Current State of Climate Science

1. Current State of Climate Science Some recent policy-relevant findings Peter Cox University of Exeter

2. New focus on non-CO2 Climate Forcing Factors

3. Radiative Forcing of Climate 1750-2005 These non-CO2 forcings are getting much more attention now IPCC 2007

4. The other forcing factors are small compared to CO2. Many of the other pollutants are short-lived compared to CO2, so emissions cuts for these gases are less urgent. Previous Rationale for Focusing on CO2 Mitigation

5. Global CO2 Emissions 10 ~ 8 GtC/yr now 8 6 Global CO2 Emissions (GtC/yr) 4 2 2300 1900 2200 1950 2000 2100 2050

6. Global CO2 Emissions - to avoid Dangerous Climate Change ? 10 ~ 8 GtC/yr now 8 Stabilisation at 450 ppmv requiresa 60% cut in global CO2 emissions by 2050 6 Global CO2 Emissions (GtC/yr) 4 ~ 3 GtC/yr by 2050 ..and continuous reductions beyond 2050…… 2 2300 1900 2200 1950 2000 2100 2050

7. ..but this ignores the effects of other pollutants... 2oC Peak Warming 0.7-1.4 Trillion Tonnes of Carbon as CO2 (and 500 GtC already burnt)

8. We aren’t making much progress on CO2! New Rationale for Mitigation of non-CO2 forcing Factors

9. Recent Trends in CO2 Emissions (Friedlingstein et al., 2010)

10. We aren’t making much progress on CO2! Reducing non-CO2 forcings could have major co-benefits (e.g. for human-health and crop yields), and “buys time” for CO2 mitigation. New Rationale for Mitigation of non-CO2 forcing Factors

11. (published 2011) • Points out that Tropospheric Ozone and Black Carbon (“soot”) contribute to climate change and have very adverse effects on human-health. • Suggests that the implementation of “simple” cost effective emission reduction measures could halve global warming by 2050. • Cautions that CO2 emissions reductions emissions are required to limit long-term climate change. • But even here I think reductions in non-CO2radiativeforcings would make the carbon mitigation problem easier....

12. We aren’t making much progress on CO2! Reducing non-CO2 forcings could have major co-benefits (e.g. for human-health and crop yields), and “buys time” for CO2 mitigation. ..and I think it also “buys carbon”... New Rationale for Mitigation of non-CO2 forcing Factors

13. The impacts of different atmospheric pollutants are typically compared in terms of Radiative Forcing or Global Warming Potential But Ecosystems and Ecosystem Services (such as land carbon storage) are affected directly by many atmospheric pollutants, as well as indirectly via the impact of these pollutants on climate change. Ecosystems and Atmospheric Pollutants

14. Impact on Land Carbon Storage of +1 W m-2(Huntingford et al., 2011) CO2 CH4 AERO O3 ….this implies the Integrated CO2 Emissions for Stabilization are extremely sensitive to non-CO2 radiative forcings

15. Permissible CO2 Emissions for +1 W m-2 Stabilization(Cox & Jeffery, 2010)

16. Some Recent Work on Climate Tipping Points (relevant to concept of “Dangerous Climate Change”)

17. “The ultimate objective [is]…. stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system…” United Nations Framework Convention on Climate Change (UNFCCC) Introduces the notion of “Dangerous” Climate Change… ….but how can this be defined ?

18. Tipping Points (Lenton et al., 2008) Map of potential policy-relevant tipping elements in the climate system, updated from ref. 5 and overlain on global population density Lenton T. M. et.al. PNAS 2008

19. Observational Constraint suggests Tropical Forests are more stable.... (relevant to “Sink Permanence”)

20. The Hadley Centre’s first coupled climate-carbon cycle model (“HadCM3LC”) simulated a dramatic dieback of the Amazon rainforest in the 21st century. Tropical Forest Dieback

21. Tropical Forest Dieback in HadCM3LC Model 2100 1850 2000

22. The Hadley Centre’s first coupled climate-carbon cycle model (“HadCM3LC”) simulated a dramatic dieback of the Amazon rainforest in the 21st century. Other coupled climate-carbon models did not project such a dramatic dieback, although all models simulated a loss of tropical land carbon as a result of warming. Tropical Forest Dieback

23. (a) Modelled Loss of Tropical Land Carbon due to Warming GtC/K

24. The Hadley Centre’s first coupled climate-carbon cycle model (“HadCM3LC”) simulated a dramatic dieback of the Amazon rainforest in the 21st century. Other coupled climate-carbon models did not project such a dramatic dieback, although all models simulated a loss of tropical land carbon as a result of warming. Until very recently it hasn’t been possible to estimate the sensitivity of the real tropical forests to climate change, but now we think we can from the year-to-year variation in the CO2 growth-rate. Tropical Forest Dieback

25. Interannual Variability in the CO2growth-rate is determined by the response of tropical land to climate anomalies Global CO2 Growth-rate Mean Temperature 30oN-30oS

26. Constraints from ObservedInterannual Variability dCO2/dt (GtC/yr) = 4.01+/-0.76 dT (K)

27. (a) Climate Impact on Tropical Land Carbon, gLT GtC/K Observed GtC/yr/K

28. Constraint suggests tropical forest dieback is unlikely Observational Constraint

29. More detailed models suggest that Permafrost Carbon is less stable...

30. Tipping Points (Lenton et al., 2008) Map of potential policy-relevant tipping elements in the climate system, updated from ref. 5 and overlain on global population density Lenton T. M. et.al. PNAS 2008

32. Rate-dependent “Compost Bomb” Instability Cs (0) = 50 kg C m-2, W m-2 K-1 Rsref = 0.5 kg C m-2 yr-1, q10 = 2.5 Ts Response 10K 8K Ta forcing 6K Time (yrs) Time (yrs) Luke and Cox, 2011.

33. A growing focus on reducing non-CO2 forcing factors is partly-motivated by slow progress on the CO2 problem, but seems to make scientific sense in its own right - because of co-benefits for health and land carbon storage (which implies a positive impact on “permissible” emissions). The observed year-to-year variability in CO2 constrains the sensitivity of tropical land carbon to climate – suggesting that tropical forests are less vulnerable than previously feared (..so sink permanence may be less of an issue..). However, recent modelling studies suggest than permafrost carbon is more vulnerable than global models typically indicate – especially when “compost self-heating” is included. Conclusions