Projections of global average temperature

When information of the kind illustrated in Figures 6.1 and 6.2 is incorporated into simple or more complex models, projections of climate change can be made. As we have seen in earlier chapters, a useful proxy for climate change that has been widely used is the change in global average temperature.

The projected increases in global average near surface temperature over the twenty-first century due to increase in greenhouse gases and aerosols as assumed by the six marker SRES scenarios is illustrated in Figure 6.4a. It shows increases for the different scenarios with best estimates for the year 2100 ranging from about 2 to 4 °C. When uncertainties are added, the overall likely range is from just over 1 to over 6 °C - that wide range resulting from the large uncertainty regarding future emissions and also from the uncertainty that remains regarding the feedbacks associated with the climate response to the changing atmospheric composition (as described in Chapter 5).

Compared with the temperature changes normally experienced from day to day and throughout the year, changes of between 1 and 6 °C may not seem very large. But, as was pointed out in Chapter 1, it is in fact a large amount when considering globally averaged temperature. Compare it with the 5 or 6 °C change in global average temperature that occurs between the middle of an ice age and the warm period in between ice ages (Figure 4.6). The changes projected for the

Simple climate models

In Chapter 5 a detailed description was given of general circulation models (GCMs) of the atmosphere and the ocean and of the way in which they are coupled together (in AOGCMs) to provide simulations of the current climate and of climate perturbed by anthropogenic emissions of greenhouse gases. These models provide the basis of our projections of the detail of future climate. However, because they are so elaborate, they take a great deal of computer time so that only a few simulations can be run with these large coupled models.

To carry out more simulations under different future emission profiles of greenhouse gases or of aerosols or to explore the sensitivity of future change to different parameters (for instance, parameters describing the feedbacks in the atmosphere which largely define the climate sensitivity), extensive use has been made of simple climate models.13 These simpler models are 'tuned' so as to agree closely with the results of the more complex AOGCMs in cases where they can be compared. The most radical simplification in the simpler models is to remove one or more of the dimensions so that the quantities of interest are averaged over latitude circles (in two-dimensional models) or over the whole globe (in one-dimensional models). Such models can, of course, only simulate latitudinal or global averages - they can provide no regional information.

Figure 6.3 illustrates the components of such a model in which the atmosphere is contained within a 'box' with appropriate radiative inputs and outputs. Exchange of heat occurs at the land surface (another 'box') and the ocean surface. Within the ocean allowance is made for vertical diffusion and vertical circulation. Such a model is appropriate for simulating changes in global average surface temperature with increasing greenhouse gases or aerosols. When exchanges of carbon dioxide across the interfaces between the atmosphere, the land and the ocean are also included, the model can be employed to simulate the carbon cycle.

Atmosphere

Surface layer

Solar radiation

Infrared radiation

Land

| Heat exchanges

Ocean t

Upwelling

Deep ocean

Deep ocean

Diffusive mixing

Diffusive mixing

Upwelling Í

Sinking of cold polar water

Figure 6.3 The components of a simple 'upwelling-diffusion' climate model.

1900 2000 2100

Year

Figure 6.4 (a) Global averages of surface warming (relative to 1980-99) for the SRES scenarios A2, A1B and B1, shown as continuations of twentieth-century simulations. Each curve is a multi-model average from a number (typically around 20) of AOGCMs; shading denotes the one standard deviation range of individual model annual means. A curve is also shown for a scenario in which greenhouse gas concentrations were held constant at year 2000 values. The grey bars at the right indicate for year 2100 the best estimate and likely range for the six SRES marker scenarios taking into account both the spread of AOGCM results and uncertainties associated with representations of feedbacks (see Chapter 5). To obtain temperature increases from pre-industrial times, add 0.6 °C.

Uncertainty ranges at 2100

1900 2000 2100

Year

Figure 6.4 (a) Global averages of surface warming (relative to 1980-99) for the SRES scenarios A2, A1B and B1, shown as continuations of twentieth-century simulations. Each curve is a multi-model average from a number (typically around 20) of AOGCMs; shading denotes the one standard deviation range of individual model annual means. A curve is also shown for a scenario in which greenhouse gas concentrations were held constant at year 2000 values. The grey bars at the right indicate for year 2100 the best estimate and likely range for the six SRES marker scenarios taking into account both the spread of AOGCM results and uncertainties associated with representations of feedbacks (see Chapter 5). To obtain temperature increases from pre-industrial times, add 0.6 °C.

twenty-first century are from one-third to a whole ice age in terms of the degree of climate change!

Figure 6.4b compares the observed global mean warming from 1990 to 2006 with model projections and their ranges from 1990 to 2025 as presented by the IPCC in its first three assessment reports. Figure 6.4c illustrates the results from 21 different models (as used in constructing the average in Figure 6.4a) for the temperature increase under SRES scenario A1B.

Beginning with the first IPCC report in 1990, the IPCC has consistently projected forecasts of global average temperature increase in the range 0.15 to 0.3 °C per decade from 1990 to 2005. This can now be compared with observed values of about 0.2 °C per decade and projections for all SRES scenarios of about

0.43 on

Observed

Observed

Uncertainty ranges for SRES

A1B A2

Commitment

A1B A2

Commitment

Uncertainty ranges for SRES

1985

1990

1995

2000

2005 Year

2010

2015

2020

2025

Figure 6.4 (b) Model projections of global mean warming compared to observed warming. Observed temperature anomalies (relative to 1960-90 average) are shown as annual (black dots) and decadal average values (black line). Projected trends and their ranges from the IPCC First (FAR) and Second (SAR) Assessment Reports in 1990 and 1995 respectively are shown as green and magenta solid lines and shaded areas and the projected range from the Third Assessment Report (TAR) in 2001 by vertical blue bars -all adjusted to start at the observed decadal average value in 1990. Multi-model mean projections to 2025 from the IPCC Fourth Assessment Report (AR4) in 2007 for the SRES scenarios B1, A1B and A2 as in Figure 6.4c are shown as blue, green and red curves with uncertainty ranges against the right-hand axis. The orange curve shows model projections of warming if greenhouse gas and aerosol concentrations were held constant from year 2000.

this value (largely independent of which scenario) over the next two or three decades (Figure 6.4b). Again, these might seem small rates of change; most people would find it hard to detect a change in temperature of a fraction of a degree. But remembering again that these are global averages, such rates of change become very large. Indeed, they are much larger than any rates of change the global climate has experienced for at least the past 10 000 years as inferred from palaeoclimate data. As we shall see in the next chapter, the ability of both humans and ecosystems to adapt to climate change depends critically on the rate of change.

M c tti

2000 2010 2020 2030

2040 2050 Year

2060 2070 2080 2090

Figure 6.4 (c) Time series for the twenty-first century from 21 models run by climate modelling centres around the world, of annual means of globally averaged surface temperature change (relative to 1980-99 average) under SRES scenario A1B. Multi-model mean is marked with black dots.

2000 2010 2020 2030

The changes in global average temperature shown in Figure 6.4 from the IPCC 2007 Report and similar ones from the 2001 Report are substantially greater than those shown in the IPCC 1995 Report. The main reason for the difference is the much smaller aerosol emissions in the SRES scenarios compared with the IS 92 scenarios. For instance, the global average temperature in 2100 relating to the IS 92a scenario is similar to that for the SRES B2 scenario even though the carbon dioxide emissions at that date for IS 92a are 50% greater than those for B2.

To complete this section on likely temperature changes in the twenty-first century, Figure 6.5 shows multi-model mean temperature changes for the A1B scenario from 1990 to 2065 and 2099 at levels in the troposphere and different depths in the ocean. It shows cooling in the stratosphere, substantial warming in the troposphere especially in the tropics and gradual penetration of warming into the ocean from the surface downwards.

2040 2050 Year

2060 2070 2080 2090

Figure 6.4 (c) Time series for the twenty-first century from 21 models run by climate modelling centres around the world, of annual means of globally averaged surface temperature change (relative to 1980-99 average) under SRES scenario A1B. Multi-model mean is marked with black dots.

0 0

Post a comment