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We now live in a 400 parts per million world. Data from the Mauna Loa observatory, Hawaii, suggests carbon dioxide concentration levels are unlikely to consistently fall back below this level in our lifetimes.
Carbon accounting is crucial to efforts to tackle climate change, providing data on where emissions emanate and where they are absorbed. Decision-makers rely on the best information about the earth’s changing sinks and sources as they seek to constrain global emissions.
This collection brings together a selection of multi-disciplinary research and commentary from across the physical and social sciences that explores the major inputs and outputs that comprise the world’s carbon account.
Estimates of carbon budgets compatible with limiting warming to below specific temperature limits are reviewed, and reasons underlying their differences discussed along with their respective strengths and limitations.
The remaining carbon budget consistent with limiting warming to 1.5 °C allows 20 more years of current emissions according to one study, but is already exhausted according to another. Both are defensible. We need to move on from a unique carbon budget, and face the nuances.
Limiting warming to 1.5 °C requires staying within an allowable carbon budget. An analysis of warming and carbon budgets from the past decade shows that the median remaining budget is 208 PgC, corresponding to about 20 years of emissions at the 2015 rate.
The drivers of the increase in atmospheric methane since 2006 remain unclear. Here, the authors use satellite and in situ measurements of CO and CH4 to show that fossil fuels and biogenic sources contribute 12–19 Tg CH4per year and 12–16 Tg CH4per year respectively to the recent atmospheric methane increase.
If CO2 emissions after 2015 do not exceed 200 GtC, climate warming after 2015 will fall below 0.6 °C in 66% of CMIP5 models, according to an analysis based on combining a simple climate–carbon-cycle model with estimated ranges for key climate system properties.
The Paris Agreement is based on emission scenarios that move from a sluggish phase-out of fossil fuels to large-scale late-century negative emissions. Alternative pathways of early deployment of negative emission technologies need to be considered to ensure that climate targets are reached safely and sustainably.
Carbon release from permafrost thaw would substantially decrease the amount of carbon emissions required to meet climate targets, according to climate simulations.
Climate feedbacks associated with wetland methane emissions and permafrost-thaw carbon release substantially reduce available carbon budgets to achieve temperature targets, suggest simulations with a climate–land-surface model system.
A combination of the level and rate of human-induced warming allows estimation of remaining emission budgets to peak warming across a broad range of scenarios, suggests an analysis of emissions budgets expressed in terms of CO2-forcing-equivalent emissions.