Global Environment.com

Climate Change

Past and Future Changes in the Earth’s Climate

Layers of ancient ice in Antarctic glaciers provide a time capsule whose contents can be analyzed to provide information about the temperature and contents of the atmosphere in the ancient past. Such analyses and other data show that the earth’s average surface temperature has fluctuated considerably over geological time. Theses data show that during the past 800,000 years several ice ages have covered much of the planet with thick ice. Each glacial period lasted 100,000 years and was followed by a warmer interglacial period of 10,000-12,500 years.

For the past 10,000 years we have enjoyed the warmth of the latest interglacial period (called the Holocene). This climatic stability has prevented drastic changes in the nature of soils and vegetation patterns throughout the world, allowing large increases in food production and thus in population. However, even small temperature changes during this period have led to large migrations of peoples in response to changed agricultural and grazing conditions.

Analysis of gases in bubbles trapped in ancient ice show that over the past 160,000 years tropospheric water vapor levels have remained fairly constant. During most of this period levels of CO2 has fluctuated between 190 and 290 parts per million. Estimated changes in the levels of tropospheric CO2 correlate fairly closely with estimated variations in the earth’s mean surface temperature during the past 160,000 years.
Since 1860, mean global temperature has risen 0.3-0.6oC. The temperature rose about 0.3°C between 1946 and 1995. Since 1860 the twelve warmest years occurred between 1979 and 1997; 1997 was the hottest of all, followed closely by 1995 and 1990. The Intergovernmental Panel on Climate Change (IPPC) believe that the observed increase over the last century is unlikely to be entirely due to natural causes but to humans influences as well.

According to the latest climate models, the IPPC projects that the earths mean surface temperature will rise 1-3.5°C between 1990 and 2100. This may not seem like much, but even at the lowest projected increase of 1.0oC, the earth would be warmer than it has been for 10,000 years.

According to the models, the northern hemisphere should warm more and faster than the southern hemisphere because the latter has more heat-absorbing ocean than land and because water cools more slowly than land. Current climate models project a more pronounced warming at the earth’s poles. Measurements reveal that the surface temperatures at nine stations north of the Arctic Circle have risen by about 5.5°C since 1968. Since 1947 the average summer temperature at Antarctica has risen by almost 2°C and the thick, massive ice shelves surrounding the continent are beginning to break up.

Climate models also project that as the earth’s atmosphere warms, the rate of water evaporation will increase and global average precipitation will rise. With a warmer climate global sea levels will rise, mainly because water expands slightly when heated. Satellite data indicate that ocean surface temperatures have been rising at a rate of about 0.1°C per year since the 1980s. Current climate models of global warming project that global sea levels will rise by 6-37 inches between 1990 and 2100.

The models also indicate that if atmospheric warming at the poles caused ice sheets and glaciers to melt even partially, global sea level would rise much more. However, this is expected to take hundreds of years to occur.

IPPC scientists warn that warming or cooling by more than 1°C over a few decades (instead of over many centuries) will cause serious disruptions of the current structure and functioning of the earth's ecosystems and of human economic and social systems.