estimate that CO will have to be captured from the atmosphere and stored in products, in the environment or underground. A 2015 study estimated that carbon budgets can only be met through carbon capture, "in all but the most optimistic cases, we also found negative emissions requirements that have not yet been shown to be achievable."[25]​.

Scientists widely agree that this research is necessary. The IPCC says: "All pathways that limit global warming to 1.5°C with limited or no overshoot project carbon dioxide removal (CDR) use on the order of 100-1000 GtCO over the century. CDR would be used to offset residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C after a peak (high confidence)."

Even for the less stringent target of 2°C warming, carbon capture is needed. The IPCC has only one Representative Concentration Pathway (RCP) that limits warming to 2°C: "RCP2.6 is representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures. Most models indicate that scenarios meeting forcing levels similar to RCP2.6 are characterized by substantial net negative emissions by 2100, on average around 2 GtCO/ year"[21].

In light of the many differences between nations, including, but not limited to, population, level of industrialization, national emissions histories, and mitigation capabilities, scientists have attempted to allocate global carbon budgets among countries using methods that follow various equity principles.[26] Allocating national emissions budgets is comparable to sharing the effort to reduce global emissions, underlined by some assumptions of state responsibility for climate change. Many authors have conducted quantitative analyzes that allocate emissions budgets,[27]​[28][29][4]​ often simultaneously addressing disparities in historical GHG emissions between nations. "Paris Agreement-compliant" national emissions budgets have also been calculated that quantify the discrepancy between emissions reductions resulting from current national mitigation pathways and those needed to meet the temperature and equity commitments enshrined in the Paris Agreement.[27]​.

A common principle that has been used to allocate global emissions budgets to nations is the principle of common but differentiated responsibilities.[26] This principle recognizes the cumulative historical contributions of nations to global emissions. Countries with higher emissions over a set period of time (for example, from the pre-industrial era to the present) have more responsibilities for mitigating emissions. Their national emissions budgets should therefore be smaller than those that have polluted less in the past. The concept of historical responsibility for climate change has been prevalent in the literature since the early 1990s[30]​[31]​ and is a fundamental part of the architecture of international agreements on climate change (the UNFCCC, the Kyoto Protocol and the Paris Agreement). Consequently, countries that have quantified the accumulated historical emissions of the states have greater responsibility in taking the most forceful actions[32]​ and in helping developing countries in their mitigation and adaptation efforts to climate change. This principle is recognized in international agreements and is part of the diplomatic strategies of developing countries, which need a larger emissions budget[33]​ to reduce inequality and achieve sustainable development.

Another common equity principle for calculating national emissions budgets is the "egalitarian" principle. This principle stipulates that people should have equal rights to pollute and therefore emissions budgets should be distributed proportionally according to each country's populations.[26]​ Therefore, some scientists have reasoned the use of national emissions per capita in national emissions budget calculations.[28]​[29][34]​ This principle may be favored by nations with larger or rapidly growing populations.[33]​.

A third equity principle that has been used in national budget calculations considers national sovereignty.[26] The "sovereignty" principle highlights the equal right of nations to pollute.[26] The grandfathering method of calculating national emissions budgets uses this principle. Grandfathering allocates these budgets proportionally according to emissions in a particular base year,[34]​ and has been used under international regimes such as the Kyoto Protocol[35]​ and the initial phase of the European Union Emissions Trading Scheme (EU ETS).[36]​ This principle is often favored by developed countries, as it allocates them larger emissions budgets.[33]​.