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The Earth's climate has changed a number of times over the course of its history, changing between ice ages and times of extreme warmth, as a result of natural factors, however, during the last 150-200 years, humans have altered this cycle by adding a tremendous amount of greenhouse gases to the atmosphere, introducing a new factor into the climate equation that has been accelerating changes. Greenhouse gases are essential to life on this planet, naturally having a mean warming effect of about 59°F, but the increase in greenhouse gases since the Industrial Revolution has been unprecedented, with CO2 reaching its highest level in possibly as long as 20 million years, with fossil fuel burning accounting for approximately 75% of this increase and deforestation making up the rest of it.

There is a big difference between climate and weather
Often, climate changes are discussed in terms of global averages, for example, the average surface area of the Earth has increased about 0.6° ± 0.2°C during the twentieth century and is projected to increase from 1.4 to 5.8 degrees by 2100, which may not seem like very much considering the daily fluctuations we typically see. This is where it is important to remember that there is a big difference between climate and weather. Climate looks at weather patterns over a long time, whereas weather changes day to day. Global averages can be deceiving, as the difference between the global average temperature today and during the last Ice Age was only about 5 degrees Celsius. This increase over the last 100 years has put the world at its highest temperature in the last 1000 years.

How We Know Now

Global temperatures have increased by 0.75 °C (1.35 °F) since between 1860 and 1900. Since 1979, the land temperatures have increased at a rate of 0.25 °C per decade, sea temperatures at a rate of 0.13 °C per decade and lower troposphere temperature increases have been between 0.12 and 0.22 °C per decade. Prior to 1850, the temperature is believed to have been relatively stable for 1,000-2,000 years and 2006 was the warmest year on record according to NOAA since reliable data became available in the late 1800s. The Earth has gone through periods of heating and cooling in the past, with eight glacial cycles in the last 800,000 years according to Antarctic ice cores, but what's concerning about this warming is that it is believed to be outside of the cycle due to human behaviors and is also occurring at a rather rapid rate.

Trying to figure out how much of this warming is due to humans and how much warming will occur in the future is done through complex climate models that try to take into account as many of the natural processes occurring as possible and also try to estimate what the future outputs of various greenhouse gases will be. Climate models have difficulty determining whether warming that occurred from 1910 to 1945 was from human or natural effects, but agree that warming since 1975 is predominantly a result of human behaviors. They also predict future warming from human behaviors, somewhere in the range of a 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) temperature increase by the end of the 21st century when compared to 1980-1999 levels.

Below are some additional, indirect evidences of climate change, in addition to the direct temperature measurements described above:
  • Pollen Analysis: The study of contemporary and fossil plant particles is used to infer the geographical distribution of plant species, which vary under different climate conditions. Different groups of plants have pollen with distinct shapes and surface textures and tend to have an outer surface that resists degradation, allowing the pollen to be used to get historical data on plant types.
  • Beetles: Similar to pollen analysis, different beetles are known to exist in different climates, so looking at the remains of beetles in an area with respect to age of the remains can help map climate.
  • Glacial Geology: Advancing glaciers leave behind a number of easily distinguishable features. Similarly, the lack of glacier cover can be identified via a number of means, allowing glacier presence or absence to be easily mapped. The IPCC considers glaciers to be one of the most sensitive climate indicators.


It is possible to use core samples to determine past temperature and CO2 levels by using air bubbles trapped in ice that have been formed by snow falls in areas where melting rarely occurs getting compressed into ice and trapping some air from the current atmosphere. Fairly accurate time scales for ice cores can be developed for at least the last 10,000 years, and estimates have been made going as far back as 420,000 years.
EPA - Climate Change
EPA's climate change homepage
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Wikipedia - Global Warming
Wikipedia article on global warming
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Exploratorium - Global Climate Change
Overview of climate change research
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Wikipedia - Global Warming
Wikipedia article on global warming
Click now to view
Wikipedia - Global Warming
Wikipedia article on global warming
Click now to view
Wikipedia - Climate Change
Wikipedia article on climate change
Click now to view
Exploratorium - Global Climate Change
Overview of climate change research
Click now to view
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