Download Current state of climate science

Current State of
Climate Science
Some recent policy-relevant
findings
Peter Cox
University of Exeter
New focus on
non-CO2
Climate Forcing
Factors
Radiative Forcing of Climate 1750-2005
These
non-CO2 forcings
are getting much
more attention now
IPCC 2007
Previous Rationale for Focusing
on CO2 Mitigation
 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.
Global CO2 Emissions (GtC/yr)
Global CO2 Emissions
10
8
~ 8 GtC/yr
now
6
4
2
1900
1950
2000
2050
2100
2200
2300
Global CO2 Emissions
Global CO2 Emissions (GtC/yr)
- to avoid Dangerous Climate Change ?
10
8
~ 8 GtC/yr
now
Stabilisation at 450 ppmv
requires a 60% cut in global
CO2 emissions by 2050
6
4
~ 3 GtC/yr
by 2050
..and continuous
reductions beyond
2050……
2
1900
1950
2000
2050
2100
2200
2300
..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)
New Rationale for Mitigation of
non-CO2 forcing Factors
 We aren’t making much progress on CO2!
Recent Trends in CO2 Emissions
(Friedlingstein et al., 2010)
New Rationale for Mitigation of
non-CO2 forcing Factors
 We aren’t making much progress on CO2!
 Reducing non-CO2 forcings could have major cobenefits (e.g. for human-health and crop yields), and
“buys time” for CO2 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 CO2 emissions reductions emissions are required
to limit long-term climate change.

But even here I think reductions in non-CO2 radiative forcings
would make the carbon mitigation problem easier....
New Rationale for Mitigation of
non-CO2 forcing Factors
 We aren’t making much progress on CO2!
 Reducing non-CO2 forcings could have major cobenefits (e.g. for human-health and crop yields), and
“buys time” for CO2 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
CO2
GtC
0
-100
CH4
-200
AERO
-300
-400
O3
….this implies the Integrated CO2 Emissions for Stabilization
are extremely sensitive to non-CO2 radiative forcings
Permissible CO2 Emissions
for +1 W m-2 Stabilization
(Cox & Jeffery, 2010)
Permissible CO2 Emissions for +1 W m -2 versus Non-CO2 RF
1200
Permissible CO 2 Emissions (Gt C)
Change in Ocean Carbon
1000
Change in Atm Carbon
Change in Land Carbon
800
600
400
200
0
-1
-0.75
-0.5
-0.25
0
0.25
-200
Non CO2 RF (W m-2)
0.5
0.75
1
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 ?
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
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 21st 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 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.
GtC/K
(a) Modelled Loss of Tropical Land Carbon due to Warming
-140
-120
-100
-80
-60
-40
-20
0
Tropical Forest Dieback
 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.
Interannual Variability in the CO2
growth-rate is determined by the response
of tropical land to climate anomalies
Global CO2 Growth-rate
Mean Temperature 30oN-30oS
Constraints from Observed
Interannual Variability
GtC/K
(a) Climate Impact on Tropical Land Carbon,
-140
-120
-100
-80
-60
-40
-20
0
12
GtC/yr/K
10
8
6
4
2
0
gLT
(b) Sensitivity of CO2 Growth-Rate to Tropical Temperature
Observational
Constraint
Constraint suggests tropical forest dieback is unlikely
More detailed models suggest
that Permafrost Carbon is less
stable...
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
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.
Conclusions
 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.