Earth's climate changes on many different time scales, ranging from tens of millions of years to decadal and even shorter time scale variations. In the last 2.5 billion years, several periods of glaciation have been identified, separated by periods of mild climate similar to that of today. Other periods are marked by global hothouse type conditions, when Earth had a very hot and wet climate, approaching that of Venus. These dramatic climate changes are caused by a number of different factors that exert their influence on different time scales. One of the variables is the amount of incoming solar radiation, which changes in response to several astronomical effects such as orbital tilt, eccentricity, and wobble. Changes in the incoming solar radiation in response to changes in orbital variations produce cyclical variations known as Milankovitch cycles. Another variable is the amount of heat that is retained by the atmosphere and ocean, or the balance between the incoming and outgoing heat. A third variable is the distribution of landmasses on the planet. Shifting continents can influence the patterns of ocean circulation and heat distribution, and placing a large continent on one of the poles can cause ice to build up on that continent, increasing the amount of heat reflected back to space and lowering global temperatures in a positive feedback mechanism. Continental collisions and the formation of supercontinents can expose many rocks to weathering, which draws CO2 out of the atmosphere and lowers global temperatures. Supercontinent breakup can be associated with large amounts of undersea volcanism, that emits huge amounts of CO2 into the atmosphere, causing global warming.
Shorter-term climate variations include those that operate on periods of thousands of years, and shorter, less regular decadal scale variations. Both of these relatively short-period variations are of most concern to humans, and considerable effort is being extended to understand their causes and to estimate the consequences of the current climate changes the planet is experiencing. Great research efforts such as that of the Intergovernmental Panel on Climate Change are being expended to understand the climate history of the last million years and to help predict the future.
Variations in formation and circulation of ocean waters may cause some of the thousands of years to decadal scale variations in climate. Thermohaline circulation is influenced by the distribution of continents and by the balance between freshwater released from melting glaciers and evaporation in different parts of the globe. If one factor changes, such as widespread melting of Arctic ice, then the patterns of thermohaline circulation can change, perhaps in a matter of years.
Changes in the thermohaline circulation rigor have also been related to other global climate changes. Droughts in the Sahel and elsewhere are correlated with periods of ineffective or reduced thermohaline circulation, because this reduces the amount of water drawn into the North Atlantic, in turn cooling surface waters and reducing the amount of evaporation. Reduced thermohaline circulation also reduces the amount of water that upwells in the equatorial regions, in turn decreasing the amount of moisture transferred to the atmosphere, reducing precipitation at high latitudes.
Atmospheric levels of greenhouse gases such as CO2 and atmospheric temperatures show a correlation to variations in the thermohaline circulation patterns and production of cold bottom waters. CO2 is dissolved in warm surface water and transported to cold surface water, which acts as a sink for the CO2. During times of decreased flow from cold, high-latitude surface water to the deep ocean reservoir, CO2 can build up in the cold polar waters, removing it from the atmosphere and decreasing global temperatures. In contrast, when the thermohaline circulation is vigorous, cold oxygen-rich surface waters downwell and dissolve buried CO2 and even carbonates, releasing this CO2 to the atmosphere and increasing global temperatures.
Major volcanic eruptions inject huge amounts of dust into the troposphere and stratosphere, where it may remain for several years, reducing incoming solar radiation and resulting in short-term global cooling. For instance, the eruption of Tambora volcano in Indonesia in 1815 resulted in global cooling and the year without a summer in Europe. The location of the eruption is important, as equatorial eruptions may result in global cooling, whereas high-latitude eruptions may cool only one hemisphere.
It is clear that human activities are changing the global climate, primarily through the introduction of greenhouse gases such as CO2 into the atmosphere while cutting down tropical rain forests that act as sinks for the CO2 and put oxygen back into the atmosphere. The time scale of observation of these human, also called anthropogenic, changes is short but the effect is clear, with a nearly one degree change in global temperature measured for the past few decades. The increase in temperature will lead to more water vapor in the atmosphere, and since water vapor is also a greenhouse gas, this will lead to a further increase in temperature. Many computer-based climate models are attempting to predict how much global temperatures will rise as a consequence of our anthropogenic influences and what effects this temperature rise will have on melting of the ice sheets (which could be catastrophic), sea level rise (perhaps 30-60 feet [tens of meters] or more), and runaway greenhouse temperature rise (which is possible).
Climate changes are difficult to measure, partly because the instrumental and observational records go back only a couple of hundred years in Europe. From these records, global temperatures have risen by about one degree since 1890, most notably between 1890 and 1940, and again since 1970. This variation however, is small compared to some of the other variations induced by natural causes, and some scientists argue that it is difficult to separate anthropogenic effects from the background natural variations. Rainfall patterns have also changed in the past 50 years, with declining rainfall totals over low latitudes in the Northern Hemisphere, especially in the Sahel, which has experienced major droughts and famine. However, high-latitude precipitation has increased in the same time period. These patterns all relate to a general warming and shifting of the global climate zones to the north. The world's alpine glaciers are shrinking and disappearing, while the desert belts are expanding. Nations of the world need to begin to plan for large-scale environmental changes that will affect patterns of agriculture, mass movements of millions of people, flooding of coastal zones, and shifting deserts, glaciers, and climate belts.
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