The Global Carbon Budget 2016
November 2016 - The Global Carbon Project provides an annual update to the amount of CO2 in the atmosphere, anthropogenic carbon dioxide emissions and natural sources and sinks of carbon.
This update provides underpinning data to inform the IPCC assessments and on-going international climate negotiations. Additionally, the Met Office forecast atmospheric CO2 for 2016 highlighting the role of ecosystems as well as human emissions in affecting the CO2 concentration in the atmosphere.
The Global Carbon Project update presents the world's carbon budget for 2015. This year's report shows that average atmospheric carbon dioxide levels in 2015 increased to 399.4ppm. The atmospheric growth rate of 2.97 above the 2014 average was a record despite near zero growth in fossil fuel emissions.
The land and ocean typically absorb approximately half of anthropogenic emissions. However, the strength of the sinks shows year-to-year inconstancy associated with climate variability, with more CO2 accumulating in the atmosphere in warm years and less in cool years. The long term trend in CO2 rise is driven by anthropogenic emissions but natural sinks moderate this: any change in the efficiency of these sinks will have significant consequences for accumulation of anthropogenic CO2 in the atmosphere.
Atmospheric CO2 increases above pre-industrial levels first began through releases of carbon to the atmosphere from deforestation and other land use activities. Starting in the 1920s, combustion of fossil fuels became the dominant source of anthropogenic emissions. Anthropogenic emissions occur on top of the natural carbon cycle that circulates carbon between the atmosphere, ocean and terrestrial biosphere.
Carbon Budget for 2015
For the year 2015, fossil fuel emissions were 9.9 ±0.5 Gigatonnes of carbon (GtC), a small increase over 2014 (a gigatonne is 1 billion tones; 1GtC (Carbon) is equivalent to 3.664GtCO2 (Carbon Dioxide)). An additional emission of 1.3 ±0.5 GtC is estimated from land-use change. Atmospheric CO2 increased by 6.3±0.2 GtC (or 2.97 ppm), a 50% increase compared to 2014 (4.2 ±0.1 GtC), due to a halving of the land sink to 1.9 ±0.9 GtC (2014: 3.9 ±0.9 GtC) resulting from the start of prevailing El Nino conditions. The rest was absorbed by the oceans - 3 ±0.5GtC (2014: 2.9 ±0.5GtC).
Cumulative emissions from pre-industrial (1750-2015) have reached 600 ±70 GtC, (555 ±55 GtC 1870-2015). This is an update to the IPCC 1750-2011 estimate of 555 [470 - 640] GtC to include 2012-2015 emissions and a revision of early 20th century land use emissions. The IPCC estimates that with cumulative emissions of 790 GtC, there is a two-thirds chance of staying below 2°C warming relative to pre-industrial temperatures. Keeping warming below two degrees requires total CO2 emissions to remain below this target. The budget is further reduced for 1.5°C. At current levels we are over half way towards this figure. If other non-CO2 gases are excluded, the emissions budget is substantially increased to approximately 1000 GtC.
Fossil fuel emissions are estimated to grow slightly in 2016, by +0.2% with a large uncertainty. At the current emission rate the remaining 2 degree carbon budget will be used in less than 30 years, and the 1.5 degree budget within the next decade, to have a 50% chance of staying within target. Any additional emissions above this ceiling will substantially reduce the chance of staying below the target global temperature. To increase this budget, negative emission technology will be required. One such example is Bio-Energy with Carbon Capture and Storage (BECCS). In this technology crops are grown for fuel and the carbon captured during combustion. BECCS can therefore offset both demand for fossil fuels, as a clean energy technology, as well removing carbon from the atmosphere. However, credibility of BECCS as a climate change mitigation option is unproven and substantial challenges remain, particularly the sustainability of production given existing issues around food security.
A successful CO2 forecast for September 2016
In November 2015 the Met Office Hadley Centre made a forecast of El Nino conditions for 2016, and then used this to produce a forecast of a record increase in atmospheric CO2. The autumn minimum at the Mauna Loa observatory in Hawaii was projected to exceed 400ppm of CO2 – a symbolic threshold for climate change. Due to ongoing emissions it is not expected that concentrations below 400ppm at Mauna Loa will be seen again in our lifetimes.
The forecast showed that a simple extrapolation of recent years would still see a seasonal minimum value below 400ppm at Mauna Loa, but the forecast predicted that the effect of El Nino would lead to a minimum of 401.48±0.53ppm. The observed average value for September was slightly lower at 401.01ppm but within the published uncertainty bounds. The success of the forecast builds confidence that we have an ability to understand the global carbon cycle and how it responds to climate.
Monthly mean CO2 concentrations at Mauna Loa since 2010, also with observed and hindcast/forecast annual mean concentrations made in November 2015 (black and orange stars and central solid lines). The observed CO2 record up to October 2016 is shown in blue. Source: Betts et al (2016) and Scripps
The 1997/1998 El Nino also resulted in a large increase in atmospheric CO2. However, anthropogenic emissions are approximately 25% larger now than in 1997/1998 enhancing the effect of climate variability on the land carbon sink.
The record CO2 growth rate in 2016 will be short-lived, and is too small in itself to induce a noticeable effect on climate. Nevertheless, it illustrates the two-way interactions between climate change and the carbon cycle.
One important feature of the 2016 CO2 rise is that we have seen a record rise in CO2 during a year where anthropogenic emissions have plateaued. This highlights the role of natural carbon sinks interacting with climate to modulate the ongoing trend of CO2 rise due to human activity – if we want to plan future CO2 emissions reductions that help us achieve climate targets then we will need to accurately understand how these natural carbon sinks continue to behave.
Met Office Hadley Centre
The Met Office Hadley Centre develops and uses complex Earth System Models that incorporate representations of the ocean and terrestrial biosphere to inform policy on future emission pathways that are compatible with climate targets. This includes ongoing research into the role of negative emission technologies in carbon budgets.
The global carbon project is an international programme of Future Earth. The annual Global Carbon Budget release is led by Professor Corinne Le Quéré of the University of East Anglia. Met Office Hadley Centre Scientist, Dr Andy Wiltshire contributed to the study.
The data from the Global Carbon Project can be explored and visualised at the Global Carbon Atlas. Cumulative emissions have been rounded to 5GtC.
Richard Betts is Met Office Fellow and Professor of Climate Impacts as the University of Exeter.
Chris Jones is head of Earth System and Mitigation Science.
Andy Wiltshire leads the Terrestrial Carbon Cycle research group.