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The Earth system response to negative emissions

Atmospheric CO2 concentration has changed from about 288 ppm in 1870 to 397 in 2014 due to emissions from fossil fuel burning and land-use change. But this rise has been offset, by approximately half, due to natural sinks on land and ocean.

October 2016 - During the last century or so, over half of our emissions of CO2 from fossil fuel burning, industry, and deforestation has been absorbed by natural sinks such as the forests and oceans. These natural sinks play a critical role in determining the climate effects of CO2 emissions.

"The more CO2 we have emitted the more natural systems have tried to absorb to compensate" said Dr Chris Jones, Head of the Earth System and Mitigation Science team at the Met Office Hadley Centre.

How will carbon sinks change as we remove CO2 from the atmosphere?

A paper published in Environmental Research Letters shows how natural carbon sinks may weaken in response to efforts to remove CO2 from the atmosphere. Understanding this effect is vital to devise pathways consistent with the UN's ambitious 1.5 °C and 2 °C climate targets. "We expect natural ecosystems to continue to absorb CO2 but as society reduces emissions, or removes CO2 from the atmosphere, this ecosystem service will itself weaken," explained Dr Jones.
The paper looks at results from several numerical climate models, including the Met Office's own Earth System model, HadGEM2-ES, which participated in the latest Coupled Model Intercomparison Project (Met Office delivers new climate simulations to international modelling activity).
For an emissions scenario that aims to limit warming to no more than 2 °C, it finds that projected land and ocean carbon sinks weaken to less than half their present day values over the latter half of the 21st century. On longer timescales they weaken to virtually zero by 2300.
"A vital requirement for low carbon pathways is to quantify the effectiveness of carbon dioxide removal," added Dr Jones. "The weakening of natural carbon sinks hinders the effectiveness of these techniques and therefore increases the required deployment of negative emissions technology to achieve global climate goals."
The model analysis revealed that in the 2 °C scenario, as fossil fuel emissions reduce in the future and as human activity to remove carbon dioxide from the atmosphere begins, atmospheric CO2 slows its growth and begins to decrease. However, the effectiveness of the removal is offset to some extent by a weakening of the natural carbon sinks caused by the reduced levels of CO2 in the atmosphere.

Sequence of 50-year periods showing that, as fossil fuel emissions decrease and carbon dioxide removal via negative emissions increases, the natural carbon sinks on land and ocean weaken.

Why are we interested in the implications of CO2 removal?

At the UNFCCC's Conference of the Parties in Paris in 2015, the parties adopted the Paris Agreement, consolidating efforts to keep the global temperature increase well below 2 °C above pre-industrial levels and pursue efforts to keep below 1.5 °C. In October 2016 enough countries ratified the agreement for it to come into force.
However, it is recognized that society may not be able to reduce fossil fuel emissions sufficiently fast to achieve these targets. Almost all emission scenarios consistent with these temperature goals therefore require intentionally removing carbon dioxide from the atmosphere. This activity is known as Carbon Dioxide Removal (CDR) or Negative Emissions Technologies (NETs).
"In the lead up to the IPCC Fifth Assessment Report many researchers started to recognize the sheer scale of carbon dioxide removal required to be consistent with ambitious policy objectives", said Dr Glen Peters from the Center for International Climate and Environmental Research - Oslo (CICERO), Norway, and co-author of the study. 
This increased recognition of the requirement for NETs led the Global Carbon Project to try to focus efforts on assessing the costs, limits and implications of negative emissions. In a scene setting paper, Fuss et al (2014) explain how the world is "betting on negative emissions" - in other words, scenarios which appear successful in avoiding 2 °C rely on large scale deployment of as-yet unproven technology.
In 2013, a conference sponsored by the International Institute for Applied Systems Analysis (IIASA) in Vienna and the Global Carbon Project, assembled scientists from around the world to bring their expertise to address the many questions around the feasibility, efficacy and consequences of negative emissions technology. It identified three key knowledge gaps on carbon dioxide removal: First, how do the natural carbon sinks respond; second, are there economic and biophysical limits; and third, what governance and institutions are required to ensure sufficient deployment. This new study responds to the first knowledge gap, and quantifies how the land and ocean carbon sinks respond to carbon dioxide removal in ambitious future scenarios. Uncertainty around their response is one of the most important issues to address in attempting to quantify the world's need for negative emissions.
The study complements another paper which resulted from the conference on the economic and biophysical limits of carbon dioxide removal published in Nature Climate Change. "The evidence suggests carbon dioxide removal has significant limitations and therefore swift and aggressive cuts in greenhouse gas emissions would be required to avoid overreliance on these, as yet, unproven technologies in the future", said Professor Pete Smith from the University of Aberdeen and author of both studies. 
Our latest results (published in Environmental Research Letters) therefore contribute to the ongoing cross-disciplinary research to help quantify the emissions reductions required to achieve the ambitious climate targets of the Paris Agreement.

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