A community of researchers has been working for the Global Carbon Project to produce a Global Carbon Budget every year since 2005. The Global Carbon Budget 2019 was the 14th edition of the annual update since the first was produced in 2006. 
The Global Carbon Project is an international research project within the Future Earth research initiative. It aims to develop a complete picture of the global carbon cycle, including the interactions and feedbacks between the world’s biophysical and human dimensions.
The global carbon budget quantifies CO2 emissions for the prior year. It apportions carbon to the atmosphere, the ocean or the land.
The budget allows the global climate research community to monitor the global carbon cycle.
The budget is a key part of work on climate policy. It provides a record of recent trends as well as updates on the amount of GHG emissions permitted to stabilise the climate. 
Carbon budgets are similar to financial ones. The diffference is that instead of income vs expenditure, a carbon budget quantifies inputs of carbon to the atmosphere (sources) against outputs (sinks). 
Generally, inputs of CO2 to the atmosphere are emissions from human activities, whereas the output is measured in terms of carbon reservoirs on land or in the ocean. Sources = amount in the atmosphere plus amounts in the sinks. Sinks = removals from the atmosphere and stored/output to the ocean or land. These concepts can be summarised as: the less carbon goes into land or ocean, the more will remain in the atmosphere.
2. 2019 global carbon budget
In 2018, it was estimated that the remaining carbon budget to keep the global climate within 2°C of warming was 1690 Gt CO2e (with 50% probability) and 1320 Gt CO2e (with 67% probability).
If emissions remain at their current levels, the more ambitious (i.e., the safer) of these two budgets expect to be exceeded around 2045.
3. National carbon budget
Countries which set carbon emission reduction targets, and in particular Net Zero commitments, must work out a carbon budget.
In the UK, the government has committed to:
reduce emissions by at least 100% of 1990 levels (net zero) by 2050
contribute to global emission reductions, to limit global temperature rise to as little as possible above 2°C 
The UK government has set five-yearly carbon budgets running until 2032. The budget restricts the amount of greenhouse gas the UK can emit in a five-year period. The UK is currently in the third carbon budget period (2018 to 2022).
The UK met its first carbon budget (2008 to 2012) and the second (2013 to 2017) Although the country is on track to outperform the third (2018 to 2022) it is not on track to meet the fourth (2023 to 2027).
The government will need to apply more challenging measures in order to meet future carbon budgets and the 100% target for 2050.
4. Corporate carbon budgets
Global carbon budgets are an important concept in setting some science-based targets for businesses.
The UK has its own system of five-year carbon budgets as part of the 2008 Climate Change Act and is currently in the third budget (2018-2022).
 ^ University of Exeter Press Release 4 December 2019 Global carbon emissions increase but rate has slowed press release
 ^ Josep (Pep) Candela and David Carlson. The Annual Global Carbon Budget. https://public.wmo.int/en/resources/bulletin/annual-global-carbon-budget
 Carbon budgets how we monitor emissions targets https://www.theccc.org.uk/what-is-climate-change/reducing-carbon-emissions/carbon-budgets-and-targets/
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