Here, we show in detail the present-day peak seen in Figs. 1.4 and 1.5, followed by projections of the temperature rise in the future as computed by climate modeling using the extensive observational database illustrated in the previous section. Figure 1.9 shows the temperature variation over the past 1,000 years as deduced by various methods (proxies). There is considerable disagreement up to about 1850, but with better data since then, all the proxies agree on the most recent temperature rise. The agreement is quite amazing since the range of the entire graph is only 1.8°C.

The decreasing uncertainly is seen more clearly in Fig. 1.10, where the weighted global average temperature deviations are shown with error bars for the calculated standard deviation.

Figure 1.11 shows the data from 1850 to the present for the northern and southern hemispheres and their average. The North has higher recent temperatures, probably because there is more industry; but other than that, the histories are similar, showing that the trend is truly global. The error bars on each point are significant: they indi­cate that there is only a 5% chance that the true value lies outside those ranges. It is quite clear from this that the earth’s temperature has risen from about -0.3 to +0.6°C (relative to 1980) since the preindustrial period.

The question is now whether the temperature increase is anthropogenic or not. Climate modelers have calculated the natural forcings and those caused by man, as shown in Fig. 1.3. Remember that these forcings depended on the “parameters” that the modelers chose to find the average over fine-scale variations in space or time.

Their projections, shown in Fig. 1.12, all agree up to year 2000 by design. The param­eters had been chosen so that the twentieth century data were correctly predicted by the models from the data from the century before that data. This is how the models are calibrated. The models can predict the future as long as the parameters do not change. Nonetheless, different models give different results for the future, and there

is a large range of uncertainty. The lowest curve in Fig. 1.12 is what would happen if the GHG level were held constant at the 2,000 levels with no further emissions. The temperature will not go down because the CO2 in the atmosphere stays there for hundreds of years. The three models shown predict a temperature rise of 1.8-3.6°C

by the year 2100. The 2007 IPCC report [6] gives the results of six scenarios ranging from optimistic to pessimistic. The most optimistic scenario predicts a temperature rise of 1.1-2.9°C in the next 100 years, and the most pessimistic one is a rise of 2.4-6.4°C. The range given for each model represents the 66% probability level.

I have chosen graphs which give an idea of the uncertainties in both the data and the models because the IPCC report has been challenged by individual scientists who have arrived at different conclusions.4 Though the ICPP’s Working Group 1 had input from over 600 scientists, only a fraction were involved with any one problem, and arguments are bound to arise. Nonetheless, it seems clear that GHG emissions will be harmful to some extent in the future, and these can be suppressed by replacing fossil fuels with other energy sources. There is no need to argue.