Tag Archives: Global emissions

Every five-year delay in meeting Paris goals could ‘add 20cm’ to global sea levels

Failure to meet the goals of the Paris Agreement within the next few decades could have long-lasting impacts on global sea level rise in the coming centuries, new research finds.

A study finds that each five-year delay in meeting the goal of reaching global peak CO2 emissions could drive sea levels to rise by an additional 20cm by 2300.

This amount of sea level rise is roughly equal to what the world has experienced since the start of the industrial revolution more than 200 years ago, the lead author tells Carbon Brief.

The findings reiterate that “peaking global CO2 emissions as soon as possible is crucial for limiting the risks of sea level rise”, the author adds.

Race to zero

Samples taken from ice cores, tide gauges and satellites show that global sea levels have risen by around 19cm from pre-industrial times to present, with recent research showing that the rate is likely to be accelerating.

The new study, published in Nature Communications, estimates how delays in meeting the goals of the Paris Agreement could affect the total amount of sea level rise by 2300.

Under the Paris Agreement, countries have pledged to cut their rates of emissions in order to keep future global temperature rise “well below” 2C. To achieve this, nations agreed to reach “peak” CO2 emissions “as soon as possible”. This will be key to achieving “net-zero emissions” within the second half of this century.

The new research shows that the speed at which the world can cut its greenhouse gas emissions is becoming “the major leverage for future sea levels,” says study lead author Dr Matthias Mengel, a postdoctoral researcher from the Potsdam Institute for Climate Impacts (PIK) in Germany. He tells Carbon Brief:

“The way that emissions will evolve in the next decades will shape our coastlines in the centuries to come: five years of delayed [CO2] peaking will lead to 0.2 metres more sea level rise in 2300. This is the same amount we have experienced so far since the beginning of the fossil economy.”

Melting prospects


RCP2.6: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP2.6 (also sometimes referred to as “RCP3-PD”) is a “peak and decline” scenario where stringent mitigation and carbon dioxide removal technologies mean atmospheric CO2 concentration peaks and then falls during this century. By 2100, CO2 levels increase to around 420ppm – around 20ppm above current levels – equivalent to 475ppm once other forcings are included (in CO2e). By 2100, global temperatures are likely to rise by 1.3-1.9C above pre-industrial levels.

RCP2.6: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP2.6 (also sometimes referred to as “RCP3-PD”) is a “peak and decline” scenario where stringent mitigation… Read More

For the study, the researchers used climate models to simulate future sea level rise by 2300 under two future scenarios. Both of the scenarios assume that the world will meet the Paris goals by the end of century and use a low-emission pathway known as RCP2.6.

The first scenario, called “net-zero greenhouse gas (GHG) emissions”, assumes that future temperature rise will be limited to well below 2C and a balance between emissions and uptake of greenhouse gases is met by the end of the century.

The second scenario, called “net-zero CO2 emissions”, is a future in which temperatures are stabilised at levels well below 2C but GHG emissions are not balanced by the end of the century. This scenario allows for the potential “overshoot” of the temperature targets before stabilisation, which some scientists suggest is likely.

The researchers used these scenarios to work out possible future global temperatures and applied them to a model of long-term sea level rise.

Global warming causes sea levels to rise in three main ways, Mengel says: “thermal expansion”, when seawater expands as the oceans absorb additional heat from the atmosphere; melting glaciers; and ice loss from the large ice sheets of Greenland and Antarctica.

The model incorporates recent research (pdf) finding that the Antarctic ice sheet may be more sensitive to climate change than previously thought, Mengel says:

“The contribution from Antarctica increases with warming, faster than all other components.”

Closing window

The charts below show the expected CO2 emissions, temperature rise and sea level rise for the net-zero CO2 emissions scenario (a-c) and the net-zero GHG emissions scenario (d-f).

On the charts, coloured lines and symbols are used to indicate the expected outcomes of reaching peak CO2 emissions in five-year intervals from 2020 to 2035. Scenarios that do not limit global warming to 2C are shown in thin grey lines.

Shading shows the 66th percentile range of each scenario in b and e and the 90th percentile range in c and f.

The charts show that, for every five-year delay in reaching peak emissions (upper charts), temperatures rise higher (middle charts), which causes more sea level rise in the long run (lower charts).

Expected CO2 emissions, temperature rise and sea level rise for a net-zero CO2 emissions scenario (a-c) and a net-zero GHG emissions scenario (d-f). Coloured lines and symbols are used to indicate simulations at five-year intervals from 2020 to 2035. Grey lines show simulations exceeding 2C. Shading shows the 66th percentile range of each scenario in b and e and the 90th percentile range in c and f. Source: Mengel et al. (2018)


RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a scenario of “comparatively high greenhouse gas emissions“ brought about by rapid population growth, high energy demand, fossil fuel dominance and an absence of climate change policies. This “business as usual” scenario is the highest of the four RCPs and sees atmospheric CO2 rise to around 935ppm by 2100, equivalent to 1,370ppm once other forcings are included (in CO2e). The likely range of global temperatures by 2100 for RCP8.5 is 4.0-6.1C above pre-industrial levels.

RCP8.5: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP8.5 is a scenario of “comparatively high greenhouse gas emissions“ brought about by rapid population growth,… Read More

The results suggest that sea levels could rise by 70-120cm by 2300 under a low emissions scenario. This level of sea rise could significantly increase the risk of flooding in coastal cities, such as New York, and island atoll nations. Currently, global emissions are tracking a higher scenario known as RCP8.5.

The results also indicate that, even once net-zero emissions are achieved and temperatures begin in fall, sea levels will continue to rise. This is because the drivers of sea level rise respond slowly to climate change, Mengel says:

“There are also unstable processes that, once triggered, will not stop contributing to sea level rise, independent of global mean temperature rise. An example of this could be the potential collapse of the West Antarctic ice sheet.”

Peak urgency

The findings show that the world must “reduce its emissions as fast as possible” in order to protect future generations from extreme sea level rise, Mengel says:

“Peaking global CO2 emissions as soon as possible is crucial for limiting the risks of sea level rise, even if global warming is limited to well below 2C.”

The results could hold relevance for today’s large infrastructure projects, which typically have a lifespan of more than 100 years, says Dr Natasha Barlow, a university research fellow specialising in sea level change from the University of Leeds, who was not involved in the research. She tells Carbon Brief:

“As a result, there is huge value in studies which consider the rate and magnitude of sea level rise beyond 2100. There are very few studies which do, largely as the uncertainties increase the further into the future we try to predict.”

However, the models used in the study do not include all of the “low probability, high risk” drivers that may contribute to sea level rise, she adds:

“There may be additional long term sea level rise from melting ice, above that predicted by the authors, due to marine ice-sheet instability driven by warmer ocean waters around West Antarctica and an albedo feedback over Greenland.”

The post Every five-year delay in meeting Paris goals could ‘add 20cm’ to global sea levels appeared first on Carbon Brief.

Guest post: How ‘enhanced weathering’ could slow climate change and boost crop yields

Prof David Beerling, director of the Leverhulme Centre for Climate Change Mitigation, and Prof Stephen Long from the Department of Crop Sciences and Plant Biology at the University of Illinois at Urbana–Champaign.

Achieving the Paris Agreement goals of keeping global warming to “well below” 2C, or to 1.5C, above pre-industrial levels will require rapid decarbonisation of human society.

But national commitments to rein in greenhouse gas emissions are currently insufficient to meet these agreed limits. It is increasingly likely that “negative emissions”, or “carbon dioxide removal”, technologies will be needed to take up the slack.

These techniques involve extracting CO2 from the atmosphere and storing it indefinitely. Scientists have proposed a range of different approaches and we now need realistic assessment of these strategies, what they might be able to deliver, and what the challenges are.

In a new paper for Nature Plants, we tackle an under-discussed technique of CO2 removal called “enhanced rock weathering”. Our research highlights the potential wider benefits for crop yields and soil health, and sets out a research agenda for the next steps.

What is enhanced weathering?

As you might remember from geography classes at school, chemical weathering is a natural process that continuously erodes away rocks in our landscapes and sequesters atmospheric CO2 over millions of years.

The process begins with rain, which is usually slightly acidic having absorbed CO2 from the atmosphere on its journey to the ground. The acidic rain reacts with the rocks and soils it lands on, gradually breaking them down into minute rock grains and forming bicarbonate in the process. Eventually, this bicarbonate washes into the oceans, where the carbon is stored in dissolve form for hundreds of thousands of years or locked up on the sea floor.

Enhanced weathering scales up this process. It involves pulverising silicate rocks such as basalt – left over from ancient volcanic eruptions – to bypass the slow weathering action. The resulting powder, with a high reactive surface area, is then spread on large areas of agricultural land where plant roots and microbes in the soil speed up the chemical reactions.

As natural rock weathering absorbs around 3% of global fossil fuel emissions, enhanced weathering can provide a boost to remove even more CO2 from our atmosphere.

But the potential benefits do not end there. As enhanced weathering makes water more alkaline, it can help counteract ocean acidification.

And adding minerals to soils can boost nutrient levels, improving crop yields and helping restore degraded agricultural soils.

Food demand

The need to cut CO2 emissions is unfolding alongside an unprecedented increase in food demand – linked to dietary changes and a growing population that may surpass 11 billion by 2100 (pdf). At the same time, farming itself a growing contributor to climate change.

Critically, enhanced rock weathering works together with existing managed croplands.  Unlike other negative emissions techniques under consideration, it doesn’t compete for land used to grow food or increase the demand for freshwater.

While enhanced weathering can be applied to any soils, arable land is the most obvious candidate as it is worked and planted throughout the year. It covers some 12m square kilometres – 11% of the global land area.

In fact, arable farms already apply crushed rock in the form of limestone to reverse acidification of soils caused by farming practices, such as the use of fertilisers. And there is a long history of small-scale farming using silicate rocks to improve crop yields in highly-weathered soils in Africa, Brazil and Malaysia.

Swapping silicate for limestone, and increasing the application rate, would do the same job to help tackle acidification, but help capture CO2 from the atmosphere at the same time.

Managed cropland, therefore, has the logistical infrastructure, such as road networks, and the machinery needed to undertake this approach at scale. These considerations could make enhanced weathering potentially straightforward to adopt.

You can see this in action in the video below from the Leverhulme Centre for Climate Change Mitigation.

Using silicate rocks as a resource in this way could also have a number of important wider benefits. These include supplying silica back into soils to improve crop health and protection from pests and diseases, and supplying nutrients to increase yields.

If realised, these benefits would reduce the usage of agricultural fertilisers and pesticides, lowering the cost of food production, increasing the profitability of farms and reducing the barriers to take up enhanced weathering for the agricultural sector.

Estimates and challenges

So, in theory, there are a lot of potential upsides for using enhanced weathering. However, like many negative emissions technologies, implementation is still in its very early stages. It needs further research, development and demonstration – not just across a range of crops and soil types, but also different climates and spatial scales.

There have been some successful field tests of using enhanced weathering – though for purposes other than capturing CO2.

For example, in a 12-year study conducted in a New Hampshire forest, scientists measured the effect of spreading silicate powder as a method of accelerating recovery from acid rain. The results confirmed some of the main impacts of enhanced weathering – a rapid increase in dissolved silicate and calcium making it into streams, and alleviation of acidification in the ecosystem.

Similarly, in Mauritius, sugarcane trials as far back as 1961 added crushed basalt to soils and increased yields by 30% over five successive harvests.

There are other challenges too. The process of mining, grinding and spreading rocks on a large-scale would likely have negative environmental and ecological impacts, and would therefore require careful management. Depending on the size of the grains of powder the rocks are pulverized down to, the energy demand could account for 10-30% of the amount of CO2 sequestered. Using renewable energy sources would minimise this.

Costs, too, need to be considered. Current cost estimates are uncertain and vary widely. The most detailed analysis to date puts operational costs at $52-480 per tonne of CO2 sequestered – though these estimates are poorly constrained and improvements in crop yields and lower fertiliser needs will offset some of these costs. This compares with a $39-100 per tonne of CO2 for another, more talked-about negative emissions technology, bioenergy with carbon capture and storage (BECCS).

Credit: Dr Ilsa Kantola, University of Illinois, Champaign-Urbana

But the potential is significant. For example, applying 50 tonnes of basalt powder per hectare per year to 70m hectares of the corn belt of North America might sequester as much as 1.1bn tonnes of CO2 in the long run – equivalent to 13% of the global annual emissions from agriculture.

Countries with considerable productive farmland have the largest potential to sequester CO2 through enhanced weathering. These include the US, China, India and Russia, which all grow crops on a massive scale and make up the highest emitters of CO2.

Scaling estimates up to a global level is tricky, but – for example – adding 10-30 tonnes of silicate per hectare per year to two-thirds of the world’s most productive cropland could take 0.5-4bn tonnes of CO2 out of the atmosphere per year by 2100. But current estimates are highly uncertain and require more research.

Putting theory into practise

Human societies have long known that volcanic plains are fertile; ideal places for growing crops without adverse human health effects but, of course, with little consideration for how adding additional rocks to soils might capture carbon.

We now need to take the theory and laboratory tests out into real crop fields to see how enhanced weathering fits – practically and economically – in the wider portfolio of options for removing CO2 from the atmosphere.

However, there is still a long way to go and research in this area remains in its infancy.  Improved assessments are required to understand how much CO2 the approach would capture, how much rock is required, how much energy is required to crush and distribute the rock, and to better understand the long-term effects on soils and water courses.

We need to undertake carefully monitored assessments on arable land. For example, can we demonstrate the expected benefits to crops amidst the seasonal and annual variations in the weather?

And finally, we need to better understand the public perception of enhanced rock weathering as a strategy for carbon capture, communicate the process, benefits and risks, and understand any public concerns about what this means for our landscapes and farmlands.

The post Guest post: How ‘enhanced weathering’ could slow climate change and boost crop yields appeared first on Carbon Brief.

Carbon emissions from Amazon wildfires could ‘counteract’ deforestation decline

The loss of carbon from wildfires fuelled by drought could “counteract” efforts to cut deforestation in the Amazon rainforest, research suggests.

A new study finds that, while rates of deforestation have sharply fallen in the Amazon over the past decade, the number of wildfires affecting the region has remained stubbornly high – particularly in drought years.

Emissions from wildfires totalled more than 1bn tonnes of CO2 from 2003-2015, the lead author tells Carbon Brief, and climate change, along with forest fragmentation, could cause a further increase in the number of forest fires in the coming decades.

The author adds that, if current efforts to curb deforestation are reversed, the combination of forest fires and deforestation could cause carbon loss from the Amazon to “escalate to proportions never experienced before”.

Three billion trees

The Amazon rainforest is the largest rainforest in the world, spanning an area that is 25 times the size of the UK.

The forest’s three billion trees absorb CO2 from the atmosphere during photosynthesis and then use it to build new leaves, shoots and roots. As they grow, these trees account for a quarter of the CO2 absorbed by the land each year.

The new study, published in Nature Communications, explores how this enormous carbon store is being affected by both deforestation and drought-driven wildfires.

The clearing of trees during deforestation causes previously locked-up carbon to be released back into the atmosphere.

The study finds that, despite remaining a major driver of forest carbon loss, rates of deforestation in the Amazon have fallen by 76% between 2003 and 2015. This reflects efforts by the Brazilian government to curb both legal and illegal deforestation, the study notes.

However, the amount of carbon loss from drought-driven wildfires has remained high, says lead author Dr Luiz Aragão, leader of the tropical ecosystems and environmental sciences group (TREES) at the National Institute for Space Research in Brazil. He tells Carbon Brief:

“This is the first time that we could clearly demonstrate how much widely-spread forest fires during recent droughts influence Amazonian carbon emissions. During these droughts, forest fires can overtake emissions from deforestation.”

Fanning the flames


El Niño: Every five years or so, a change in the winds causes a shift to warmer than normal sea surface temperatures in the equatorial Pacific Ocean – known as El Niño. Together with its cooler counterpart, La Niña, this is known as the El Niño Southern Oscillation (ENSO) and is responsible for most of the fluctuations in temperature and rainfall patterns we see from one year to the next.

El Niño: Every five years or so, a change in the winds causes a shift to warmer than normal sea surface temperatures in the equatorial Pacific Ocean – known as El Niño. Together with… Read More

Although wildfires occur regularly in the Amazon, fire incidence is at its highest in drought years.

“Drought years” happen on average every five years in the Amazon and are typically a result of changes to wind and weather patterns brought about by warming in the Atlantic Ocean during events of the climate phenomenon El Niño.

Although droughts have been a natural part of the year-to-year variations in the Amazon’s climate, both the frequency and severity of droughts in the rainforest have been increasing over the last decade because of climate change, Aragão says:

“These natural swings can be intensified by global warming. Another catalyst of this Atlantic oceanic warming is the declining Northern Hemisphere aerosol production, which is also influenced by climate change.”

Previous research (pdf) shows that aerosols influence cloud formation in the rainforest and, therefore, the amount of regional rainfall.

Wildfires are usually sparked by humans clearing land, either for small-scale farming or major deforestation, Aragão says. During drought years, these small fires can quickly spread to large areas of the rainforest.

A lack of rainfall during drought years causes large sections of the lush canopy to dry out and die. These dry, dead leaves can then act as tinder, allowing small fires to spread, says Aragão:

“These forests would not burn naturally. Most of the fires that happen during droughts are human-driven. When the climate is drier, fires that are set for land management are more likely to leak into surrounding forests.”

As large areas of forest burn, huge stores of previously stored carbon are released into the atmosphere. The amount of carbon loss is greatest when fires “leak into” previously pristine forest, which may have been storing carbon for decades or even centuries, Aragão adds.

Assessing the damage

The researchers used satellite data to record the number and regional spread wildfires in the Amazon from 2003-15. The figure below shows the annual fire counts of each year of the study (red bars and numbers), with bold text signifying drought years (2005, 2010 and 2015). The numbers on the x-axis correspond to the fire season months from June (month 6) to December (month 12).

The length of the grey bars corresponds to the sum of all months with more than 10,000 fires. The colour within the grey bar shows the number of fire counts during the year’s “peak month”, with dark red showing a count of more than 40,000 and green showing a count of 10,000 to 15,000.

The chart also displays annual deforestation rates in the Amazon (khaki bars), which were derived from the Brazilian government.

The chart shows how the number of forest fires tends to spike in drought years. For example, during the 2015 drought, fire incidence was 36% higher compared to the average for the previous 12 years, the study finds.

Annual absolute fire counts (red bars) and deforestation rates (khaki bars) in the Amazon from 2003-15. Number of fires during drought years (2005, 2010, 2015) are shown in bold. The length of the grey bars corresponds to the sum of all months with more than 10,000 fires. The colour within the grey bar shows the number of fire counts during the year’s “peak month”, with dark red showing a count of more than 40,000 and green showing a count of 10,000 to 15,000. Source: Aragão (2018)

The results show that, while deforestation rates are decreasing, forest fire counts have not seen a similar decline.

This suggests that the role that deforestation plays in sparking forest fires is diminishing over time, says Aragão:

“Before we used to observe that fires in Amazonia were strongly related to deforestation. Now this relationship is weak. We think that there are few explanations, but a strong one is that because the Amazon is more fragmented, there are more edges between the deforested land and forests. This increases the chances of fires from open lands to propagate into forests.”

The effect of fragmentation could be magnified by future climate change, which is expected to bring more extreme droughts to the Amazon, he says:

“We expect that drought intensity and frequency will increase towards the end of the century. So, with more droughts it is very likely that fire incidence will also increase if no policy actions are taken to curb ignition sources.”

The researchers also used satellite data to record the total amount of CO2 released as a result of deforestation and wildfires over the study period.

The results are shown on the chart below, which shows annual CO2 emissions in teragrams (one teragram is equal to 1m tonnes of CO2) for forest fires (dark green) and deforestation (light green).

Over the course of the study period, emissions from wildfires in drought years alone totalled more than 1bn tonnes, Aragão says. The release of CO2 during forest fires could increase further as the climate warms, he adds.

Annual CO2 emissions in teragrams (1m tonnes) of forest fires (dark green) and deforestation (light green) in the Amazon from 2003 to 2015. Source: Aragão (2018)

‘Compelling’ results

The “comprehensive” study raises concerns over the growing threat of wildfires in the Amazon, says Prof Guido van der Werf from Vrije University in the Netherlands, who was not involved in the study. Van der Werf was part of a team that developed the Global Fire Emissions Database. He tells Carbon Brief:

“What is particularly compelling to me is the observed decoupling of fire and deforestation; they used to go hand in hand as fire was the cheapest tool in the deforestation process, but in recent years fires are observed more outside the deforestation regions, especially during drought years.

“Controlling these forest fires may be more difficult than controlling deforestation as it only takes one simple ignition to burn down a large area and many fires occur in secondary forests which may be less well monitored.”

The research uses a “sound methodology” but relies on statistics from the Brazilian government, which research suggests could be overstating recent deforestation reductions, says Dr Richard Birdsey, a senior scientist specialising in quantifying forest carbon budgets from the Woods Hole Research Centre, who also wasn’t involved in the research. He tells Carbon Brief:

“Some studies question the official statistics of deforestation in Brazil, suggesting that the reduction over previous decades may be overestimated. If deforestation is higher than the estimate used in this paper, then the relative effect of drought-induced fire would not be as large as stated here, and the decoupling of fire incidence from deforestation rates would not be as significant.”

Another factor not considered in the study is that forests tend to recover more quickly from wildfires than deforestation, he adds:

“Fires create young forests that will rather quickly recover significant amounts of lost biomass and usually have higher productivity compared with old-growth for several decades. Thus, considering the effects of fires over decades is an important consideration not addressed in this paper.”

Further carbon loss as a result of drought-driven wildfires could “counteract” efforts to curb deforestation in the Amazon, says Aragão. And, if efforts to stop deforestation are unsuccessful, the scale of carbon loss from the Amazon could reach “unprecedented” levels, he says:

“The main worry is that if deforestation increases, in combination with the increase fragmentation, increase in drought probability [caused by climate change] and the use of fires by humans, carbon emissions could escalate to proportions never experienced before.”

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Negative emissions have ‘limited potential’ to help meet climate goals

The potential for using negative emissions technologies to help meet the goals of the Paris Agreement could be more “limited” than previously thought, concludes a new report by European science advisors.

Negative emissions technologies (NETs) describe a variety of methods – many of which are yet to be developed – that aim to limit climate change by removing CO2 from the air.

Some of these techniques are already included by scientists in modelled “pathways” showing how global warming can be limited to between 1.5C and 2C above pre-industrial levels, which is the goal of the Paris Agreement.

However, the new report says there is no “silver bullet technology” that can be used to solve the problem of climate change, scientists said at a press briefing held in London.

Instead, “the primary focus must be on mitigation on reducing emissions of greenhouse gases,” they added.

Zero emissions

The 37-page report was produced by the European Academies Science Advisory Council (EASAC), an independent group made up of staff from the national science academies of EU member states, Norway and Switzerland, which offers scientific advice to EU policymakers.

Drawing on the results of recently published research papers, the report assesses the feasibility and possible impacts of NETs.

The report splits these “technologies” into six categories:

  • Afforestation and reforestation
  • Land management to increase soil carbon
  • Bioenergy with carbon capture and storage (BECCS)
  • Enhanced weathering
  • Direct capture of CO2
  • Ocean fertilisation.

A Carbon Brief article published in 2016 explained how these proposed technologies might work.

Though differing in approach, all of the proposed NETs aim to slow climate change by removing CO2 from the atmosphere and storing it underground or in the sea.

Some scientists argue that such technologies could be used to soak up some of the CO2 that is released by human activity, which could, in turn, help the world to achieve “net-zero” greenhouse gas emissions.

Net-zero emissions is a term used to describe a scenario where the amount of greenhouse gases released by humans is balanced by the amount absorbed from the atmosphere.

Achieving net-zero emissions within this century will be key to limiting global warming to between 1.5C and 2C above pre-industrial levels, says Prof Michael Norton, EASAC environment programme director and member of the expert group behind the report. At a press briefing, he said:

“Indeed, without assuming that technologies can remove CO2 on a large, that’s gigatonne [billion tonne] scale, IPCC scenarios have great difficulty in envisaging an emission reduction pathway consistent with the Paris targets.”

However, the new report suggests that there is currently no “silver bullet” technology that can absolve the world of its greenhouse gas emissions, scientists said at a press briefing. The report concludes:

“We conclude that these technologies offer only limited realistic potential to remove carbon from the atmosphere and not at the scale envisaged in some climate scenarios.”

The report also shows that many of the NETs could have large environmental impacts, says Prof John Shepherd FRS, emeritus professor of ocean and Earth sciences at the University of Southampton and member of the expert group behind the report. He told the press briefing:

“Some of these techniques would have adverse environmental impacts, including some of the ones that appear to be natural. There is an emotional response in most people to prefer natural appearing solutions, but, in many cases, the environmental are as great as the more engineering-type applications.”

The “pros and cons” of each proposed technology are summed up on the table below. The top half of the table includes: the technical status of each technology; the amount of carbon that could be removed if the technology were to be implemented on a wide scale; the potential cost of implementing the technology (low/medium/high); and the likely efficacy of each method.

The bottom half of the table assesses: the relative security of the carbon storage of each technology; the possibility that the technology may actually contribute to climate change; and the possibility that the technology could have environmental impacts.

A summary of the strengths, weaknesses and uncertainties of negative emissions technologies (NETs). Technologies include afforestation and reforestation (AR), land management (LM), bioenergy with carbon capture and storage (BECCS), enhanced weathering (EW), direct air capture and storage (DACCS), ocean fertilisation (OIF) and carbon capture and storage (CCS). Source: EASAC (2018)

Blow for BECCS?

One of the techniques under scrutiny in the new report is BECCS. Put simply, BECCS involves burning biomass – such as trees and crops – to generate energy and then capturing the resulting CO2 emissions before they are released into the air.

BECCS has been labelled one of the “most promising” NETs and is already included by scientists in many of the modelled “pathways” showing how global warming can be limited to 2C above pre-industrial levels.

However, BECCS has yet to be demonstrated on a commercial basis, the report finds, and its ability to effectively store large amounts of carbon is still “uncertain”.

Recent research has revealed a number of “drawbacks” to using BECCS on a wide scale, Norton said:

“These include the reality that even if all the carbon emitted when the biomass is burnt were to be captured, extensive emissions across the supply chain will not be captured, thus severely limiting its effectiveness as a negative carbon technology.”

In other words, the emissions resulting from the different stages of BECCS, including on transportation and on applying nitrogen fertilisers, may significantly reduce the technology’s overall ability to reduce the amount of CO2 in the atmosphere. In some cases, biomass energy might have higher emissions than fossil fuels.

On top of this, implementing BECCS on a large scale would require large amounts of land to be converted to biomass plantations, which could have considerable environmental impacts, the report notes. Carbon Brief recently covered new research investigating how using BECCS could affect different aspects of the natural world.

Forest fall out

The report also assesses the potential of afforestation, creating new forests on land that

was not previously forest, and reforestation, planting new trees on land that was once forest, to remove carbon from the atmosphere.

Trees absorb carbon from the atmosphere during photosynthesis and then use it to build new leaves, shoots and roots. By doing this, trees are able to store carbon for long periods.

However, implementing afforestation on a large scale could have “significant” environmental impacts, the new report finds. This is because growing new forests would require a large amount of land-use change and the application of nitrogen fertilisers. The production of nitrogen fertilisers releases a group of potent greenhouse gases known as nitrous oxides, along with CO2.

On top of this, new trees take many years to grow and so will not be able to immediately absorb large amounts of carbon from the atmosphere, Norton said:

“We can also see many scenarios in which the land-use change involved in extending forestry would be counterproductive for decades or even centuries.”

Norton said that curbing the rate of deforestation, which is causing the release of large stores of carbon from the world’s tropical regions, should be a priority for policymakers. He added:

“Forestation and reforestation offer simple ways of increasing carbon stocks, but it would be a mistake to be distracted from the reality of the current situation which is that…carbon is being lost by continuing deforestation.”

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Cutting to the chase

Despite potential drawbacks, there may be some scenarios where the use of NETs will be “necessary” to balance the release of greenhouse gases, said Shepherd:

“They are especially likely to be necessary to deal with intractable sources of greenhouse gases, in particular aviation and agriculture.”

In other words, NETs may be needed to compensate for industries that are unable to radically cut their rate of greenhouse gas emissions.

Such industries could include cattle ranching and rice production, says Dr Phil Williamson, associate fellow at the University of East Anglia, who was not an author of the new report. At the sidelines of the press briefing, he told Carbon Brief:

“There’s a whole lot of things that are going to be very difficult to control, including methane from cattle and methane from rice. We’re not going to stop growing rice, so we’re still going to have methane emissions. In order to have that balance, we’re still going to need some negative emissions technologies.”

However, the “primary focus” of policymakers should be on rapidly cutting greenhouse gas emissions, said Prof Gideon Henderson FRS, professor of Earth sciences at the University of Oxford and reviewer of the report. He told the press briefing:

“The primary focus must be on mitigation, on reducing emissions of greenhouse gases. That’s not going to be easy, but it’s undoubtedly going to be easier than doing NETs at a substantial scale.”

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