current state of climate science

Current State of Climate Science

Peter CoxUniversity of Exeter

Some recent policy-relevant findings

New focus on non-CO2

Climate ForcingFactors

IPCC 2007

Radiative Forcing of Climate 1750-2005

Thesenon-CO2 forcings are getting much

more attention now

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.

10

8

6

4

2

2050200019501900 2100 23002200

Glo

bal C

O2 E

mis

sion

s (G

tC/y

r)Global CO2 Emissions

~ 8 GtC/yr now

10

8

6

4

2

2050200019501900 2100 23002200

Glo

bal C

O2 E

mis

sion

s (G

tC/y

r)Global CO2 Emissions

~ 8 GtC/yr now

~ 3 GtC/yr by 2050

- to avoid Dangerous Climate Change ?

Stabilisation at 450 ppmv requires a 60% cut in global CO2 emissions by 2050

..and continuous reductions beyond 2050……

2oC Peak Warming

0.7-1.4 Trillion Tonnes of Carbon as CO2

(and 500 GtC already burnt)

..but this ignores the effects of other pollutants...

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 co-benefits (e.g. for human-health and crop yields), and “buys time” for CO2 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 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....

(published 2011)

New Rationale for Mitigation of non-CO2 forcing Factors

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”...

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)

-400

-300

-200

-100

0

100

200G

tCCO2

O3

AEROCH4

….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

-200

0

200

400

600

800

1000

1200

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1

Non CO2 RF (W m-2)

Perm

issi

ble

CO

2 Em

issi

ons

(Gt C

)

Change in Ocean Carbon

Change in Atm Carbon

Change in Land Carbon

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 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.

(a) Modelled Loss of Tropical Land Carbon due to Warming Gt

C/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 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 anomaliesGlobal CO2 Growth-rate Mean Temperature 30oN-30oS

Constraints from ObservedInterannual Variability

dCO 2/dt (G

tC/yr) = 4.01+/-0.76 dT (K)

(a) Climate Impact on Tropical Land Carbon, LT

(b) Sensitivity of CO2 Growth-Rate to Tropical Temperature

GtC/

yr/K

GtC/

K

Observed

Obs

erva

tiona

l Co

nstr

aint

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

Cs (0) = 50 kg C m-2, W m-2 K-1

Rsref = 0.5 kg C m-2 yr-1, q10 = 2.5

Luke and Cox, 2011.

Ta

forcing6K

10K 8K

Ts

Response

Time (yrs) Time (yrs)

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