• GG 312: Global Climate Change and Environmental Impacts (Fall 2002)


    Chapter 7: Biotic Responses and Sea Level Changes


    7.1. Terrestrial Biotic Responses

    Terrestrial ecosystems influence climate through two broadly defined processes:

    • Biogeochemical - exchange of carbon dioxide, methane, nitrous oxide, all of which are radiative active gases
    • Biogeophysical - exchange of water and energy leading to changes in surface temperatures, precipitation and atmospheric circulation.

    Changes in structure and function of terrestrial ecosystems can lead to changes in climate. Various kinds of feedbacks can operate at different spatial and temporal scales, making it difficult to accurately evaluate the impact on climate.

    Global Land Cover Map

    7.1.A. Land Atmosphere Exchange of CO2

    Terrestrial ecosystems are thought to contain about 2500 GtC (giga tons of carbon), with approximately 500 Gt C (likely an over estimate) in standing vegetation and about 2000 Gt C in the soils. These stocks can change in time for various reasons; for example, the area of agricultural land, age structure of the forest stands, changes in ecosystem metabolism, etc.

    • Area of Agricultural Land
      • Cultivation represents about 20 percent of the Earth's vegetated area.
      • Most of this land was once forested; hence, contained abundant carbon.
      • The conversion of forested area to cultivation releases carbon to the atmosphere because of biomass burning and decay of litter. Conversely, aforestation or reforestation withdraws carbon from the atmosphere.
      • The net flux per year during the 1980s and 90s has been estimated between 0.6 to 2.5 GtC.

    • Age Structure of the Forest
      • Young and middle-aged forests accumulate carbon while older forests accumulate little, if any. This may not be true anymore, in light of new evidence which indicates 500 yr old forests in Siberia accumulating carbon.
      • Forests of the mid-Northern latitudes harvested in the early years of this century, are now known to be rapidly accumulating carbon. Massachusetts is a good example.
      • Fires, insect outbreaks and other disturbances can change the rate of carbon accumulation by forests. As an example, the boreal forests of Canada were estimated to be a source of carbon (0.05 GtC/year), as a result of disturbances from fire and insect damage.

      How Will Climate Change Impact the World's Forests?

    • Changes in Ecosystem Metabolism
      • Gross Primary Production or GPP, of terrestrial vegetation is the total amount of carbon fixed, and is currently estimated at about 100-120 GtC/year.
      • Plant or Autotrophic Respiration or AR, is the loss of carbon fixed (GPP) due to respiration by the living biomass of the vegetation. It is estimated at about 50-60 GtC/year.
      • Net Primary Production or NPP, is the net carbon fixed by the vegetation globally, and is equal to GPP minus AR, or about 50-60 GtC/year. This represents the net drawdown of carbon from the atmosphere by the vegetation of the Earth in a year.
      • Soil or Heterotrophic Respiration or HR, is the carbon released to the atmosphere by heterotrophs in the soil from decay and breakdown of litter. This is estimated to be somewhat less than NPP.
      • Net Ecosystem Production or NEP, is the difference between NPP and HR. This is typically about 5-10 GtC/year. Much of this is lost by nonrespiratory p rocesses such as fire, insect damage, and harvest.
      • Net Biome Production or NBP, is the final balance. This is typically about 1-2 GtC/year, and can be positive (sink) or negative (source).
      • Various factors can affect the carbon metabolism of global vegetation; for example, climate change, CO2 fertilization, nitrogen deposition from acid rain, etc. These will discussed next in some detail.

      Carbon Content of Northern Vegetation

    7.1.B. Climate Change and Responses

    Considering the role of vegetation in the global carbon cycle, the following question is important: how will the relationship between between net primary production and heterotrophic respiration change as the Earth warms in response to the accumulation of greenhouse gases?

    • Precipitation and Cloudiness: In dry areas , NPP decreases from decreased soil moisture; but if precipitation increases, then, NPP also increases. In moist regions , cloudiness decreases incident radiation; so, NPP may decrease, and precipitation is less of a factor.
    • Warming may increase NPP by enhancing photosynthesis or through increased nutrient availability if decomposition and mineralization are accelerated.
    • Warming may decrease NPP by decreasing soil moisture, which reduces photosynthesis through decreased stomatal conductance, or by increasing plant (autotrophic) respiration.
    • Warming may cause heterotrophic respiration to go up more than NPP, so, as a result, climate change may lead to reduction of carbon storage in terrestrial ecosystems.
    • A longer growing season due to an early spring and/or delayed fall can result in NPP increases.
    • In this fashion, climate change can differentially affect NPP and heterotrophic respiration, thus changing the carbon content of vegetation.

    7.1.C. CO2 Fertilization

    • Short-term increases in CO2 increases leaf-level photosynthesis. Leaves then soon acclimatize, so that longer-term increases are usually less.
    • At the whole-plant level, other factors come into play. For example, if photosynthesis is happening at a rate faster than needed for plant growth, it is considerably slowed.
    • CO2 Experiments at Flakaliden, Sweden
    • From various sources, the total observed range is from -43 percent to 375 percent; observations almost always from controlled chamber experiments.
    • At all levels (tissue, leaf, plant, ecosystem), experimental evidence points to increased proportionate response of NPP to CO2, when soil moisture is limiting. That is, increased CO2 decreases stomatal conductance, and thus, transpiratory water loss; hence, increased water use efficiency, or, photosynthates fixed per unit water lost.
    • The evidence for the CO2 fertilization effect is weak.

    7.1.D. Nitrogen Fertilization

    • Most mid-latitude forests have been reported to have a higher than normal growth rates during the 1950 to 1980 time period, primarily from Nitrogen fertilisation (deposition from pollution).
    • When mid-latitude forests at an experimental site were given 50 Kg of Nitrogen per hectare per year, it resulted in 10 to 20 percent higher above ground woody biomass Carbon storage.
    • Obviously if the forests are NOT Nitrogen limited, no effect of Nitrogen fertilisation can be observed.
    • Nitrogen fertilization effect is now believed to make only a minor contribution to carbon storage.

    7.1.E. Terrestrial Carbon Sink

    • Attempts to balance the global carbon budget, usually requires aa carbon sink of the order of 3 to 4 GtC/year.
    • The ocean sink is estimated consistently to be about 2 GtC/year, which leaves about 1 to 2 GtC/year of missing carbon.
    • This sink is now increasingly attributed to vegetation in the temperate and boreal regions.
    • These numbers are extremely significant because the sink represents 15-35% of emissions from fossil fuels and industrial activities.
    • It is also clear that any future predictions of atmospheric CO2 must accurately account for the role of global vegetation in the global carbon cycle.
    • Learn about the Kyoto Protocol

    7.1.F. Land cover Changes

    Changes in climate and atmospheric CO2 affect species composition and structure of ecosystems because the environment limits the kinds of plants and amount of tissue they can support. The ecosystems of the world are usually classified into 15 to 20 land covers; their geographical distribution is controlled by temperature and water.

    Example Satellite Land cover Maps
    Global Land cover at Last Glacial Maximum

    • Cold Tolerance: Some tropical forests are damaged by temperatures less than 10oC, compared to boreal forests, in their leafless winter stage can survive almost unlimited cold periods.
      • Warmer temperatures would result in a poleward movement of a diverse type of woody species, in general.
      • Warm summer temperatures and longer growing seasons would allow some cold adapted trees to move northward, especially at the polar treeline, where warmth is limiting.
      • Chilling requirements keep continental woody plants out of maritime climates. Warmer winters will force these to retreat from their low-latitude and maritime limits.

    • Water Tolerance: Rain forests can tolerate only a few weeks of drought, while hot desert species can persist in a completely dry climate.
      • In general, evergreen trees survive drought by reducing water loss from transpiration; deciduous trees shed their leaves altogether. As water availability declines further, trees give way to fire-adapted grasses and shrubs; then ultimately to desert-brush vegetation and other drought-adapted forms. Climate change induced water availability, whether positive or negative, will result in vegetation changes along this continuum.

    • Competitive Balance:
      • Wet tropical climates support ever-green rain forests; longer dry seasons favor drought-deciduous trees in seasonal and dry forests. Any change in precipitation will affect these natural boundaries.
      • Savannas represent equilibrium between trees and grasses. In seasonal tropical climates, grasses exists on clayey soils and summer rainfall, while woodlands surviveon sandy soils and winter rainfall. Changes in rainfall seasonality and land-use will affect this dynamical process.

    • C3 and C4 Dominance:
      • At the present levels of atmospheric CO2 concentration, temperatures above 22C favor C4 over C3 because of higher light-use efficiency.
      • Increasing the CO2 concentration increases this temperature limit. Therefore, C4 species are expected to benefit with enhanced warming.
      • However, C3 plants are generally CO2 limited; therefore, increasing CO2 benefits C3 plants more than C4 plants.
      • Thus, the dynamics between C3 and C4 species depends on CO2 and temperature interactions.

    7.1.G. Biogeophysical Feedbacks

    • Vegetation mediates the exchange of water and energy between the land surface and the atmosphere. Typically, this exchange is expressed in the form of an heat budget equation,
      • R_N = S + LE + G + M
      • where, R_N is the net radiation (shortwave plus longwave), S is sensible or convective heat flux, LE is the latent heat flux, G is soil heat flux and M is energy used for metabolic activities.
      • Because of the layering of leaves along the vertical in vegetation canopies, the total one-sided leaf area, per unit ground area, in contact with the atmosphere can range from 1-2 in grasses to over 5 in forests. Thus, compared to a bare surface, vegetated surfaces interact with the atmosphere more intimately, and this interaction is dynamic, because plants act to optimize the amount of photosynthate produced per unit water lost.
      • LAI Images
    • The most important land surface parameters influenced by vegetation structure are,
      1. Albedo (that is, reflectivity of showtwave radiation)
      2. Roughness Length (affects boundary layer transport of fluxes)
      3. Canopy Conductance (integral of leaf stomatal conductance)
      4. Rooting Depth
    • Albedo: It ranges from 0.15 over dense forests, to about 0.4-0.5 over bring sandy soils, and to greater than 0.8 for snow.
    • Roughness Length: Tall vegetation presents a much deeper diffusion layer than short vegetation, facilitating faster exchanges of mass (water and CO2) and momentum with the atmosphere.
    • Canopy Conductance: Depends on ambient CO2 concentration, hydrological conditions, foliage density and stress. For natural vegetation, the typical values are about 3-6 mm/s, while for crops it is about twice as that, 12 mm/s.

    7.1.H. Land Surface Changes

    The sensitivity of global climate to changes in land surface parameters has been investigated with global climate models, in a number of instances. The following is a brief summary of these works.

    • Large scale deforestation tends to reduce moisture convergence and therefore, precipitation, by raising albedo and/or lower evapo-transpiration. Therefore, the potential area of tropical forest is further reduced.
    • Albedo changes are also important in the high latitudes. If the boreal forests are replaced by tundra, albedo increases dramatically tundra can be buried under snow. This results in much colder winter and a longer snow season (Bonan et al., Nature, Vol. 359, pages 716-718, 1992).
    • Climate induced changes in biome redistribution would also include some feedbacks. For example, Foley et al. indicated that the poleward expansion of boreal forests into what is now tundra, about 6000 BP (before present), could have doubled the effect of orbital forcing on climate. Note that the axial tilt of the Earth at that time was 24.1 degrees, compared to 23.5 degrees now, and the perihelion was in mid-September
    • Replacing the current orbital parameters with those from the mid-Holocene (12,000 to 5000 years BP) changes summer precipitation by 12 percent between 15N-22N. And, replacing the desert with grassland and the sandy soil with a loamy soil (thought to have resulted from the increased precipitation in that zone), further increases precipitation by 6 percent and 10 percent, respectively.
      • Thus, the total increase in precipitation is about 28 percent. Under such conditions, the size of the Sahara shrinks by 20 percent of its present extent, and the current border between the Sahara and the Sahel, moves northward by about 5 degrees. This is in good agreement with data from fossil pollen, ancient lake sediments, and archeaological evidence.
    • CO2 induced changes in stomatal conductance are expected to have further feedback on climate. If stomatal conductance is halved globally, surface air temperatures over vegetated areas would increase by about 0.5C; compare this to the 1.1 to 2.5C expected from the combined effects of traces gases and aerosols.

    7.2. Marine Biotic Responses

    Marine biogeochemical processes both respond to and influence climate. The oceans contain about 40,000 GtC in dissolved, particulate, and living forms. By contrast, land biota, soils and detritus total about 2,200 GtC. Living and dead biogenic matter in the ocean contains at least 700 GtC, almost equal to the amount of CO2 in the atmosphere (about 750 GtC). Because of the complexity of biological systems, we cannot yet say whether some likely feedbacks from the marine biota in response to climate related changes will be positive or negative.

    • New nutrients, including iron, coming from outside the ocean (increased atmospheric deposition and coastal runoff) would increase organic carbon production, its export to the deep ocean (the Biological Pump), and the drawdown of atmospheric CO2 (probably less than 1 GtC/yr).
    • Calcium carbonate (CaCO3) is fixed during photosynthesis by those organisms that have hard parts of CaCO3. This CaCO3 sinks from the surface layer with the exported organic carbon, but each molecule removed is accompanied by creation of a molecule of CO2 in the surface ocean. Globally, the ratio of C_org to CaCO3 exported from the surface ocean is about 4 to 1, but a shift in phytoplankton species which can change this ratio to 1:1 would neutralize the effect of the biological pump.

    Coupled time-dependent general circulation models are principal tools in probing the possible response of land-atmosphere-ocean system to climate change. At present, models constructed to consider biogeochemical processes are severly limited by their inability to parameterize important biological activities.

    7.3. Sea Level Changes

    The possible climate related factors contributing to sea level rise include thermal expansion of the ocean, melting of glaciers, ice caps and ice sheets, and changes in surface water and ground water storage. With respect to the past 100 years, recent analysis suggests that -

    • Global mean sea level has risen 10-25 cm over the past 100 years
    • There has been no detectable acceleration of sea level rise during this century. However, the average rise is significantly higher than the average rise of the several thousand years.
    • It is likely that the rise in sea level has been largely due to the concurrent increase in global temperature over the past 100 years.
    • With respect to both the Greenland and the Antarctic ice sheets, the observational evidence is insufficient to say with any certainity that the ice sheets are in balance or have increased or decreased in volume over the past 100 years (IPCC, 1995). However, recent results do indicate several alarming changes in the cryosphere!

    Projections of future changes in sea level as a consequence of greenhouse gas induced warming were made for the IPCC emission scenarios.

    • For the business-as-usual scenario, sea level is projected to be about 50 cm higher than today by the year 2100, with an uncertainity range of 20-86 cm.
    • For the range of emission scenarios, sea level is projected to be 38-55 cm higher than today by the year 2100.
    • The extreme range of projections, taking into account both emission and model uncertainities, is about 13-94 cm.
    • Most of the projected rise in sea level is due to thermal expansion, followed by an increased melting of glaciers and ice caps.
    • The changes in future sea level rise will not occur uniformly over the globe. Recent coupled model experiements suggest that regional responses could differ significantly due to regional differences in heating and circulation changes.

    Further Readings
    Impacts of Global Climate Change
    Responses of Primary Production and Total Carbon Storage ...
    Biodiversity and Ecosystem Response To Climate Change
    Impacts of Global Climate Change ...
    Exploring the Capacity of the Ocean to Retain Artificially Sequestered CO2
    Global sea level change: Determination and interpretation
    Potential Impacts of Sea-Level Rise on Populations and Agriculture
    Sea Level Rise
    The Probability of Sea Level Rise