1. EVIDENCE - How do we know?
2. Solutions
3. CAUSES
3.1. greenhouse effect
3.1.1. On Earth, human activities are changing the natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2. To a lesser extent, the clearing of land for agriculture, industry, and other human activities has increased concentrations of greenhouse gases.
3.1.1.1. Greenhouse gases
3.1.1.1.1. Water vapor
3.1.1.1.2. Carbon dioxide (CO2).
3.1.1.1.3. Methane.
3.1.1.1.4. Nitrous oxide
3.1.1.1.5. Chlorofluorocarbons (CFCs)
3.1.2. The consequences of changing the natural atmospheric greenhouse
3.1.2.1. Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer.
3.1.2.2. On average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not.
3.1.2.3. A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise.
3.1.2.4. Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities.
3.1.3. The role of human activity
3.1.3.1. In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change, a group of 1,300 independent scientific experts from countries all over the world under the auspices of the United Nations, concluded there's a more than 95 percent probability that human activities over the past 50 years have warmed our planet. The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million in the last 150 years. The panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in Earth's temperatures over the past 50 years.
3.2. Solar irradiance
3.2.1. It's reasonable to assume that changes in the sun's energy output would cause the climate to change, since the sun is the fundamental source of energy that drives our climate system. Indeed, studies show that solar variability has played a role in past climate changes. For example, a decrease in solar activity is thought to have triggered the Little Ice Age between approximately 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the 1720s and glaciers advanced in the Alps.
3.2.1.1. Several lines of evidence show that current global warming cannot be explained by changes in energy from the sun:
3.2.1.1.1. Since 1750, the average amount of energy coming from the sun either remained constant or increased slightly.
3.2.1.1.2. If the warming were caused by a more active sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere. That's because greenhouse gases are trapping heat in the lower atmosphere.
3.2.1.1.3. Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.