Introduction to Climate Science

Transcription

1 Introduction to Climate Science Vegetation and the Carbon Cycle Mike Unsworth Atmospheric Science

2 Outline: Global carbon budget : role of vegetation How does weather and climate affect vegetation? How does vegetation affect weather and climate? Carbon and water cycles are linked Feedbacks from vegetation to climate Modeling the future

3 Observing the biosphere breathing Rate of increase averages about 1.5ppm/yr Peaks occur in the N. hemisphere winter Amplitude of the annual peak-peak variation is about 5ppm, of which 1-1.5ppm is associated with the ocean and ppm is associated with the terrestrial biosphere

4 Global Carbon Dioxide Budget (in Pg C yr -1 ) Emissions (fossil fuel, cement) Land-use change 1.7 Range 0.6 to 2.5 Atmospheric increase Atmosphere-ocean flux Atmosphere-land flux 1.9 Range 3.8 to Emissions + Land-use change = Atmosphere-ocean + Atmosphere land fluxes + Atmospheric increase =

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6 Even when fully open, stomata occupy only 0.5 to 5% of the leaf surface area

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9 The biosphere is not a steady breather Rate of increase averages about 1.5ppm/yr But some years increase more or less than the average. WHY?

10 Annual rate of change of CO2 concentration in the atmosphere ppm CO 2 /yr Agung El Chichon Pinatubo Volcanic eruptions (closed arrows) are associated with low rates of increase of CO 2 in the atmosphere. WHY? ENSO events (open arrows) associated with high rates of increase.

11 Possible explanations of minima for CO 2 retained in the atmosphere after volcanic eruptions: Volcanic eruptions > aerosols > global cooling > reduced terrestrial respirationie reduced natural CO 2 emission less CO 2 stored in the atmosphere OR..aerosols -> globally increased diffuse radiation -> increased photosynthesis > ie increased CO 2 sink strength less CO 2 stored in the atmosphere

12 Vegetation canopies use diffuse radiation more efficiently than direct radiation Pinatubo erupts From Gu et al, Science, 299, , 2003

13 How do we investigate how the carbon cycle is related to weather and climate?

14 Micrometeorological Pros measurements Direct measurement Evaluates fluxes on diurnal, seasonal and interannual time scales Reveals how processes (e.g. evaporation, photosynthesis, respiration) depend on the weather Remedy Validate with leaf and plant scale measurements. Compare with other watershed scale measurements Cons Unreliable at nighttime Measure only small area (< 1 km) Not applicable in complex terrain Network of towers is discrete in space. How do we scale up?

15 Simplified Methodology Eddy Covariance Method is use to measure Fluxes of CO 2, water vapor and energy exchange. The method measures the Net Ecosystem Exchange NEE = w' ρ c ' w is vertical velocity fluctuation; ρ c is a density fluctuation (e.g. CO 2, water vapor.

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17 Tonzi Ranch, United States

18 Old-growth ponderosa pine site (~ 250y) Intermediate ponderosa pine site (~60y) Young ponderosa pine site (~ 20 y)

19 About 400 towers (2007)

20 Mean Diurnal Pattern of CO 2 Flux CO 2 Flux Density µmol m -2 s time (hours) N e : measured (-4.84 gc m -2 day -1 ) N e : computed (-5.09 gc m -2 day -1 ) F wpl +Storage: measured (-5.96 gc m -2 day -1 ) F wpl : measured (-6.12 gc m -2 day -1 )

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22 Harvard Forest CO 2 Flux Density (gc m -2 d -1 ) NEE Reco GEE Year Data of Wofsy, Munger, Goulden et al.

23 Spatial Variations in C Fluxes

24 Feedbacks and forcing from vegetation in climate models Early methods of representing the land surface in climate models treated the surface as a green slime with surface temperature and soil moisture determined by a bucket model of hydrology. In the late 1990s improved land surface schemes included better treatment of soil moisture, and included physiological attributes for vegetation that improved the simulation of water vapor and heat fluxes as soils dried. Feedback therefore included: albedo, and variation in the partitioning of net radiation at the surface into latent and sensible heat fluxes to the atmosphere But the vegetation distribution was fixed and the influence of rising CO 2 on the carbon cycle was missing

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26 In around 2000, climate models that included carbon cycles on land and in the ocean were developed. Most models have fixed vegetation as the climate changes. A few have dynamic vegetation that evolves as climate changes according to ecological rules (eg from forest to shrubs, grassland and finally desert) These models allow new feedbacks called physiological feedback, and they also generate new physiological forcing. In particular: the carbon cycle responds to increased CO 2 by partially closing stomates and thus increasing the water use efficiency (more productivity per unit of water). Thus more water may be available for run-off, or drought may hit less rapidly than in simpler models. The validity of this behavior in the field is debatable. A reduced evaporative flux to the atmosphere requires an increased heat flux, and in regions like the Amazon this may decrease precipitation (also see next slide). If vegetation dies, its carbon decays to the atmosphere, creating additional radiative forcing. New vegetation types may have different albedos, further altering forcing

27 Fig. 1. Deforestation and dry season cumulus cloud cover in northern Costa Rica and southern Nicaragua. (A) 16:15 UTC (10:15 local time) false color Landsat image showing the long-deforested (pink) portions of the San Carlos plains (SC) and Tortuguero plains (T) upwind of the cloud forests (dark red) of Monteverde (M) and the Cordillera Volcanica Central (CC). Deforested lee (Pacific) slopes southwest of Monteverde and in the Meseta Central (MC) appear gray-green. Remnant forest (RF) remains south of the Costa Rica-Nicaragua border, whereas intact lowland forest covers southeast Nicaragua (N). (B)16:15 UTC false color Landsat image from a day of extensive cumulus cloud field development. Clouds have not developed over the parts of the San Carlos or Tortuguero plains that have been deforested the longest, and are less well-developed over the remnant forests of northern Costa Rica than above the adjacent forested Nicaraguan lowlands (N). (C) Frequency of cumulus cloud field coverage at 16:15 UTC during March 1999, compiled from daily GOES imagery, showing poor cumulus development over deforested parts of the San Carlos and Tortuguero plains From: Lawton et al. Climatic impact of tropical lowland deforestation on nearby montane cloud forests. Science 294, , 2001

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29 The next 3 slides are from a paper in Nature by Cox et al. (vol 408, , 2000 They ran three related versions of the Hadley Centre climate model, all using a typical scenario of increasing CO 2 1. A model with interactive CO 2 and dynamic vegetation 2. A standard simulation with fixed vegetation and no coupled carbon cycle 3. A simulation in which the interactive CO 2 and dynamic vegetation were in the model, but the radiative forcing by CO 2 was switched off (i.e. no direct CO 2 -induced climate change

30 Results from the fully coupled model with dynamic vegetation From 1900 to about 2050 the land is a net sink for atmospheric CO 2, but after 2050 the land becomes a net source. The ocean in contrast becomes an increasingly strong sink as the atmospheric CO 2 concentration increases

31 Note that the increased CO 2 concentration from vegetation and soil respiration provides extra forcing that increases mean temperatures

32 Note that most of the loss of carbon from vegetation is from Amazon die-back.

33 Summary The terrestrial surface absorbs (sequesters) about 30% of annual global CO 2 emissions This percentage is influenced by natural phenomena such as volcanic eruptions (aerosol) and El Nino Field studies of sequestration help to calibrate remote sensing of vegetation growth, and improve our confidence in predicting how future climate change will affect sequestration Most climate models have land surface schemes that simulate vegetation feedback in term of energy balance components A few climate models have a coupled carbon cycle, but very few have dynamic vegetation Model experiments with fully coupled dynamic carbon cycles suggest that additional physiological forcing and feedbacks occur that might increase warming, alter plant distributions, and affect hydrology