The Signs of Global Warming

Updated 7/11/2011, published online 8/9/2010

 

The earth’s climate fluctuates naturally, with cold and warmer periods. However, many climatic, biological and chemical indicators point to an unusual level of global warming that started in the early 20th century.

The Earth's Climate Changes Naturally

The earth goes through periods of climatic cooling and warming that can extend over several tens or hundreds of thousands of years.
There are two main causes for these variations:

       • Changes in the chemical composition of the atmosphere;

      • Changes in the amount of solar energy received by the earth. These depend on the shape of the earth’s rotation around the sun and on the angle of the earth’s rotational axis. But fluctuations in the sun’s activity can also explain why the intensity of the sun’s rays reaching the earth can vary. The sun goes through phases of high activity and lesser activity.

So, during glacial periods, a large part of the earth’s surface is covered in ice. When the climate warms up, this ice retreats and the earth enters into what is known as an interglacial period. Currently the earth is in an interglacial period that started 10,000-12,000 years ago. This is known as the Holocene. Derived from the Greek words holos (whole) and kainos (new), this term means "entirely new". It refers to an unprecedented development of human civilization, particularly the invention of writing and farming. In climate terms, this period is characterized by stable temperatures that have favored the development of human societies.

However, the Holocene has been marked by episodes that are a little colder or a little warmer in some regions (fluctuations in solar activity due to unusually long cycles are one of the possible causes for these variations):

  • In the 10^th and 11^th centuries, the Vikings began settling in the south of Greenland, which, due to a brief warm period was partially free of ice and enabled them to colonize the region;

   • Between the 16^th and 19^th centuries, the northern hemisphere went through a cooling period called the Little Ice Age, during which agricultural crops in Europe fell significantly.

Unusual Levels of Global Warming

In the last few decades, scientists have been collecting data to trace the changes in the global climate. They do this work on behalf of:

  • Scientific and agronomic research organizations;

  • Meteorology, climatology and glaciology organizations.

Apart from natural climate variations, researchers have also noted an abnormal rise of 0.74°C in average temperatures on the earth’s surface over the period 1906-2005¹ compared to those recorded between 1860 and 1905 (there are no reliable, systematic temperature readings for previous periods). This rise may seem too low to have any real impact on the earth’s climate.

To understand the consequences, we need to remember that many climate phenomena depend on specific temperature conditions. For example, cyclones form and circulate above oceans where the temperature is above 27°C so even a slight temperature rise can change the frequency, intensity, or the paths these depressions usually take.

Global Warming Indicators

As well as temperature readings, experts also have various climatic, biological and chemical indicators to help them pinpoint the changes affecting the planet’s climate (changes in sea levels or rainfall, changes in ecosystems, etc.). Since 1988, these indicators have been assessed and summarized by the IPCC (Intergovernmental Panel on Climate Change) in various reports.

In drawing up its 2007² report, the IPCC used 29,000 series of observations drawn from 75 studies. According to IPCC experts, 89% of these indicators show that many physical and biological systems are currently experiencing disruption, signs of global warming.

Climatic Indicators³

Biological and Chemical Indicators⁴

[1, 2, 3, 4] Source: IPCC Synthesis Report on Climate Change (2007).