Review

Climate, ecosystems, and planetary futures: The challenge to predict life in Earth system models

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Science  02 Feb 2018:
Vol. 359, Issue 6375, eaam8328
DOI: 10.1126/science.aam8328

Figures

  • The various models used for climate projections and mitigation and VIA analyses overlap in scope and would benefit from a broad perspective of Earth system prediction.

    Shown are the domains of ESMs, mitigation models, and VIA models along axes from VIA to climate processes (horizontal) and from primarily serving the research community to informing societal needs (vertical). Panels show forests and agriculture (left) and marine ecosystems (right) as represented across modeling domains.

  • Fig. 1 Representation of the biosphere in Earth system models (ESMs).

    The top panel shows land and ocean as included in climate models, and the bottom panel shows the additional processes included in ESMs. ESMs simulate atmospheric CO2 in response to fossil fuel emissions and terrestrial and marine biogeochemistry. Some ESMs also simulate atmospheric chemistry, aerosols, and CH4. Terrestrial processes shown on the left side of the diagram include biogeophysical fluxes of energy, water, and momentum; biogeochemical fluxes; the hydrologic cycle; and land-use and land-cover change (13). The carbon cycle includes component processes of gross primary production (GPP), autotrophic respiration (RA), litterfall, heterotrophic respiration (RH), and wildfire. Carbon accumulates in plant and soil pools. Additional biogeochemical fluxes include dust entrainment, wildfire chemical emissions, biogenic volatile organic compounds (BVOCs), the reactive nitrogen cycle (Nr), and CH4 emissions from wetlands. Ocean processes are shown on the right side of the diagram. Physical processes include sea ice dynamics, ocean mixing and circulation, changes in sea surface temperature (SST), and ocean-atmosphere fluxes. The gray shaded area depicts the marine carbon cycle, consisting of the phytoplankton-based food web in the upper ocean, export and remineralization in the deep sea and sediments, and the physiochemical solubility pump controlled by surface-deep ocean exchange (100).

  • Fig. 2 Schematic depiction of Earth system prediction of the biosphere.

    The synergies between climate feedback processes, internal climate variability, and ecosystem impacts determine model outcomes. Subseasonal to seasonal forecasts and decadal climate prediction are initial value problems. Earth system projections are a boundary value problem driven by anthropogenic forcing scenarios. Uncertainty arises from inexactness of initial conditions, model imperfections, and scenarios.

  • Fig. 3 Ocean and land forced trends relative to internal variability and model uncertainty.

    Data are from a range of ESMs contributed to CMIP5. (A to C) Multimodel time of emergence for SST, O2, and net primary production (NPP) (89). Time of emergence is defined as the year at which the signal exceeds the noise, which, as used here, includes both internal variability and model uncertainty. The forced SST signal emerges rapidly in many locations, O2 time of emergence is regionally variable, and the forced NPP signal does not statistically emerge by 2100. (D to F) Signal-to-noise ratio for cumulative land carbon uptake in a business-as-usual scenario at 2030 for three different ESMs (92). Positive (negative) values indicate carbon gain (loss). In these panels, the noise is strictly internal variability, and a ratio greater than 2 or less than –2 indicates that the signal has emerged from the internal variability. There are considerable differences among models in the sign of the terrestrial carbon flux and whether the change has emerged from natural variability by 2030. CCSM4, Community Climate System Model version 4; HadGEM2-ES, Hadley Centre Global Environmental Model version 2; CanESM2, second-generation Canadian Earth System Model.

  • Fig. 4 Ocean and land carbon cycle uncertainty.

    The percentage of total variance attributed to internal variability, model uncertainty, and scenario uncertainty in projections of cumulative global carbon uptake from 2006 to 2100 differs widely between (A) ocean and (B) land. The ocean carbon cycle is dominated by scenario uncertainty by the middle of the century, but uncertainty in the land carbon cycle is mostly from model structure. Data are from 12 ESMs using four different scenarios (94).

Tables

  • Table 1 Planetary stresses faced by terrestrial and marine ecosystems.

    StressReference
    Terrestrial ecosystems
    Greening of the biosphere
        Earlier springtime and longer growing season (101)
        Higher leaf area index (55)
        Greater productivity (56, 91)
        Higher water-use efficiency (102)
        Increased nitrogen deposition (5, 26, 28)
        Diffuse radiation (103)
    Browning of the biosphere
        Tree mortality (104)
        Extreme events (105)
        Wildfire and insects outbreaks (36, 106)
        Ozone damage (29, 30)
        Community assemblages (107)
        Land-use and land-cover change (4, 40)
        Human appropriation of net primary production (108)
    Marine ecosystems
    Vertical stratification, nutrient supply and phytoplankton productivity (86, 109)
    Plankton seasonal phenology (46)
    Coral bleaching (110)
    Polar marine ecosystems and sea ice loss (111, 112)
    Community assemblages (46, 49, 113)
    Acidification (114)
    Deoxygenation (57)
    Aerosol deposition (28)

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