Some recent policy-relevant findings
University of Exeter

2

Radiative Forcing of Climate 1750-2005
These non-CO
2 forcings are getting much more attention now
IPCC 2007

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
10
8
6
4
~ 8 GtC/yr now
2
1900 1950 2000 2050 2100 2200 2300

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

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

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 cobenefits (e.g. for human-health and crop yields), and
“buys time” for CO
2 mitigation.

(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 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....

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 cobenefits (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)
Change in Land Carbon
(Climate+Physiology)
200
100
CO
2
0
-100
CH
4
-200
AERO
-300
-400 O
3
….this implies the Integrated CO
2
Emissions for Stabilization are extremely sensitive to non-CO
2 radiative forcings

1200
1000
800
600
400
200
0
-200
-1
Permissible CO
2
Emissions for +1 W m -2 Stabilization
(Cox & Jeffery, 2010)
Permissible CO
2
Emissions for +1 W m
-2
versus Non-CO
2
RF
Change in Ocean Carbon
Change in Atm Carbon
Change in Land Carbon
-0.75
-0.5
-0.25
0
Non CO
2
RF (W m
-2
)
0.25
0.5
0.75
1

(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 ?

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

(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
1850 2000
2100

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.

-140
-120
-100
-80
-60
-40
-20
0
(a) Modelled Loss of Tropical Land Carbon due to Warming

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-rate Mean Temperature 30 o N-30 o S

Constraints from Observed
Interannual Variability

-140
-120
-100
-80
-60
-40
-20
0
(a) Climate Impact on Tropical Land Carbon, g
LT
12
10
8
2
0
6
4
(b) Sensitivity of CO
2
Growth-Rate to Tropical Temperature

Constraint suggests tropical forest dieback is unlikely

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

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
T s
Response
10K
8K
6K
T a forcing
Time (yrs) Time (yrs)
Luke and Cox, 2011.

 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.