1. Avoided Deforestation and Anthropogenic CO2 Sinks
Stephen W. Pacala

2. Disclaimer
My remarks represent my own views and are specifically not conclusions of the National Academy panel that I chair. The panel has not yet reached final conclusions.

3. 2/3 chance of keeping warming below 2 °C .
Source: Smith and Friedmann. Chapter UNEP emissions gap report.

4. Outline From Steve: Constraints on land options.
Misconceptions about negative emissions.
Avoided deforestation
From Pete:
4. Negative Emissions Options.

5. Negative emissions options
Increase mass of living and dead wood.
Increase soil carbon in cropland and pasture soils.
BECCS – biofuel or biopower with CCS
Increase carbon stored by sediments in coastal lands.
DAC-CCS – direct air capture of CO2 with CCS
Accelerate CO2 mineralization
Fertilize oceans to accelerate deep-sea C storage
High Capacity
Med. Capacity
High Capacity
Low Capacity
Expensive
Untried
Controversial
Blue = technologically ready.

6. Food
Food demand doubles by midcentury. Source: Foley et al. 2011
Biodiversity
Extinction rates already elevated X (Millennium Ecosystem Assessment 2005).
11% of species expected to go extinct eventually because of historical habitat lost to agriculture. Doubling agricultural area to meet doubled food demand would cause 60% of species to go extinct (method in Taylor et al Nature 427: ).

7. Existing land options for negative emissions with the high capacities are land hungry: afforestation, reforestation and BECCS. 600 million hectares of land is devoted to bioenergy – 40% of today’s cropland - in RCP 2.6 (IPCC AR5 WG III, Chapter 6).
Sources: van Vuuren, et al., “The representative concentration pathways: an overview,” Climatic Change, 109: 5-31, van Vuuren, et al., “RCP2.6: exploring the possibility to keep global mean temperature increase below 2°C,” Climatic Change, 109: , 2011.

8. If we cannot afford to devote hundreds of millions of hectares to negative emissions to afforestation, deforestation and BECCS, then we need technological progress on other options like DAC and accelerated weathering. I estimate < 10GtCO2/y from land.

9. Outline From Steve: Constraints on land options.
Misconceptions about negative emissions.
Avoided deforestation
From Pete:
4. Negative Emissions Options.

10. Things about negative emissions technologies and methods (hereafter NET’s) that everyone knows.
NET’s represent a MORAL HAZARD, but:
We will almost certainly overshoot the goals of Paris. NET’s will offer the only way to reduce the atmospheric concentration of CO2 back to a safe level.
Natural sinks will make this difficult. Although they absorbed half the CO2 we emitted on the way up, this will all come out on the way down.
NET’s will be needed once there are no remaining options for reducing fossil emissions, but they will compete with mitigation options before that.

11. Fate of anthropogenic CO2 emissions (2006-2015)
16.4 GtCO2/yr
44%
34.1 GtCO2/yr
91%
Sources = Sinks
31%
11.6 GtCO2/yr 9%
3.5 GtCO2/yr
26%
9.7 GtCO2/yr
Source: CDIAC; NOAA-ESRL; Houghton et al 2012; Giglio et al 2013; Le Quéré et al 2016; Global Carbon Budget 2016

12. We will almost certainly overshoot the goals of Paris, and thus will need to draw down atmospheric CO2.
Atmospheric CO2 consistent with Paris Agreement long-term goals. What emissions lead to this?
Jones et al Environ. Res. Lett

13. Positive Anthropogenic Emissions
Jones et al Environ. Res. Lett

14. Decline in Atmospheric CO2
Jones et al Environ. Res. Lett

15. Because of Persistent Sinks
Jones et al Environ. Res. Lett

16. Fossil fuel use for centuries, and so mitigation competes with NET’s for centuries.
Anthropogenic emissions are positive from , even though atmosphere declines by 75 ppm.

17. Why do the sinks behave this way?
Because disequilibrium dominates!

18. True statements about negative emissions technologies and methods:
NET’s = Mitigation. NET’s are best thought of as mitigation options that have received too little attention by scientists, engineers and policy people.
NET’s will compete on price with other mitigation options for centuries.
NET’s would significantly reduce the cost of meeting the goals of Paris, just like wind, solar and every other mitigation option.
What went in need not all come out:
The next 100 ppm worth of net positive emissions will increase the atmospheric CO2 from 400 to 450 ppm. To reverse this will NOT require negative emissions of 100 ppm. Instead the 50 ppm reduction could be accomplished even with positive net emissions.

19. Outline From Steve: Constraints on land options.
Misconceptions about negative emissions.
Avoided deforestation
From Pete:
4. Negative Emissions Options.

20. Two Categories of Emissions Offsets
Reduce non-fossil emissions of CO2 or other greenhouse gases, including methane.
Create a carbon sink that generates negative CO2 emissions.

21. Avoided deforestation of primary forest reduces emissions by ~ 900 tCO2/ha.
Cropland
2 tC
50 tC
In 1 ha of land:

22. Avoided Deforestation
Avoided deforestation offsets have important co-benefits, but also unintended negative consequences and risks that must be managed.
Co-benefits
Biodiversity.
Forest Ecosystem Services.
Concerns
Additionality.
Permanence.
Leakage.
Cost and governance including monitoring and verification.
Avoided Deforestation

23. Outline From Steve: Constraints on land options.
Misconceptions about negative emissions.
Avoided deforestation
From Pete:
4. Negative Emissions Options.

24. Greenhouse gas removal through carbon sequestration in vegetation and soils
Pete Smith
Professor of Soils & Global Change, FRS, FRSE, FNA, FRSB,
Institute of Biological & Environmental Sciences
University of Aberdeen,
Scotland, UK.
BP HQ, London, 25th April 2018

25. Smith et al. UNEP EGR (2017)

28. The Paris Agreement (COP21) was a game changer…
The Paris Agreement commits the 190+ signatories of the United Nations Framework Convention on Climate Change (UNFCCC) to keeping the increase in global average temperature to well below 2°C above pre-industrial levels, with an aim to limit the increase to 1.5°C.
It is necessary that global emissions peak as soon as possible, recognizing that this will take longer for developing countries, and that rapid reductions occur thereafter.
In order to be consistent with a 2°C target, emissions across all sectors need to decrease by over 80% by 2050, with even greater reductions required for a 1.5°C target.

29. Negative Emissions Technologies (NETs)
The ones I will focus on today….
Direct Air Capture (DAC)
Enhanced weathering of minerals (EW)
Afforestation
Bioenergy with carbon capture and storage (BECCS)
Soil carbon sequestration
Biochar

30. Summary of the carbon cycle impacts of different NETsJ K
Summary of the carbon cycle impacts of different NETs
Smith et al. (2016b)

31. Smith (2016); Smith et al. (2016)

32. Impact / limit summary for NETS
SCS
Biochar
Smith et al. (2016); Smith (2016)

33. Synthesis of comparison across GGR options from systematic review
Fuss et al. ERL (2018)

35. Global potential for agricultural GHG mitigation
Paustian et al. Nature (2016)

36. “Natural” C sequestration
Griscom et al. (2017)

37. A recent TNC study concludes that several GtCO2/y of reforestation negative emissions are achievable, but at higher cost than reduced deforestation offsets. Some previous studies estimate lower costs.
(GtCO2e/y)
Dark Grey = <$10/tCO2
Light Grey = <$100/tCO2
Griscom et al. (2017) – PNAS

38. Concerns:
Food production.
Permanence.
Biophysical Impacts.
Costs of monitoring and verification.
Co-benefits:
Biodiversity preservation.
Reforestation or afforestation produces a sink of 4-15 tCO2/ha/year, that eventually totals ~900 tCO2/ha.
Primary
Forest
100 tC
200 tC
Cropland
2 tC
50 tC

39. Smith (2008) International Journal of Agricultural Sustainability 6(3),169–170
“There are a number of well rehearsed arguments against reliance on carbon sequestration for tackling climate change, involving saturation of the carbon sink (the carbon is only removed from the atmosphere while the tree is growing or until the soil reaches a new equilibrium soil carbon level; Smith, 2005), permanence (carbon sinks can be reversed at any stage by deforestation or poor soil management; Smith, 2005), leakage/displacement (e.g. planting trees in one area leads to deforestation in another; Intergovernmental Panel on Climate Change (IPCC), 2000), verification issues (can the sinks be measured; Smith, 2004), and total effectiveness relative to emission reduction targets (only a fraction of the reduction can be achieved through sinks; IPCC, 2007)”.

40. Saturation – the time course of carbon sequestration
Sink saturation ~ years
Sink strength declines towards new equilibrium
Smith (2004a)

41. Permanence
Manure treatment in red,
Woodland in blue
Smith (2005)

42. Obersteiner et al. (2018)

43. Summary of negative emissions options favours soil C restoration (SCS).
Smith, Friedmann et al. (2017) - unpublished
Courtesy of Prof Pete Smith

44. Conclusions (I)
Negative emissions of 3.3 GtC-eq./yr in 2100 are possible with BECCS (and DAC)
EW, AR, SCS and biochar can provide less negative emissions than this in 2100
All NETs have limits / downsides and none is a magic bullet
Improve governance to ensure sustainable implementation of NETs - safe storage needed
Need more R&D and pilot projects – then to see if technology is scalable
Smith et al. (2016); Smith (2016)

45. Conclusions (II)
An over-reliance on NETs in the future, if used as a means to allow continued use of fossil fuels in the present, is extremely risky since our ability to stabilise the climate at <2C declines as cumulative emissions increase (Kriegler et al., 2014, Luderer et al., 2012)
A failure of NETs to deliver expected mitigation in the future, due to any combination of biophysical and economic limits examined here, leaves us with no “Plan B”. “Plan A” must be to reduce GHG emissions aggressively now.
Seems impossible to meet Paris targets without NETs
Some land-based NETs are “no regrets”, cost-effective and ready to implement now
We might need to consider implementing “difficult” NETs (like BECCS) soon to avoid overshoot – this will be controversial
Smith et al. (2016); Smith (2016); Obersteiner et al. (2018)

46. Thank you for your attention (pete.smith@abdn.ac.uk)

47. Two Questions for Q&A
IAM teams, including many participating in IPCC Working Group 3, continue to estimate potential negative emissions form afforestation, reforestation and BECCS at over 10 GTCO2/y, where as other recent analyses conclude that the total is likely to be significantly lower because of constraints imposed by food production and biodiversity conservation.  What do you think is a realistic potential for 2050 negative emissions?
One reason that forestry projects (avoided deforestation, afforestation and reforestation) seem so attractive is their low price.  Given the price of timber, and alternative uses of the land, how can the price be so low?  Does a low price signal problems of additionality and permanence?