Mountains

There is now little doubt that the presence of mountain ranges on the Earth can dramatically influence global climate, and that episodes of mountain building through Earth History have acted as mechanisms of global climate change. The process of mountain building is very slow however, being associated with continental drift and taking tens or even hundreds of millions of years. Consequently, changes in climate as a result of such tectonic movements develop only over such long periods of time.

The temperature difference between the equator and the poles generates wind which carries heat gradually north and south from low latitudes to high latitudes on the Earth. Global atmospheric circulation patterns however, maintain a more east-west trend on account of the Earth's rotation, which deflects moving air due to the Coriolis force. North-south orientated mountain ranges therefore, have the ability to influence such circulation patterns which regulate the global climate system. The Rocky Mountains for example, which extend along the western side of the North American continent, influence significantly the climate on their eastern flank. Air originating from the Pacific Ocean is deflected north around them before returning southwards, bringing with it exceptionally cold temperatures in winter to much of the central US and Canada.

Scientists have suggested that the uplift of the Tibetan Plateau and the Himalayas, resulting from the collision of India into Asia 20 million years ago, may have contributed to the long term global cooling that has taken place during the last 40 million years. In addition to its effect on global atmospheric circulation, the mountain uplift exposed greater volumes of rock to the effects of chemical and physical weathering. During chemical weathering, carbon dioxide dissolved in rainwater helps decompose rock minerals to form bicarbonates. These bicarbonates are soluble and are transported via rivers downstream to the coasts where they are deposited on ocean floors as sediment. This locks away the carbon dioxide for millions of years, thereby reducing the amount of carbon dioxide remaining in the atmosphere. Since carbon dioxide is a greenhouse gas, increased rates of mountain building can diminish the strength of the Earths natural greenhouse effect, contributing to global cooling.

Air is colder at higher altitudes, where precipitation may fall as snow rather than rain. Mountain uplift increases the amount of high altitude land area and therefore the surface area covered by snow the year round. Snow, of course, has a much greater reflectivity or albedo than most other snow- and ice-free surfaces. The subsequent increase in the amount of reflected sunlight reduces the amount of energy absorbed at the Earths surface, encouraging further cooling. This cooling process is known as the ice-albedo feedback effect.