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The terrestrial carbon cycle

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My presentation at the Norwegian Academy of Science and Letters on the Terrestrial Carbon Cycle (2 October 2017). I do not using present so detailed on the carbon cycle, so the slide deck is not that well developed. I mainly focused on aspects of uncertainty, and the interplay between the land sources and sinks.

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The terrestrial carbon cycle

  1. 1. The terrestrial carbon cycle and carbon uptake on land Glen Peters (CICERO) The Norwegian Academy of Science and Letters, 02/10/2017
  2. 2. Source: IPCC AR5 WG1 Chapter 6
  3. 3. 31% 11.6 GtCO2/yr Source: CDIAC; NOAA-ESRL; Houghton et al 2012; Giglio et al 2013; Le Quéré et al 2016; Global Carbon Budget 2016 Fate of anthropogenic CO2 emissions (2006-2015) 26% 9.7 GtCO2/yr 34.1 GtCO2/yr 91% 9% 3.5 GtCO2/yr 16.4 GtCO2/yr 44% Sources = Sinks
  4. 4. Carbon dioxide sources from fossil fuels, industry, and land-use change emissions are balanced by the atmosphere and carbon sinks on land and in the ocean Source: CDIAC; NOAA-ESRL; Houghton et al 2012; Giglio et al 2013; Joos et al 2013; Khatiwala et al 2013; Le Quéré et al 2015; Global Carbon Budget 2015 Global Carbon Budget
  5. 5. If we have strong future mitigation, the carbon sinks will also decline There is uncertainty on how the carbon cycle will respond to climate change, efficiency may decline Source: Rockström et al (2017) Carbon sinks decline with mitigation
  6. 6. • Knowledge gaps on the terrestrial carbon cycle – Verification of carbon dioxide emissions – Significant interannual & decadal variability – Unclear response to temperature & nutrient limits – Carbon cycle dynamics affects mitigation requirements • Mitigation implications – Preserving forest and land sinks – Expanding the sink (e.g. afforestation) – Source of (large-scale) bioenergy Why the terrestrial carbon cycle?
  7. 7. Atmospheric concentrations
  8. 8. The global CO2 concentration increased from ~277ppm in 1750 to 399ppm in 2015 (up 44%) 2016 will be the first full year with concentration above 400ppm Globally averaged surface atmospheric CO2 concentration. Data: NOAA-ESRL after 1980; the Scripps Institution of Oceanography before 1980 (harmonised to recent data by adding 0.542ppm) Source: NOAA-ESRL; Scripps Institution of Oceanography; Global Carbon Budget 2015 Atmospheric concentration emissions carbon sinks variability El Nino Drivers of recent changes (carbon-cycle feedback) CO2 driver of long-term changes
  9. 9. The CO2 concentration is driven by changes in: the seasonal cycle (short), volcanos (short), ENSO (interannual), CO2 emissions (long) Source: Robbie Andrew (CICERO) Drivers of CO2 concentrations
  10. 10. Source: Betts et al (2016); Robbie Andrew (CICERO) The march beyond 400ppm
  11. 11. CO2 concentration is given by a component due to emissions and a component due to El Niño Forecast of the annual value and seasonal cycle very accurate in even a simple model Source: Betts et al (2016); Betts et al (2017); Robbie Andrew (CICERO) Forecasting CO2 concentrations
  12. 12. Verification of carbon dioxide emissions
  13. 13. Emissions growth has been flat for the last three years, perhaps ending a long period of near continuous growth Emissions from land-use change have no significant trend, but are a declining share of the total emissions. Estimates of fossil fuel and industry emissions are preliminary 2017 estimates. Land-use change is from 2016 and is to be updated. Source: CDIAC; Le Quéré et al 2016; Global Carbon Budget 2016 Three years with near-zero growth! 2017 data (preliminary) 2016 data (to be updated)
  14. 14. CO2 emissions a record high, so expect record high increase in CO2 concentrations (plus El Niño) How many years would it take to verify flat CO2 emissions in atmospheric measurements? Source: Peters (2017), Can we trust emission statistics? Are the emissions data bogus? (no)
  15. 15. Land and Ocean Sinks
  16. 16. The ocean carbon sink continues to increase 9.7±1.8 GtCO2/yr for 2006-2015 and 11.1±1.8 GtCO2/yr in 2015 Source: Le Quéré et al 2016; Global Carbon Budget 2016 Individual estimates from: Aumont and Bopp (2006); Buitenhuis et al. (2010); Doney et al. (2009); Hauck et al. (2016); Landschützer et al. (2015); Oke et al. (2013); Rödenbeck et al. (2014); Sérérian et al. (2013); Schwinger et al. (2016). Full references provided in Le Quéré et al. (2016). Ocean sink this carbon budget individual ocean models data products
  17. 17. The residual land sink decreased to 6.9±3.2 GtCO2/yr in 2015, due to El Niño conditions Total CO2 fluxes on land (including land-use change) are constrained by atmospheric inversions Source: Le Quéré et al 2016; Global Carbon Budget 2016 Individual estimates from: Chevallier et al. (2005); Clarke et al. (2011); Jain et al. (2013); Kato et al. (2013); Krinner et al. (2005); Melton and Arora (2016); Oleson et al. (2013); Peters et al. (2010); Reick et al. (2013); Rodenbeck et al. (2003); Sitch et al. (2003); Smith et al. (2014); Stocker et al. (2013); Tian et al. (2010); Woodward et al. (1995); Zaehle and Friend (2010); Zhang et al. (2013). Full references provided in Le Quéré et al. (2016). Terrestrial sink this carbon budget individual land models atmospheric inversions this carbon budget individual land models fire-based estimate this carbon budget individual land models Land source Land sink Total land + =
  18. 18. Key uncertainties
  19. 19. Updates of well-known datasets can lead to significant revisions (land use data, carbon densities, etc) The latest estimates (bold line) have a similar cumulative total, despite big difference in certain time periods Source: Houghton and Nassikas (2017) Land-use emissions
  20. 20. Different datasets can have significant differences, but hard to determine which may be more correct Source: Le Quéré et al 2016; Global Carbon Budget 2016; Houghton and Nassikas (2017); Hansis et al (2015) “Bookkeeping” models of land-use
  21. 21. DGVMs suggest that CO2 emissions from land-use change have been substantially underestimated This is also an opportunity, as it means the land sink must be stronger (mass balance) Source: Arneth et al (2017) Dynamic vegetation models of land-use
  22. 22. • Mass balance – If land-use emissions are larger, land sinks is larger – A new source means a new sink (somewhere) • Uncertainty in land-use source and sink is as large as ever, leads to more uncertainty in mitigation opportunities Opportunities with uncertainty?
  23. 23. Land-use accounting and reporting
  24. 24. Land-use change is the sum of deforestation, degradation, and afforestation Land-use sinks has no land-use change and is due to CO2 fertilization, recovery from disturbance, etc. Source: Pan et al (2011) Differentiating sources and sinks
  25. 25. UNFCCC and carbon cycle community have very different definitions Convert: Land-use change; Remain: A subset of the land sink (“managed”); HWP: Harvested Wood Products Source: CDIAC; UNFCCC National Inventory Reports; own calculations UNFCCC emissions reporting
  26. 26. For land-use change (source), governments and scientists get similar values For the land sink, governments and scientists get very different values because of definitions Source: Grassi et al (2017) Definitions have a big impact
  27. 27. UNFCCC/IPCC defines bioenergy as “carbon neutral” in the energy sector, but includes in the land-use sector This is problematic because 1) not all countries report, 2) land use and bioenergy are not linked. Why is bioenergy “carbon neutral”?
  28. 28. CO2 from biomass is a sizeable share of CO2 flux in the Nordics, assumed carbon-neutral with flux in LULUFC sector Could this lead to an attribution problem in regional carbon budgets? Source: UNFCCC National Inventory Reports; own calculations CO2 from biomass (report memo)
  29. 29. Emission Scenarios
  30. 30. IPCC assessed about 1200 scenarios, and about 120 different “2°C scenarios” Different scenarios cover different models, policy start dates, technology portfolios, etc Light lines: The IPCC Fifth Assessment Report assessed about 1200 scenarios using Integrated Assessment Models (IAMs) Dark lines: Detailed climate modelling was done on four Representative Concentration Pathways (RCPs) Source: Fuss et al 2014; CDIAC; IIASA AR5 Scenario Database; Global Carbon Budget 2016 There are many options to stay below 2°C
  31. 31. IPCC assessed about 1200 scenarios, and about 120 different “2°C scenarios” Different scenarios cover different models, policy start dates, technology portfolios, etc Light lines: The IPCC Fifth Assessment Report assessed about 1200 scenarios using Integrated Assessment Models (IAMs) Dark lines: Detailed climate modelling was done on four Representative Concentration Pathways (RCPs) Source: Fuss et al 2014; CDIAC; IIASA AR5 Scenario Database; Global Carbon Budget 2016 There are many options to stay below 2°C Lack of understanding of negative emissions and their consequences… Emission pledges
  32. 32. Most Integrated Assessment Models only report net deforestation (deforestation plus afforestation) Large variation between models, all consistent with keeping below 2°C Source: Based on IIASA AR5 Scenario Database Afforestation
  33. 33. Today, robust scientific debate over 1EJ/yr, scenarios are 100-300EJ/yr between 2050 and 2100 Need to have a clear and accessible narrative on why 100-300EJ/yr is carbon neutral Source: Based on IIASA AR5 Scenario Database Bioenergy use
  34. 34. Source: MCC 2016 Carbon dioxide removal
  35. 35. Source: Smith et al. 2016 Land, water & energy requirements
  36. 36. Discussion
  37. 37. • Terrestrial carbon cycle critical, yet highly uncertain • Why do we need to invest to reduce uncertainty? – Verification of emission statistics – Understand future climate impacts – Understand the value of maintaining forests (e.g. REDD+) – Prioritize where to invest in afforestation / reforestation – Prioritize where to invest in bioenergy • Lots of science to be done! Discussion
  38. 38. Peters_Glen cicero.oslo.no cicerosenterforklimaforskning glen.peters@cicero.oslo.no Glen Peters

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