Global Carbon Cycle Feedbacks:

From pattern to process

Dave SchimelNEON inc

Fate of Anthropogenic CO2 Emissions (2000-2007)

Canadell et al. 2007, PNAS (updated); Slide from global Carbon Project

1.5 Pg C y-1

+7.5 Pg C y-1

Atmosphere46%

4.2 Pg y-1

Land29%

2.6 Pg y-1

Oceans26%

2.3 Pg y-1

The Basic Narrative of Climate Change and the Carbon Cycle

Energy Balance

Water vapor

Temperature

We are changing the Carbon Cycle

Which causes a change in the distribution of the energy reaching the Earth

Which warms the planet

Which changes the water cycle

Which amplifies the climate change, mainly because of increased water vapor

Eventually feeding back on the carbon cycle

Carbon cycle feedbacks

• Terrestrial uptake and release depend on temperature and precipitation

• Terrestrial uptake depends on atmospheric CO2 concentration

• The above feedbacks depend on nutrient cycles• Terrestrial carbon storage depends on

ecosystem type, which depends on climate• And then there are the oceans (for another talk)

– And this all leads to model uncertainty…..

The C4MIP uncertainty figure

Status of global models ?

• Current models match local and some global observations but often predict variables and scales that are hard to validate (same problem as in night 1 talks)

• Current models give drastically different predictions under climate change despite similar skill levels for the present day

What can we constrain from observations?

Pattern: the interhemispheric gradient implies a Northern

Hemisphere sink

TFT 1990

What does a Northern Hemisphere land sink imply?

• CO2 fertilization implies a global sink

• The Northern hemisphere is perturbed by historical land use, air pollution, nonstationary disturbance regimes (fire)

• Therefore, this spatial pattern suggests weak CO2 and strong land use+climate effects

• For the purposes of this talk, I will make that assumption, although recent observational data suggests it may be false…

Patterns in time:

The impact of carbon–climate feedback on carbon storageIncreased in the north, reduced in the south.

Fung I Y et al. PNAS 2005;102:11201-11206

©2005 by National Academy of Sciences

Regional differences in the change in hydrologic regime and ecosystem productivity with global warming: positive effects of warming on pant growth dominate in the North, drought in

the South.

Fung I Y et al. PNAS 2005;102:11201-11206

©2005 by National Academy of Sciences

Observational support for the Fung and Doney hypothesis

Middle sites are in the middle

Extratropic mean normalized anomalies in the net spring uptake [dotted black line, expressed by inverted early summer (June) detrended CO2 concentration] and spring (MAM)

temperature (dotted red line, weighted by NPP) (a) and in the net growing-season uptake (dotted black line, expressed by inverted seasonal minimum detrended CO2 concentration

taken from the GLOBALVIEW “reference marine boundary layer matrix”) and growing season (MAMJJA) temperature (dotted red line, weighted by NPP) (b).

Angert A et al. PNAS 2005;102:10823-10827

©2005 by National Academy of Sciences

Model-data fusion

• Detailed analyses at a specific site

Niwot Ridge, Colorado

Self-consistent parameter sets

CS,0 (g m-2)

KH (

g g

-1 y

-1)

Self-consistent parameter sets

Fit to the diurnal cycle (~12 hour time steps)

Fit to daily data: 24 hour time steps

Observed variability of fluxes

Analyzed variability of processes

Analysis of controls

Warm springs accelerategrowth but also evaporation, consistent with information from spatial flux patterns and atmospheric CO2 trends

Patterns in time: clues to mechanisms

Emissions from Indonesian wildfire estimated by inversion of global CO2 data

One more factor:: nutrient loading

Terrestrial stoichiometry:equilibration of plant growth with water/energy and nutrients: at equilbrium, nutrients and climate co-vary, in the transient, not.

Conclusions

Changes to water balance driven by temperature and moisture dominate the terrestrial carbon feedback.

Some of these effects are due to effects on photosynthesis and respiration.

Others are due to changes in disturbance and vegetation structure.

Requilibration of limiting nutrients will influence, or dominate, the transient.

Conclusions: the road forward

In the spirit of communicating about climate change, what does this mean?

1. The terrestrial carbon-climate feedback is net positive, though internal (nutrient) feedbacks make it less positive than in simple models

2. Part of this is because of ecosystem destruction and structural change, which is partly under direct human control

3. A net positive carbon climate feedback is not just an esoteric feedback on global mean temperature, but implies damages to agriculture, forestry and associated ecosystem services that may be far more serious for human society than the (small to moderate) additional gain in the coupled climate system.

We are changing the Carbon Cycle…

Upscaling from experiments:

DRI Reno Face study, Nevada