3.1.1 Global warming
Since the mid-19th century, human industrial and agricultural activity has caused a change in the concentrations of atmospheric gases. In particular, there have been dramatic increases in the production of carbon dioxide by combustion of fossil fuels. Figure 12 shows deviations in mean surface temperatures from recent average temperatures for two time periods. The first (Figure 12a) shows changes in the global mean annual surface temperature since 1860. The second (Figure 12b) covers the last 1000 years for the Northern Hemisphere, and has been dubbed the 'hockey-stick' trend from its shape; it represents the main evidence for anthropogenic global warming.
Since about 1915, there has been an increase in the annual mean surface temperature in the Northern Hemisphere of about 0.7 °C. Global warming seems to have a more significant effect on night-time rather than daytime temperatures. Since 1950 the minimum daily temperature over most of the landmass of the Northern Hemisphere has risen three times as fast as the maximum temperature. In the UK, nights are on average 0.84 °C warmer than they were in 1950, whereas days are only 0.28 °C warmer — a trend that applies to all northern continents and all seasons.
In North America and Europe, cloud cover has increased along with the warmer nights. Low-altitude clouds limit the loss of heat from the ground by radiation at night, but reflect sunlight and limit the warming effect by day. There are two reasons for increased cloud cover. One is particulate pollution (dust and smoke), since small particles encourage the formation of denser and more numerous clouds. The other is ocean evaporation: the enhanced greenhouse effect may be increasing evaporation from the oceans, leading to increased cloud cover over land.
The fossil fuels burned in the 200 years since the Industrial Revolution took many millions of years to accumulate. The sudden return of ancient carbon to the atmosphere upsets the outer Earth's energy balance, which now gives rise to much concern among scientists, environmentalists and politicians. With no change in the current 'blend' of energy sources, industrial greenhouse gas emissions may result in doubling of the pre-industrial CO2 level by 2028. The global mean surface temperature could increase at about 0.3 °C per decade; faster than any rise seen over the last 10 000 years. By 2030, temperatures could be 0.7-2.0 °C higher than at present, and by the end of the 21st century the rise could be 6 °C.
Increased surface temperatures are likely to be accompanied by a rise in global sea-level, because of thermal expansion of the oceans and melting of land-based ice. A 'business-as-usual' scenario suggests a sea-level rise of 0.2 m by 2030 and 0.65 m by the end of the 21st century.
To stabilise CO2 levels would require an immediate reduction of emissions from human activities. Yet even if fossil fuel burning stopped immediately, the temperature rise would continue because of the time lags in the gas-energy balance in the atmosphere. To eventually achieve a balance that does not destabilise global climate requires finding alternative sources of energy for transport and for electricity generation.
Figure 13 returns to the World Energy Council's scenarios of primary energy use during the 21st century (Figures 8 and 9), and shows the changes in atmospheric CO2 concentration that would ensue from the three cases shown in Table 1. Whichever scenario is used, global mean surface temperature is predicted to rise over the next century by 1 to 2.7 °C (minimum for Case C to maximum for Case A2).