We begin by considering the long-term rising trend referred to as global warming, and then consider the effect of rising surface temperatures on sea levels and patterns of rainfall.
Global land and sea surface temperatures are rising
In the 100 years between 1905 and 2005, the average global surface temperature has increased by 0.74 °C. Most of this increase has occurred since 1950 and the upward trend is continuing (Bates et al., 2008). The first decade of the 21st century was the warmest since humans began recording surface temperature (IPCC, 2013). If present trends continue, the average global surface temperature is projected to increase by between 0.3 °C and 4.8 °C by the end of the 21st century (IPCC, 2013).
The surface temperature of the oceans has also been rising and the increase is expected to accelerate during the 21st century. Figure 9.1 shows global averages for the annual combined land and ocean surface temperature and the average in each decade from 1850 to 2012.
Look at the general pattern of surface temperatures shown in Figure 9.1. How would you describe the trend during this period?
The global surface temperature fluctuated around a relatively stable average from 1880 to about 1920, but after that date it began to rise. Around 1950, the temperature increase seems to have levelled for about 30 years, but it has been increasing rapidly in the decades since 1970.
Effects of rising global temperature on sea levels
Increased global temperature affects the amounts of snow, ice and glaciers that remain frozen. Several reports indicate that the frozen water in the northern hemisphere is melting faster than expected, particularly in the last two decades (IPCC, 2013). As you can see from Figure 9.2, there has been a sharp decrease since about 1960 in the extent of Arctic sea ice during the summer months and in the spring snow cover in the northern hemisphere. About 60–70% of Africa’s glaciers have also been lost since the early 1900s (UNEP, 2012).
When there is increased melting of polar ice sheets and snow cover, the melted water flows into the oceans and contributes to the long-term rising trend in global sea levels, shown in Figure 9.3. The global sea level has risen by an estimated 190 millimetres (mm) since the start of the 19th century.
Rising sea levels erode coastlines and allow seawater to flood inland, damaging coastal towns and cities.
What other effects will seawater flooding have on coastal communities and environments?
Sea water contaminates fresh water and soils with salt, which reduces the productivity of agricultural land, kills trees and destroys habitats for animals and plants. Salt damage to the environment causes shortages of food and fresh drinking water, leading to malnutrition, displacement of human populations and loss of biodiversity.
Global warming also increases the evaporation of surface water into the atmosphere, which in turn leads to changes in the patterns and intensity of rainfall in different parts of the world. Changes in rainfall cause an increase in floods in some regions and more frequent droughts in others. You will learn more about these changes and their effects on humans and the environment in Study Sessions 10 and 11. Next we discuss the causes of the global climate changes we have just described.