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

New focus on non-CO 2 Climate Forcing Factors

IPCC 2007 Radiative Forcing of Climate These non-CO 2 forcings are getting much more attention now

Previous Rationale for Focusing on CO 2 Mitigation  The other forcing factors are small compared to CO 2.  Many of the other pollutants are short-lived compared to CO 2, so emissions cuts for these gases are less urgent.

Global CO 2 Emissions (GtC/yr) Global CO 2 Emissions ~ 8 GtC/yr now

Global CO 2 Emissions (GtC/yr) Global CO 2 Emissions ~ 8 GtC/yr now ~ 3 GtC/yr by to avoid Dangerous Climate Change ? Stabilisation at 450 ppmv requires a 60% cut in global CO 2 emissions by and continuous reductions beyond 2050……

2 o C Peak Warming Trillion Tonnes of Carbon as CO 2 (and 500 GtC already burnt)..but this ignores the effects of other pollutants...

New Rationale for Mitigation of non-CO 2 forcing Factors  We aren’t making much progress on CO 2 !

Recent Trends in CO 2 Emissions (Friedlingstein et al., 2010)

New Rationale for Mitigation of non-CO 2 forcing Factors  We aren’t making much progress on CO 2 !  Reducing non-CO 2 forcings could have major co- benefits (e.g. for human-health and crop yields), and “buys time” for CO 2 mitigation.

 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  Cautions that CO 2 emissions reductions emissions are required to limit long-term climate change.  But even here I think reductions in non-CO 2 radiative forcings would make the carbon mitigation problem easier.... (published 2011)

New Rationale for Mitigation of non-CO 2 forcing Factors  We aren’t making much progress on CO 2 !  Reducing non-CO 2 forcings could have major co- benefits (e.g. for human-health and crop yields), and “buys time” for CO 2 mitigation. ..and I think it also “buys carbon”...

Ecosystems and Atmospheric Pollutants  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.

Impact on Land Carbon Storage of +1 W m -2 (Huntingford et al., 2011) CO 2 O3O3 AERO CH 4 ….this implies the Integrated CO 2 Emissions for Stabilization are extremely sensitive to non-CO 2 radiative forcings

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

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

United Nations Framework Convention on Climate Change (UNFCCC) “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…” Introduces the notion of “Dangerous” Climate Change… ….but how can this be defined ?

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 Tipping Points (Lenton et al., 2008)

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

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

Tropical Forest Dieback in HadCM3LC Model

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

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

Tropical Forest Dieback  The Hadley Centre’s first coupled climate-carbon cycle model (“HadCM3LC”) simulated a dramatic dieback of the Amazon rainforest in the 21 st 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 CO 2 growth-rate.

Interannual Variability in the CO 2 growth-rate is determined by the response of tropical land to climate anomalies Global CO 2 Growth-rateMean Temperature 30 o N-30 o S

Constraints from Observed Interannual Variability dCO 2 /dt (GtC/yr) = 4.01+/-0.76 dT (K)

(a) Climate Impact on Tropical Land Carbon,  LT (b) Sensitivity of CO 2 Growth-Rate to Tropical Temperature GtC/yr/K GtC/K Observed

Observational Constraint Constraint suggests tropical forest dieback is unlikely

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

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 Tipping Points (Lenton et al., 2008)

Rate-dependent “Compost Bomb” Instability C s (0) = 50 kg C m -2, W m -2 K -1 R sref = 0.5 kg C m -2 yr -1, q 10 = 2.5 Luke and Cox, T a forcing 6K 10K 8K T s Response Time (yrs)

 A growing focus on reducing non-CO 2 forcing factors is partly-motivated by slow progress on the CO 2 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 CO 2 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