The Earth's natural climate system is a sensitive combination involving the land, including vegetation, snow and glaciers, the oceans and seas, including the ice in polar regions and the atmosphere. Circulation of the atmosphere and in the oceans, along with the hydrological cycle between water, air and land are responsible for climate and climate variability. Humans have adjusted agricultural and other activities to the current climatic configuration of the Earth, i.e. the present patterns of temperature, precipitation, wind, over the days, months and seasons of the year, that we think of as "normal".
Global climates are influenced by incoming solar radiation, the arrangement of landmasses and water bodies, and the composition of the atmosphere. These conditions, and therefore climate, can change over time. Paleoclimatic data indicates that the Earth has experienced both warm phases and ice ages, with the cold phases lasting generally for shorter periods. In the mid cretaceous period (120 to 90 million years ago), dinosaurs roamed in northern areas and sea-levels were much higher than at present, as less water was held as ice.
The most recent geological period, the Quarternary, has seen numerous oscillations in temperature and icecaps. These are called glacial/interglacial cycles. The Earth is at present in the Holocene period, which began about 15 thousand years ago. The warming was interrupted by a cold phase called the Younger Dryas, but about 11,800 years ago, an abrupt warming brought the climate into the interglacial phase we are still experiencing today.
In the 20th century, the influence of human activities, burning fossil fuels, deforestation, and similar activities have changed the chemical composition of the atmosphere, which has impacted on the natural system. For example, the development of urban areas has created different ground characteristics that have resulted in urban heat islands in which cities are warmer, particularly at night, than the surrounding countryside. The input of carbon dioxide (CO2) and other greenhouse gases into the atmosphere through human activities enhances the natural greenhouse effect, leading to an increase in the Earth's average surface temperatures. Climate change will influence the Earth's temperature, sea level and weather patterns, which will dramatically change the Earth as we know it today.
The influence of global warming trends on such disparate elements as climate change, sea level rise, El Nino/La Nina phenomena, ozone depletion, desertification, coral reefs and the Ozone Hole are featured here.