Since the formation of the earth around 4.5 billion years ago, it’s climate has undergone vast changes due to the constant evolution of its physical elements like its landmasses (continents) and oceans, changes in the levels of greenhouse gases contained within its dense atmosphere, the intensity of radiation emanating from the sun, as well as the earth’s orbit around it. Though during the last billion years, the temperature consistency of the planet has been broken by periods of frost starting from about 925 million years ago and even up to about 2 million years ago, overall, for most of its history, its mean temperature was about 8 to 15 degrees warmer than at present. During the past million years, the average temperature of the earth’s surface has hovered at 5 degrees Celsius more or less through the course of 10 ice ages.
The last 8000 years or so have seen temperatures on the warmer side, leading to a slow melting of the polar ice caps. However, this noticeable increase in mean temperature has been a recent development, preceded by climate changes that have occurred very slowly, over tens of thousands or even millions of years. The only known evidence of any naturally fast and drastic changes in temperature and climate that equaled the effect of the greenhouse gas emissions caused by human intervention took place about 56 million years ago. At this time, the global temperature increased by about 5 degrees Celsius, accompanied by a significant release of greenhouse gases into the atmosphere.
The period from about 2 million to about 14,000 years ago is known as the Pleistocene or Ice Age. During this period, most of our presently populated continents were covered under thick sheets of ice for long periods of time. However, there was a constant advance and retreat of glaciers caused by regional shifts in temperatures towards the warmer climes as well as the shifting of ocean currents. The most recent glacial retreat, caused by an average increase in temperatures, began around 12000 B.C. Between 10000 and 8500 B.C., there was a period of coolness caused by the release of fresh water trapped behind ice in North America into the Atlantic ocean, which caused an alteration in ocean currents that subsequently released heat energy into the atmosphere. From 8500 B.C. onwards, temperatures began to shift upwards steadily and remained so till about 3000 B.C., when they were at their warmest, about 1 – 2 degrees hotter than the average temperature today. This warm period and the resultant increase in water level in rivers and other water bodies caused by ice melt caused several civilizations to begin and flourish. This phase also saw a heavy increase in tropical vegetation.
From 3000 B.C. onwards, there have been several alternating phases of cooling and heating, which lasted till the year 1850, after which the average temperature has remained on the warmer side. The period between 1990 to 2006 has observed 10 of the warmest years in the last 100 years of recorded climate history, and the year 2005 is considered the warmest in 1200 years of the earth’s history. Scientists are of the opinion that human activities have resulted in the recent spurt in temperatures consistently due to the development of the greenhouse effect.