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


    Chapter 5: Projections of Future Climate Change

    5.1. Introduction
    5.2. Global Mean Response
    5.2.A. 1% CO2 Increase Experiments
    5.2.B. Projections of Climate from Forcing Scenarios
    5.3. Patterns of Future Climate Change
    5.4. Temperature Response to Emission Scenarios
    5.4.A. Emission Scenarios
    5.4.B. Temperature Responses
    5.5. Summary of Changes

    5.1. Introduction

    The purpose of this chapter is to assess and quantify projections of possible future climate change from climate models. The IPCC-1995 assessment included results from the first two global coupled models run with a combination of increasing CO2 and sulfate aerosols for the 20th and 21st centuries.

  • The combination of the warming effects on a global scale from increasing CO2 and the regional cooling from the direct effect of sulfate aerosols produced a better agreement with observations of the time evolution of the globally averaged warming and the patterns of 20th century climate change.
  • The two global coupled model runs with the combination of CO2 and direct effect of sulfate aerosols both gave a warming at mid-21st century relative to 1990 of around 1.5C.
  • With increasing greenhouse gases the land was projected to warm generally more than the oceans, with a maximum annual mean warming in high latitudes associated with reduced snow cover and increased run-off in winter.
  • Including the effects of aerosols led to a somewhat reduced warming in middle latitudes of the northern hemisphere and the maximum warming in northern high latitudes was less extensive since most sulfate aerosols are produced in the northern hemisphere.
  • All models produced an increase in global mean precipitation.

    5.2. Global Mean Response

    5.2.A. 1% CO2 Increase Experiments

    Figure 5.1a shows the global average of temperature and precipitation change from 19 coupled model simulations. At the time of CO2 doubling at year 70, the 20 year average (years 61-80) global mean temperature change for these models is 1.1 to 3.1C with an average of 1.8C and a standard deviation of 0.4C. Likewise, at the time of CO2 doubling at year 70, the 20 year average (years 61-80) percentage change of the global mean precipitation for these models ranges from -0.2% to 5.6% with an average of 2.5% and a standard deviation of 1.5% (Figure 5.1b).

    The relative agreement on seasonal climate changes is slightly lower. Only 10-20% of the inter-experiment variance in temperature changes is attributed to internal variability, which indicates that most of this variance arises from differences in the models themselves. The estimated contribution of internal variability to the interexperiment variance in precipitation changes is larger, from about a third in the annual mean to about 50% in individual seasons. Thus there is more internal variability and model differences and less common signal indicating lower reliability in the changes of precipitation compared to temperature.

    Figure 5.1: The time evolution of the globally averaged (a) temperature change relative to the years (1961-1990) of the CMIP2 simulations. [Unit: degreesC]. (b) same for precipitation. [Unit: %].

    5.2.B. Projections of Climate from Forcing Scenarios

    These experiments include changes in greenhouse gases (GHG) plus the direct effect of sulfate aerosol using IS92a (business-as-usual) type forcing. The temperature change for the 30 year average 2021-2050 compared to 1961-90 is +1.3C with a range of +0.8C to +1.7C as opposed to +1.6C with a range of +1.0C to 2.1C for GHG-only (Figure 5.2).

  • The former is smaller because the positive forcing is smaller (since there is the addition of negative radiative forcing associated with the inclusion of the direct effect of sulfate aerosols in the former).
  • Additionally, in these simulations CO2 would double around year 2060. Thus for the averaging period being considered, years 2021-2050, the models are still short of the CO2 doubling point seen in the idealised 1% CO2 increase simulations.
  • These sensitivity ranges could be somewhat higher (about 30%) if the positive feedback effects from the carbon cycle are included interactively but the magnitude of these feedbacks is uncertain.
  • The globally averaged precipitation response for 2021-2050 for GHG plus sulfates is +1.5% with a range of +0.5% to +3.3% as opposed to +2.3% with a range of +0.9% to +4.4% for GHG-only (Figure 5.3).

    Figure 5.2: The time evolution of the globally averaged temperature change relative to the years (1961-1990). G: greenhouse gas only (top), GS: greenhouse gas and sulfate aerosols (bottom). The observed temperature change (CRU) is indicated by the black line.
    Figure 5.3: The time evolution of the globally averaged precipitation change relative to the years (1961-1990). G: greenhouse gas only (top), GS: greenhouse gas and sulfate aerosols (bottom).

    5.3. Patterns of Future Climate Change

    For the change in annual mean surface air temperature in the various cases, the model experiments show maximum warming in the high latitudes of the NH and a minimum in the Southern Ocean (due to ocean heat uptake) evident in the zonal mean for the coupled models (Figure 5.4) and the geographical patterns for all categories of models (Figure 5.5). For the zonal means in Figure 5.4, there is consistent midtropospheic tropical warming and stratospheric cooling. Ocean heat uptake also contributes to a minimum of warming in the North Atlantic, while land warms more rapidly than ocean almost everywhere. The large warming in high latitudes of the Northern Hemisphere is connected with a reduction in the snow (not shown) and sea-ice cover (Figure 5.6 ).

    Figure 5.4: Multi-model annual mean zonal temperature change [Units: degrees C].
    Figure 5.5: The multi-model ensemble annual mean change of the temperature (colour shading) and its range (isolines) [Unit: degrees C] at the time of CO2-doubling.
    Figure 5.6: Change in sea-ice thickness between the periods 1971-1990 and 2041-2060 as simulated by four of the most recent coupled models. The upper panels show thickness changes in the northern hemisphere, the lower panels show changes in the southern hemisphere. All models were run with similar forcing scenarios: historical GHG and aerosol loading, then future forcing as per the IS92a scenario. The colour bar indicates thickness change in metres-negative values indicate a decrease in future ice thickness.

    The relative change of the mean precipitation (Figure 5.7) for all models in all categories shows a general increase in the tropics (particularly the tropical oceans and parts of northern Africa and south Asia) and the mid and high latitudes, while the rainfall generally decreases in the subtropical belts. Areas of decrease show a high intermodel variability and therefore little consistency among models, while in the tropics the change can exceed the variability of the signal by a factor of 2. This is particularly evident over the central and eastern tropical Pacific where the El Nino-like surface temperature warming is associated with an eastward shift of positive precipitation anomalies.

    Figure 5.7: The multi-model ensemble annual mean change of the precipitation (colour shading) and its range (isolines) [Unit: %] at the time of CO2-doubling.

    Summary: First we note results assessed here that reconfirm results from the IPCC-1995.

    • As the climate warms, Northern Hemisphere snow cover and sea ice extent decreases. The globally averaged precipitation increases.
    • As the radiative forcing of the climate system changes, the land warms faster than the ocean. The cooling effect of tropospheric aerosols moderates warming both globally and locally.
    • The surface air temperature increase is smaller in the North Atlantic and circumpolar southern ocean regions.
    • Most tropical areas, particularly over ocean, have increased precipitation, with decreases in most of the subtropics, and relatively smaller precipitation increases in high latitudes.
    • The signal to noise ratio (from the multi-model ensemble) is greater for surface air temperature compared to precipitation.
    A second category of results assessed here are those that are new since the IPCC-1995.
    • There are many more model projections for a given scenario, and more scenarios. The greater number of model simulations allows us to better quantify patterns of climate change for a given forcing and develop a measure of consistency among the models.
    • Including the direct effect of sulfate aerosols according to an IS92a type estimate reduces global mean mid-21st entury warming. The indirect effect, not included in most AOGCM experiments to date, is acknowledged to be uncertain.
    • The geographic details of various forcing patterns are less important than differences among the models' responses. This is the case for the global mean as well as for patterns of climate response. Thus, the choice of model makes a bigger difference to the simulated response than the choice of scenario.

    5.4. Temperature Response to Emission Scenarios

    5.4.A. Emission Scenarios

    IS92a: Business-as-usual scenario, that is, emissions continue to grow at the present rate.
    IS92c: Lower-end scenario of emissions, with the assumption of low population growth, low economic growth, and constraints on fossil fuel supplies.
    IS92e: Higher-end scenario of emissions, with the assumption of moderate population growth, high economic growth, high fossil fuel availability and phase-out of nuclear power.
    SRES-A1: The A1 describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B).
    SRES-A2: The A2 describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing population. Economic development is primarily regionally oriented and per capita economic growth and technological change more fragmented and slower than other storylines.
    SRES-B1 The B1 describes a convergent world with the same global population, that peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid change in economic structures toward a service and information economy, with reductions in material intensity and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social and environmental sustainability, including improved equity, but without additional climate initiatives.
    SRES-B2 The B2 describes a world in which the emphasis is on local solutions to economic, social and environmental sustainability. It is a world with continuously increasing global population, at a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented towards environmental protection and social equity, it focuses on local and regional levels.

    5.4.B. Temperature Responses

    Estimated total historical anthropogenic radiative forcing from 1765 to 1990 followed by forcing resulting from the illustrative scenarios are shown in Figure 5.8a. It is evident that the the six SRES (special report on emission scenarios) scenarios considered cover nearly the full range of forcing that results from the full set of SRES scenarios. The latter is shown on the figure as an envelope since the forcing resulting from individual scenarios cross with time. For comparison, radiative forcing is also shown for the IS92a, IS92e and IS92c scenarios. It is evident that the range for the new SRES scenarios is wider and shifted higher compared to the IS92 scenarios. The range is wider due to more variation in forcing from non-CO2 greenhouse gases and sulphate aerosols. The shift to higher forcing is mainly due to the reduced future sulphur dioxide emissions of the SRES scenarios compared to the IS92 scenarios but also due to the revised forcing calculations.

    Figure 5.8bshows the simple climate model simulations representing AOGCM-calibrated global-mean temperature change results for the six illustrative SRES scenarios and for the full SRES scenario envelopes. The individual scenario timeseries and inner envelope (darker shading) are the average results obtained from simulating the results of seven AOGCMs, denoted 'ensemble'. The average of the effective climate sensitivity of these AOGCMs is 2.8C. The range of global-mean temperature change from 1990 to 2100 given by the six illustrative scenarios for the ensemble is 2.0 to 4.5C ( Figure 5.9).

    Figure 5.8: (a) Estimated historical anthropogenic radiative forcing followed by radiative forcing for the four illustrative SRES marker scenarios and for two additional scenarios from the A1 family illustrating different energy technology options. The dark blue shading shows the envelope of forcing that encompasses the full set of thirty five SRES scenarios. (b) Historical anthropogenic global mean temperature change and future changes for the six illustrative SRES scenarios using a simple climate model tuned to seven AOGCMs. Also for comparison, following the same method, results are shown for IS92a. The dark blue shading represents the envelope of the full set of thirty-five SRES scenarios using the simple model ensemble mean results. The light blue envelope is based on the GFDL and DOE parameter settings. The bars show the range of simple model results in 2100 for the seven AOGCM model tunings.
    Figure 5.9: As for Figure 5.8b but results are relative to 1990 and shown for 1990 to 2100.

    Temperature change results from the simple climate model tuned to individual AOGCMs using the six illustrative SRES scenarios are shown in Figure 5.10. For comparison, analogous results are shown for the IS92a scenario. Note that the range of temperature change for the SRES scenarios tends to be shifted higher compared to the range for the IS92 scenarios largely due to the reduced sulphur dioxide emissions in the new scenarios.

    A second feature of the illustrative SRES scenarios is that their relative ranking in terms of global-mean temperature changes varies with time. The temperature-change values of the scenarios cross in about mid-century because of links between the emissions of different gases. In particular, for scenarios with higher fossil-fuel use, and therefore carbon dioxide emissions (for example A2), sulphur dioxide emissions are also higher. In the near term (to around 2050) the cooling effect of higher sulphur dioxide emissions more than offsets the warming caused by increased emissions of greenhouse gases in scenarios such as A2.

    The opposite effect is seen for scenarios B1 and B2, which have lower fossil fuel emissions, but also lower sulphur dioxide emissions. This leads to a larger near-term warming. In the longer term, however, the level of emissions of long-lived greenhouse gases such as carbon dioxide and nitrous oxide becomes the dominant determinant of the resulting global-mean temperature changes. For example, by the latter part of the 21st century, the higher emissions of greenhouse gases in scenario A2 result in larger climate changes than in the other three marker scenarios (A1B, B1 and B2) even though this scenario also has higher sulphur dioxide emissions.

    In view of the fact that not all the AOGCMs are represented, it is fair to say that the range due to differences in emissions and the range due to different model responses contribute similar amounts to the range in future global temperature change. Further uncertainties arise due to uncertainties in the radiative forcing, especially that due to sulphates.

    Figure 5.10: Temperature changes from (a) 1990 to 2030 and from (b) 1990 to 2100 for the six illustrative SRES scenarios and IS92a. The bottom axis indicates the AOGCM to which the simple model is tuned. For comparison results are also shown for the SAR (IPCC-1995) version of the simple climate model using SAR forcing with some of the IS92 scenarios. IS92a H/M/L refers to the IS92a scenario with climate sensitivity of 1.5, 2.5 and 4.5C respectively. Also shown are the IS92e scenario with a sensitivity of 4.5C and the IS92c scenario with a sensitivity of 1.5C.

    5.5. Summary of Changes

    Figure 5.11 summarizes some of the model results for projections of future climate change for the end of the 21st century. This figure can be compared to one for observations from the 20th century in Chapter 3 (Figure 3.20 and Figure 3.21). A number of the observed changes are qualitatively consistent with those projected for future climate changes from climate models. A confidence scale is provided for the model projections in Figure 5.11. Since there is considerable agreement between the observations and model results, we conclude that many of the larger observed climate changes to date are qualitatively consistent with those changes in climate models for future climate with increases of GHGs.

    Figure 5.11: Schematic of changes in the temperature and hydrological indicators from projections of future climate changes with AOGCMs.

    Climate Impact of Quadrupling Atmospheric CO2
    Forecasting the Future
    Climate Change in the 21st Century
    Future Climate
    Quantifying the Uncertainty in Climate Predictions
    Climate is expected to Continue to Change (IPCC)
    Uncertainties Plague Climate Forecasting, Forum Finds
    Climate Change and Sea Level Rise
    Global Warming in the 21st Century:An Alternate Scenario